Patent Publication Number: US-6213584-B1

Title: Dual head multicolor printing

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to printing systems capable of bidirectional printing. Specifically, the present invention concerns a system in which either bidirectional or unidirectional printing is selectively performed based on whether data to be printed includes data other than black print data. 
     2. Description of the Related Art 
     A conventional serial printing system prints images on a recording medium by scanning a printing unit across the medium. During scanning, the printing unit places ink on the medium according to received print data. After completion of one scan, the medium is advanced with respect to the printing unit. This process continues until an entire page of print data is printed. 
     In unidirectional serial printing, the printing unit, while printing, scans across the recording medium from a home end of the medium to a far end of the medium. After reaching the far end of the medium, the unit moves, without printing, back to the home end. Bidirectional printing differs from the foregoing in that printing occurs also during movement of the printing unit from the far end of the medium to the home end. Accordingly, it takes more time to print a particular page of data using unidirectional printing than when using bidirectional printing. 
     Although bidirectional printing provides greater throughput than unidirectional printing, bidirectional printing has some drawbacks. Most notable is that data printed during a backward scan is often of different quality than data printed during a forward scan. Several factors may contribute to this difference. For example, a leading edge of the printing unit during the forward scan becomes the trailing edge during the backward scan, therefore printing elements of the printing unit must be extremely symmetrical to avoid creating differences between forward-printed data and backward-printed data. As another example, the printing unit may be disposed at a distance from the recording medium during the forward scan which is different from the corresponding distance during the backward scan, due to vibrations or other mechanical factors. 
     In addition, in the case of ink jet printing systems, a droplet of ink ejected to a desired location is often accompanied by smaller “satellite” droplets which, during a forward scan, often merge with the droplet at the desired location but which, during a backward scan, extend past the desired location in the direction of the backward scan. It should be noted that differences in forward-printed and backward-printed portions of an image due to such satellites are most noticeable in the case of inks having a high optical density. 
     In view of the foregoing problems, what is needed is a system for quickly printing data onto a recording medium while minimizing image gradations caused by bidirectional printing. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing problems by providing a printing system to print bands of print data in a forward direction and in a backward direction. The system includes a step to print print data other than black print data included in the bands of print data using bidirectional printing and a step to print black print data included in the bands of print data using unidirectional printing. By virtue of the foregoing, a desirable tradeoff is achieved between print speed and print quality. 
     This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of computing equipment used in connection with the printer of the present invention. 
     FIG. 2 is a front perspective view of the printer shown in FIG.  1 . 
     FIG. 3 is a back perspective view of the printer shown in FIG.  1 . 
     FIG. 4 is a back, cut-away perspective view of the printer shown in FIG.  1 . 
     FIG. 5 a front, cut-away perspective view of the printer shown in FIG.  1 . 
     FIG. 5A is a top-down plan view of the printer shown in FIG.  1 . 
     FIG. 5B shows a face-on view of clutch plate and gears operated by both line feed motor and carriage motor of the printer shown in FIG.  1 . 
     FIG. 5C is a flow diagram which depicts operation of the automatic sheet feeder process for the printer of the present invention. 
     FIG. 5D is a flow diagram which depicts operation of the capping and purge process for the printer of the present invention. 
     FIG. 6 shows an example of a disposable ink cartridge used with the present invention. 
     FIG. 7 shows a face-on view of head configurations for print heads used with the present invention. 
     FIG. 8 is a block diagram showing the hardware configuration of a host processor interfaced to the printer of the present invention. 
     FIG. 9 shows a functional block diagram of the host processor and printer shown in FIG.  8 . 
     FIG. 10 is a block diagram showing the internal configuration of the gate array shown in FIG.  8 . 
     FIG. 11 shows the memory architecture of the printer of the present invention. 
     FIG. 12 shows an overall system flowchart detailing the operation of the printer of the present invention. 
     FIG. 13 is a flowchart showing print control flow in accordance with the present invention. 
     FIG. 14 depicts a table showing command flow during a printing sequence. 
     FIG. 15 is a flow diagram which depicts a hard power-on sequence for the printer of the present invention. 
     FIG. 16 is a flow diagram which depicts a soft power-on equence for the printer of the present invention. 
     FIG. 17 is a flow diagram which depicts a soft power-off sequence for the printer of the present invention. 
     FIG. 18 illustrates communication according to the preferred embodiment of the invention between an application program and other operations running on a host processor and various tasks running on a printer according to the preferred embodiment of the invention. 
     FIG. 19 is a flow diagram illustrating controller timer control according to a cyclic handler for controlling timer operations. 
     FIG. 20 is a flow diagram which depicts printer driver software process flow. 
     FIG. 21A is a flow diagram which depicts automatic sheet feed sequence of the present invention. 
     FIG. 21B is a continuation of the automatic sheet feed sequence shown in the automatic sheet feed sequence of FIG.  21 A. 
     FIG. 21C is a flow diagram which depicts the early success logic shown in the automatic sheet feed sequence of FIG.  21 A. 
     FIG. 21D is a flow diagram which depicts the load speed select for the automatic sheet feed sequence shown in FIG.  21 A. 
     FIG. 21E is a flow diagram which depicts the recovery sequence as shown in the automatic sheet feed sequence of FIG.  21 A. 
     FIG. 22 is a flow diagram which depicts an automatic sheet feed sequence for a first page within a printer. 
     FIG. 23 is a flow diagram which depicts an automatic sheet feed sequence after an eject sequence in a printer. 
     FIG. 24 is a flow diagram which depicts printer driver logic for the selection of line feed, paper load and eject speeds. 
     FIG. 25 is a flow diagram which depicts eject speed override logic of the present invention. 
     FIG. 26 is a flow diagram which depicts line feed speed override logic of the present invention. 
     FIG. 27A is a representative view of for describing carriage control for printing text, continuous images, and color images. 
     FIG. 27B is a representative view for describing carriage direction control for scan lines which include both non-color continuous and color images. 
     FIGS. 27C to  27 G are tables for determining print direction and other print information based on print mode, head type, paper type, and print data type. 
     FIG. 28 is a representative view for explaining movement of print heads according to the invention. 
     FIG. 29 is a flowchart for describing a SKIP command issued by a printer driver according to the invention. 
     FIG. 30 is a flowchart for describing a PRINT command issued by a printer driver according to the invention. 
     FIG. 31 is a flowchart for describing a DIRECTION command issued by a printer driver according to the invention. 
     FIG. 32 is a flowchart for describing an EDGE command issued by a printer driver according to the invention. 
     FIG. 33 is a flowchart for describing determination of a scan margin by a printer driver according to the invention. 
     FIG. 34 is a flowchart for describing a NEXT_MARGIN command issued by a printer driver according to the invention. 
     FIG. 35 is a flowchart for describing an AT_DELAY (automatic delay) command issued by a printer driver according to the invention. 
     FIG. 36 is a flowchart for describing a carriage task performed by a printer control according to the invention. 
     FIG. 37 is a flowchart for describing a first carriage scan control called by the carriage task of FIG.  36 . 
     FIG. 38 is a flowchart for describing a second carriage scan control called by the carriage task of FIG.  36 . 
     FIGS. 39 a  and  39   b  are representative views for describing satellite control according to the invention. 
     FIG. 40 is a flowchart for describing carriage motor start performed by a printer control according to the invention. 
     FIG. 41 is a flowchart for describing a carriage interrupt process performed by a printer control according to the invention. 
     FIG. 42 is a flowchart for describing automatic trigger delay performed by a printer control so as to alleviate satelliting according to the invention. 
     FIG. 43 is a flow diagram which depicts a printer driver software alignment process of the present invention. 
     FIG. 44 is a series of print mode tables for printing with alignment and without alignment pursuant to the printer driver software alignment process of FIG.  43 . 
     FIG. 45 is a flow diagram of processor-executable process steps to print color data. 
     FIG. 46 illustrates printing of color data and black data using two different ink jet print heads. 
     FIG. 47 is a diagram for describing prefire control in which a prefiring operation is performed at a predetermined interval. 
     FIGS. 48 and 49A to  49 C are diagrams for describing image degradation that can result from inadequate prefiring. 
     FIG. 50 is a diagram for describing prefire control according to the invention. 
     FIG. 51 is a flowchart for describing prefire control timing according to the invention. 
     FIG. 52 is a flowchart for describing an update of prefire timers by a printer controller according to the invention. 
     FIG. 53 is a flowchart for describing a prefire check operation performed by a printer controller according to the invention. 
     FIG. 54 is a flowchart for describing generation of a nozzle-number-change prefire request by a printer driver according to the invention. 
     FIG. 55 is a flowchart for describing scan prefire processing by a printer controller according to the invention. 
     FIG. 56 is a flowchart for describing a prefire (print) function according to the invention. 
     FIG. 57 is a diagram for describing a relationship between ink jet nozzle heat pulse width and output images. 
     FIG. 58 is a diagram for describing a heat pulse width modulation. 
     FIG. 59 is a flowchart for explaining control of nozzle heat pulse driving times. 
     FIG. 60 is a diagram showing exploded views of tables for heat-up coefficients and tables for driving times stored in a printer. 
     FIG. 61 is a flowchart for describing use of a real-time environmental temperature for determination of driving times. 
     FIG. 62 is a diagram for describing heat pulse width modulation during printing of plural scan lines. 
     FIG. 63 is a diagram for describing heat pulse width modulation according to the invention in which a heat pulse width is maximized after a first time interval since a previous prefire operation. 
     FIG. 64 is a flowchart for describing heat pulse width modulation according to the invention in which a heat pulse width is maximized after a first time interval since a previous prefire operation. 
     FIG. 65 is a flow diagram of computer-executable process steps to produce binarized data for five different inks based on RGB data of a pixel. 
     FIG. 66 illustrates a graph of input values versus output values for performing output correction on input values corresponding to five different types of ink. 
     FIG. 67 is a functional block diagram showing computing equipment communicating with the printer. 
     FIG. 68 is a flow diagram illustrating how print driver obtains status from printer and modifies processing of print data generation. 
     FIG. 69 illustrates a flow sequence executed by print controller. 
     FIG. 70 illustrates process steps for bleed reduction. 
     FIG. 71 is a graph of color values. 
     FIG. 72 illustrates values stored in Color Table 1 as opposed to values stored in Color Table 2. 
     FIGS. 73A and 73B are flow diagrams for implementing smear control processing. 
     FIG. 74 is a flow diagram illustrating how the print driver sets the value for the smear timer. 
     FIG. 75 is a flow diagram illustrating how the print driver sets the density threshold for smear control. 
     FIGS. 76 and 77 are flow diagrams for explaining how the print driver modifies speed at which the printer feeds sheets from the feed tray. 
     FIG. 78 is a flow diagram for explaining how the print driver modifies the operational parameter of the printer that controls the timing for pre-fire operations. 
     FIG. 79 shows a portion of user interface displayed by the print driver on the display. 
     FIG. 80 is a flow diagram for explaining how the print driver modifies its own operation based on status of the printer. 
     FIG. 81 illustrates modification of purge speed in the printer. 
     FIG. 82 illustrates modification of print driver operations. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This detailed description of the preferred embodiment is organized into sections, as follows: 
     1.0 Mechanical 
     1.1 Structure 
     1.2 Cleaning 
     1.3 Ink Cartridge 
     1.4 Print Head Structure 
     1.5 Print Modes 
     2.0 Electrical 
     2.1 System Architecture 
     2.2 System Function 
     2.3 Control Logic 
     2.4 General Operation 
     3.0 Architecture of Printer Software 
     3.1 Operating System 
     3.2 Initialization 
     3.3 Tasks 
     3.4 Interrupt Handlers 
     3.5 Cyclic Handlers 
     3.6 Commands To And From The Host Processor 
     3.6.1 Control Commands 
     3.6.2 Setting Commands 
     3.6.3 Maintenance Commands 
     4.0 Automatic Sheet Feed Control 
     4.1 ASF, Line Feed and Eject Speed Selection 
     4.2 Early Determination of Paper Load Success 
     4.3 Print Head Maintenance During Paper Load 
     5.0 Carriage Control 
     5.1 Margin And Direction Control 
     5.1.1 Printer Driver Initiated Operation 
     5.1.2 Print Control Operation 
     5.2 Automatic Ink Ejection and Satelliting Control 
     6.0 Printer Control Based On Head Alignment 
     7.0 Dual Head Multicolor Printing 
     8.0 Prefiring and Pulse Width Modulation 
     8.1 Prefire Control 
     8.1.1 Prefire Timing 
     8.1.2 Embodiment 
     8.2 Pulse Width Modulation Control 
     9.0 Color Printing Using Multiple Inks 
     10.0 Status-Based Control Over Printer 
     10.1 Obtaining Status 
     10.2 Bleed Reduction 
     10.3 Smear Reduction 
     10.4 Automatic Sheet Feed (ASF) Speed 
     10.5 Prefire Timing 
     10.6 Delay of Manual Feed 
     10.7 Purge Speed 
     10.8 Compression Mode 
     1.0 Mechanical 
     This section describes the mechanical layout and functionality of a printer which includes the inventions described herein. 
     1.1 Structure 
     FIG. 1 is a view showing the outward appearance of computing equipment used in connection with the inventions described herein. Computing equipment  1  includes host processor  2 . Host processor  2  comprises a personal computer (hereinafter “PC”), preferably an IBM PC-compatible computer having a windowing environment, such as Microsoft® Windows95. Provided with computing equipment  1  are display  4  comprising a color monitor or the like, keyboard  5  for entering text data and user commands, and pointing device  6 . Pointing device  6  preferably comprises a mouse for pointing and for manipulating objects displayed on display  4 . 
     Computing equipment  1  includes a computer-readable memory medium, such as fixed computer disk  8 , and floppy disk interface  9 . Floppy disk interface  9  provides a means whereby computing equipment  1  can access information, such as data, application programs, etc., stored on floppy disks. A similar CD-ROM interface (not shown) may be provided with computing equipment  1 , through which computing equipment  1  can access information stored on CD-ROMs. 
     Disk  8  stores, among other things, application programs by which host processor  2  generates files, manipulates and stores those files on disk  8 , presents data in those files to an operator via display  4 , and prints data in those files via printer  10 . Disk  8  also stores an operating system which, as noted above, is preferably a windowing operating system such as Windows95. Device drivers are also stored in disk  8 . At least one of the device drivers comprises a printer driver which provides a software interface to firmware in printer  10 . Data exchange between host processor  2  and printer  10  is described in more detail below. 
     In preferred embodiments of the invention, printer  10  is a multi-head serial printer. Accordingly, although the inventions described herein are not limited to use with such a printer, the inventions will be described in the context of a such a printer. 
     In this regard, FIGS. 2 and 3 show close-up perspective front and back views, respectively, of printer  10 . In physical structure, the preferred embodiment of printer  10  is similar to the printer disclosed in U.S. patent application Ser. No. 08/972,113, entitled “Multi-Head Printing With Differing Resolutions”, filed on Nov. 17, 1997, which is incorporated herein by reference. 
     As shown in these FIGS. 2 and 3, printer  10  includes housing  11 , access door  12 , automatic feeder  14 , automatic feed adjuster  16 , manual feeder  17 , manual feed adjuster  19 , media eject port  20 , ejection tray  21 , tray receptacle  22 , indicator light  23 , power button  24 , resume button  26 , power supply  27 , power cord  29 , and parallel port connector  30 . 
     Housing  11  is approximately 498 mm in width by 271 mm in depth by 219 mm in height, and houses the internal workings of printer  10 , including the print engine described below which prints images onto recording media. Included on housing  11  is access door  12 . Access door  12  is manually openable and closeable so as to permit a user to access the internal workings of printer  10  and, in particular, to access print cartridges installed in printer  10  so as to allow the user to change or replace print cartridges. 
     Disposed on the top of access door  12  is a front panel comprising indicator light  23 , power button  24 , and resume button  26 . Power button  24  is a control by which a user can turn printer  10  on and off. Additional functions, however, are also available through power button  24 . For example, a test print function can be selected by holding down power button  24  until a speaker (not shown) in printer  10  emits a sound, such as one beep. In response to this test print function, printer  10  prints a test pattern. 
     Resume button  26  provides control by which an operator can resume printing after an error condition has occurred. In addition, resume button  26  can be used to activate other functions. For example, a print head cleaning function can be activated by holding down resume button  26  until the speaker in printer  10  produces a beep. 
     In this regard, printer  10  is able to provide a variety of consecutive beeping sounds. Each of these sounds indicates a different type of error, such as paper empty, paper jam, etc. 
     Indicator light  23  is comprised of a single light pipe, a green light emitting diode (hereinafter “LED”), and an orange LED. Indicator light  23  provides a user with an indication of the operational state of printer  10 . Specifically, when indicator light  23  is off, this indicates that printer  10  is powered off. When indicator light  23  is illuminated green (i.e., the green LED is activated), this indicates that printer  10  is powered on and is ready for printing. When indicator light  23  is green and blinking, this indicates an operational state of the printer, such as that the printer is currently powering on. 
     Indicator light  23  can also be illuminated orange by the orange LED. When indicator light  23  is illuminated orange, this indicates that a recoverable error, i.e., an operator call error, has occurred in printer  10 . Recoverable errors comprise paper empty, paper jam, defective cartridge installed in printer  10 , cartridge replacement in process, etc. It is possible to distinguish the type of recoverable error based on a number of beeps from printer  10 &#39;s speaker. By counting these beeps when indicator LED is continuously orange, a user can determine which error has occurred and act accordingly. 
     When indicator light  23  is orange and blinking, this indicates that a fatal error, i.e., a service call error, has occurred in printer  10 . It is possible to distinguish the type of fatal error that has occurred merely by counting how many times the orange light has blinked. 
     As shown in FIGS. 2 and 3, automatic feeder  14  is also included on housing  11  of printer  10 . Automatic feeder  14  defines a media feed portion of printer  10 . That is, automatic feeder  14  stores recording media onto which printer  10  prints images. In this regard, printer  10  is able to print images on a variety of types of recording media. These types include, but are not limited to, plain paper, high resolution paper, transparencies, glossy paper, glossy film, back print film, fabric sheets, T-shirt transfers, bubble jet paper, greeting cards, brochure paper, banner paper, thick paper, etc. 
     Automatic feeder  14  is able to accommodate a recording media stack which is approximately 13 mm thick. This means that automatic feeder  14  can hold, e.g., approximately 130 sheets of paper having a density of 64 g/m 2  or approximately 15 envelopes. During printing, individual sheets which are stacked within automatic feeder  14  are fed from automatic feeder  14  through printer  10 . Specifically, rollers (described below) in printer  10  draw individual media from automatic feeder  14  into printer  10 . These individual media are then fed in a “J” type path through the rollers to eject port  20  shown in FIG.  2 . 
     Automatic feeder  14  includes automatic feed adjuster  16 . Automatic feed adjuster  16  is laterally movable to accommodate different media sizes within automatic feeder  14 . Automatic feeder  14  also includes backing  31 , which is extendible to support recording media held in automatic feeder  14 . When not in use, backing  31  is stored within a slot in automatic feeder  14 , as shown in FIG.  2 . 
     Individual sheets also can be fed through printer  10  via manual feeder  17  shown in FIG. 3, which also defines a media feed portion of printer  10 . In preferred embodiments, manual feeder  17  can accommodate media having a density of at least between 64 g/m 2  and 550 g/m 2 , and having a thickness of 0.8 mm. Sheets fed through manual feeder  17  are fed straight through the rollers in printer  10  to eject port  20 . As was the case with automatic feeder  14 , manual feeder  17  includes manual feed adjuster  19 . By sliding manual feed adjuster  19  laterally, a user can vary the media which manual feeder  17  can accommodate. 
     Using manual feeder  17  and automatic feeder  14 , printer  10  can print images on media having a variety of different sizes. These sizes include, but are not limited to, letter, legal, A4, A3, A5, B4, B5, tabloid, #10 envelope, DL envelope, banner, wide banner, and LTR full bleed. Custom-sized recording media can also be used with printer  10 . 
     As noted above, media are fed through printer  10  and ejected from eject port  20  into ejection tray  21 . Ejection tray  21  includes spring-biased flaps which support media ejected from printer  10 , and which move downwardly as more media are piled thereon. When not in use, ejection tray  21  is stored within tray receptacle  22  of printer  10 , as shown in FIG.  2 . 
     Power cord  29  connects printer  10  to an external AC power source. Power supply  27  is used to convert AC power from the external power source, and to supply the converted power to printer  10 . Parallel port  30  connects printer  10  to host processor  2 . Parallel port  30  preferably comprises an IEEE-1284 bi-directional port, over which data and commands, such as those described below in section 3.0, are transmitted between printer  10  and host processor  2 . 
     FIGS. 4 and 5 show back and front cut-away perspective views, respectively, of printer  10 . As shown in FIG. 4, printer  10  includes rollers  32 , noted above, for transporting media from either automatic feeder  14  or manual feeder  17  through printer  10  to media eject port  20 . Rollers  32  rotate in a counterclockwise direction during media transport, as indicated by arrow  32   a  shown in FIG.  4 . 
     Line feed motor  34  controls the rotation of rollers  32 . The arrangement shown in FIG. 4 for depicting the operational relationship between line feed motor  34  and rollers  32  is a simplified arrangement for purposes of the present discussion. A more detailed description of this relationship can be found in FIGS. 5A and 5B and in the corresponding descriptions for these figures presented below. Line feed motor  34  preferably comprises a 96-step, 2 phase pulse motor and is controlled in response to signal commands received from circuit board  35 . Line feed motor  34  is driven by a motor driver having four-level current control, with the four levels preferably set at 0, 40, 70 and 100 percent of maximum current. 
     In preferred embodiments, line feed motor  34  is able to cause rollers  32  to rotate so that a recording medium is fed through printer  10  at 238 mm/sec at the maximum speed of line feed motor  34 . In a primary mode of operation for printer  10 , line feed resolution is (1/720)inches/pulse (2-2 phase), and in a 1440 dpi mode, line resolution is (1/1440)inches/pulse (1-2 phase). Print modes are described in more detail below. 
     As shown in FIG. 5, printer  10  is a dual-cartridge printer which prints images using two print heads (i.e., one head per cartridge). Specifically, these cartridges preferably are held side-by-side by cartridge receptacles  37   a  and  37   b  such that respective print heads on the cartridges are offset horizontally from each other. Carriage motor  39 , shown in FIG. 4, controls the motion of cartridge receptacles  37   a  and  37   b  in response to signal commands received from circuit board  35 . Specifically, carriage motor  39  controls the motion of belt  40 , which in turn controls the movement of cartridge receptacles  37   a  and  37   b  along carriage  41 . In this regard, carriage motor  39  provides for bi-directional motion of belt  40 , and thus of cartridge receptacles  37   a  and  37   b.  By virtue of this feature, printer  10  is able to print images from both left to right and right to left. 
     Carriage motor  39  comprises a 96-step, 2 phase pulse motor resulting in a carriage resolution of (9/360)inches/pulse. Carriage motor  39  is driven by a motor driver having four-level current control. When printer  10  is printing in a 360 dpi standard default mode, carriage motor  39  is driven to cause cartridge receptacles  37   a  and  37   b  to move along carriage  41  at a speed of 22.5 inches/sec, which corresponds to a print head heat pulse frequency of 6.51 KHz. When printer  10  is printing in a 360 dpi draft mode, carriage motor  39  is driven to cause cartridge receptacles  37   a  and  37   b  to move along carriage  41  at a speed of 27.5 inches/sec, which corresponds to a print head heat pulse frequency of 10.0 KHz. In contrast, when printer  10  is printing in a 720 dpi mode, carriage motor  39  is driven to cause cartridge receptacles  37   a  and  37   b  to move along carriage  41  at a default speed of 13.8 inches/sec (10.0 KHz). 
     Cartridge receptacles  37   a  and  37   b  are used to hold ink cartridges  43   a  and  43   b  (which each include a print head and can include one or more removable ink reservoirs for storing ink) in printer  10 . A representative ink cartridge is described below in Section 1.3 with reference to FIG.  6 . 
     Returning to FIG. 5, printer  10  preferably includes pre-fire receptacles  42   a  and  42   b,  wipers  44   a  and  44   b  and ink cleaning mechanism  45 . Ink cleaning mechanism  45  is disposed at home location  46  and comprises a rotary pump (not shown) and print head connection caps  47   a  and  47   b.  Print head connection caps  47   a  and  47   b  connect to print heads of cartridges installed in cartridge receptacles  37   a  and  37   b,  respectively, during print head cleaning and at other times, such as when printer  10  is powered off, so as to protect the print heads. 
     Line feed motor  34  drives the rotary pump of ink cleaning mechanism  45  so as to suction excess ink from a print head connected to either of print head connection caps  47   a  and  47   b.  Preferably, ink can be suctioned from one cartridge at a time. 
     Wipers  44   a  and  44   b  can comprise blades or the like which are driven by carriage motor  39  to wipe excess ink from cartridge print heads. Specifically, wipers  44   a  and  44   b  are lifted to contact a print head after a predetermined condition has occurred. For example, wipers  44   a  and  44   b  can be lifted after a predetermined number of dots have been printed by a print head. 
     FIG. 5A shows the interoperation of line feed motor  34  and of carriage motor  39  for the operation of the automatic feeder rollers  32  and the ink cleaning mechanism  45 . Specifically referring to FIG. 5A, the line feed motor  34  operates line feed roller  165  through gears  160 ,  161  and  162 . Clutch unit  140  is driven by line feed roller  165  through gears  150  and  151 . Clutch unit  140  and control rod  141  operate in cooperation with line feed motor  34  and carriage motor  39  to position clutch unit  140  in one of several positions corresponding to either: (1) a neutral position for normal printing; (2) a position for operation of the automatic feeder; or (3) a position for operation of the ink cleaning mechanism. 
     As shown in FIG. 5A, carriage motor  39  drives belt  40  to move cartridge receptacle  37   b  in a linear motion along carriage  41 . The movement of cartridge receptacle  37   b  past the home position  46  towards the right end of carriage  41  allows cartridge receptacle  37   b  to translate control rod  141  away from clutch unit  140  so as to disengage the pin-shaped end of control rod  141  from clutch unit  140 . Line feed motor  34  is then turned for a limited rotation in a given direction to re-engage clutch unit  140  in a new position so as to drive either the automatic feed rollers  32  or the ink cleaning mechanism  45 . 
     FIG. 5B provides a more detailed view of clutch unit  140  and the surrounding gears provided for the operation of automatic feeder rollers  32  or for the operation of ink cleaning mechanism  45 . Specifically, clutch unit  140  consists of two separate and mutually exclusive slots,  145  and  146 , for the engagement of the pin-shaped end of control rod  141 , gear  147  for rotation by line feed roller  165  through gears  150  and  151 , and gear  148  for rotation by gear  147 . Gear  148  is the driving gear of clutch unit  140  and either spins freely in the neutral position, or is engaged with input gear  152  when driving the purge pump (not shown) in ink cleaning mechanism  45  or is engaged with gear  153  when driving automatic feeder rollers  32 . 
     During neutral operation of clutch unit  140 , slot  145  of clutch unit  140  is engaged by control rod  141 . In this position, gear  148  is disengaged from both of gears  152  and  153 , thereby preventing the operation of ink cleaning mechanism  45  and automatic feeder rollers  32 . During operation of ink cleaning mechanism  45 , slot  146  of clutch unit  140  is engaged by control rod  141 , thereby biasing gear  148  to engage with input gear  152 . Input gear  152  thereupon operates ink cleaning mechanism  45  to remove excess ink from the print heads. During operation of automatic feeder rollers  32 , control rod  141  is positioned directly on front plate  167  of clutch unit  140 , thereby biasing gear  148  to engage with gear  153  so as to drive automatic feeder rollers  32  via gears  153  through  156 . 
     FIG. 5C provides the detailed steps for engaging clutch unit  140  so as to operate automatic feeder rollers  32 . As shown in FIG. 5C, the first step S 501  consists of disengaging clutch unit  140 . This is performed by moving the carriage receptacle  37   b  past home position  46  so as to disengage control rod  141  from clutch unit  140 . Next, step S 502  consists of moving line feed motor  34  in the forward direction so as to engage gear  148  of clutch unit  140  with gear  153  for driving automatic feeder rollers  32  via gears  153  through  156 . In step S 503 , cartridge receptacle  37   b  is moved to the left of home position  46  so as to allow control spring  142  to bias control rod  141  against front plate  167  of clutch unit  140 . In step S 504 , line feed motor  34  is then operated in forward, thereby causing the rotation of automatic feeder rollers  32 . Line feed motor  34  is then operated in the reverse direction in step S 506  so as to align neutral slot  145  of clutch unit  140  with control rod  141 , thereby disengaging automatic feeder rollers  32  from line feed motor  34 . Control rod  141  is then biased by spring  142  (step S 507 ) to engage neutral slot  145  so as to return clutch unit  140  to a neutral position. 
     FIG. 5D provides the detailed steps for engaging clutch unit  140  so as to operate ink cleaning mechanism  45 . As shown in FIG. 5D, step S 551  consists of disengaging clutch unit  140 . This is performed by moving carriage receptacle  37   b  past home position  46  so as to disengage control rod  141  from clutch unit  140 . Next, step S 552  consists of moving line feed motor  34  in the reverse direction to align slot  146  of clutch unit  140  with control rod  141 , thereby engaging gear  148  of clutch unit  140  with input gear  152  for driving ink cleaning mechanism  45 . Step S 553  then comprises moving cartridge receptacle  37   b  to the left of home position  46  so as to allow control spring  142  to bias control rod  141  for engagement with slot  146  of clutch unit  140 . In step S 554 , line feed motor  34  is then operated in the reverse position for one-quarter rotation so as to raise print head connection caps  47   a  and  47   b  for engagement with the print heads. In step S 555 , line feed motor  34  is operated in the reverse position for one-half rotation so as to drive the rotary pump of ink cleaning mechanism  45  to remove excess ink from the print heads. Print head connection caps  47   a  and  47   b  are then lowered in step S 556  by operating line feed motor  34  in the reverse position for one-quarter rotation. Clutch unit  140  is returned to the neutral position in step S 557  by moving cartridge receptacle  37   b  past home position  46  to disengage control pin  141  from clutch unit  140 . Line feed motor  34  is then operated in the forward direction in S 558  so as to align neutral slot  145  of clutch unit  140  with control rod  141 . Cartridge receptacle  37   b  is then moved to the left of home position  46  in step S 559 , thereby allowing control rod  141  to engage slot  145  so as to return clutch unit  140  to a neutral position. 
     1.2 Cleaning 
     Printer  10  includes a manual cleaning function which can be activated via its front panel. Specifically, manual cleaning is activated by pressing resume button  26  until printer  10  emits a beep which is two seconds long. To indicate that manual cleaning has been activated, indicator light  23  blinks. Any medium in the process of printing is then ejected from eject port  20 . Ink cleaning mechanism  45  then cleans, e.g., suctions ink from and wipes ink off of, the print heads of ink cartridges stored in cartridge receptacles  37   a  and  37   b,  and the suctioned and wiped ink is stored in a waste ink storage area. Thereafter, indicator light  23  stops blinking and is turned on if no errors have occurred. In the event that a waste ink error has occurred, e.g., the waste ink storage area is near capacity, the orange LED will illuminate indicator light  23  and printer  10  will emit six beeping sounds. 
     1.3 Ink Cartridge 
     The printer described herein can use ink cartridges which include removable ink reservoirs for storing different types of ink. 
     FIG. 6 shows the configuration of ink cartridge  43   a  which may be installed within cartridge receptacle  37   a  (see FIG.  5 ). Ink cartridge  43   b  may be configured identically to ink cartridge  43   a.  Therefore, for the sake of brevity, only ink cartridge  43   a  is described herein. 
     As shown in FIG. 6, ink cartridge  43   a  comprises print head  51 , ink reservoirs  52 , and cartridge hole  54 . At this point, it is noted that the present invention can also be used with ink cartridges that do not contain removable ink reservoirs, but instead store all ink internally. 
     Ink reservoirs  52  are removable from ink cartridge  43   a  and store ink used by printer  10  to print images. Specifically, ink reservoirs  52  are inserted within cartridge  43   a  and can be removed by pulling along the direction of arrows  56 , as shown in FIG.  6 . Reservoirs  52  can store color (e.g., cyan, magenta and yellow) ink and/or black ink, as described in more detail below. Print head  51  includes a plurality of nozzles (not shown) which eject ink from ink reservoirs  52  during printing. Cartridge hole  54  mates to a pin (not shown) on cartridge receptacle  37   a  so as to hold ink cartridge  43   a  in place. 
     In general, printer  10  can operate with a variety of different cartridge types. For example, printer  10  can use a cartridge which stores dye-based black ink and which has a print head with 128 nozzles extending in the vertical direction. An example of such a cartridge is a Canon BC-20 cartridge. A similar type cartridge may also be used which stores pigment black ink. An example of such a cartridge is a Canon BC-23 cartridge. In this regard, generally speaking, dye-based black ink has high penetration characteristics relative to a recording medium. On the other hand, pigment-based black ink generally has low penetration characteristics (and in some cases no penetration) relative to a recording medium. 
     Printer  10  can also operate with color ink cartridges. For example, printer  10  can operate with an ink cartridge which stores cyan, magenta, yellow and black inks, and which includes 136 nozzles extending in the vertical direction. In such a cartridge, 24 nozzles print with cyan ink, 24 nozzles print with magenta ink, 24 nozzles print with yellow ink, and 64 nozzles print with black ink. An example of such a cartridge is a Canon BC21(e) cartridge. 
     Still another example of an ink cartridge that may be used with printer  10  stores reduced optical density (e.g., “photo”) ink, and includes 136 nozzles arranged in the vertical direction. Such a cartridge also has the same nozzle configuration as the color cartridge described above. An example of such a cartridge is a Canon BC-22 cartridge. 
     1.4 Print Head Structure 
     With regard to the physical construction of the print heads of cartridges that may be used with the present invention, FIG. 7 shows a close-up, face-on view of nozzle configurations for a case in which printer  10  includes print head  61  having 128 nozzles and arranged near-vertical, with each nozzle closely spaced to adjacent nozzles. Such an arrangement is preferred for single color (such as black) printing. The nozzles are preferably arranged at a slight oblique slant so that as the print head is moved across the recording medium, it is possible to fire the nozzles in rapid succession, rather than all at once, so as to print a vertical line. The power and control requirements for firing nozzles in rapid succession are significantly reduced relative to those for firing all at once. One preferable arrangement of slant angle would correspond to a one pixel horizontal change for every 16 vertical nozzles, at 360 dpi resolution. 
     Print head  62  has 136 nozzles, with 24 nozzles preferably for yellow ink, 24 nozzles preferably for magenta ink, 24 nozzles preferably for cyan ink, and 64 nozzles preferably for black ink, arranged at a slight slant angle to vertical, one on top of another. Each color group of nozzles is separated from an adjacent group by a vertical gap corresponding to 8 nozzles. The slight slant angle is, again, arranged to provide one pixel of horizontal change for every 16 vertical nozzles, at 360 dpi. 
     1.5 Print Modes 
     During its operation, printer  10  includes different modes which may be set via commands issued to printer  10  by host processor  2  (see FIG.  1 ). In these modes, cartridges installed in printer  10  may eject different-sized ink droplets to form images having different resolutions. Whether certain modes of printer  10  are available depends, in part, on the type of cartridge installed in printer  10 . That is, print heads on some types of cartridges are capable of ejecting different-sized droplets, e.g., large or small ink droplets, whereas print heads on other types of cartridges are capable of ejecting droplets having a single size. 
     As noted above, different ink droplet sizes are used during different printer operational modes to form images having different resolutions. More specifically, ink jet printers create images by forming dots on a page. The resolution of a formed image corresponds in part to the number of dots formed and in part to the arrangement in which those dots are formed. In the printer of the present invention, images can be formed at a variety of different resolutions using either the large or small ink droplets described above. 
     At this point, it is noted that dot allocation and arrangement during printing is limited, in part, based upon the type of paper used during printing. Specifically, plain paper can absorb approximately a maximum of four small droplets in a 360 dpi pixel, whereas high resolution (hereinafter “HR-101”) paper can absorb a maximum of 6 small droplets in a 360 dpi pixel. 
     2.0 Electrical 
     As described in section 1.0 above, printer  10  may use multiple print heads in different combinations, such as black-black, black-color, color-color, or color-photo, so that several print modes may be executed at different resolutions (e.g., 180 dpi, 360 dpi, 720 dpi). Further, print head combinations may be changed for different print modes, such as text, text and color, color and high quality color. As a result, printing tasks for the different modes require complex operations that vary based on the print head combination, recording media and print quality. In the information processing system of FIG. 1, printer parameters relating to print head configuration, print head alignment, etc. are stored in printer  10  and sent to host processor  2  based on data obtained by printer  10 . Preferably, a printer driver in host processor  2  performs the complex processing of print data and printer set up for the various print modes and sends dictated command sequences to the printer that simplify printing execution. 
     2.1 System Architecture 
     FIG. 8 is a block diagram showing the internal structures of host processor  2  and printer  10 . In FIG. 8, host processor  2  includes a central processing unit  70  such as a programmable microprocessor interfaced to computer bus  71 . Also coupled to computer bus  71  are display interface  72  for interfacing to display  4 , printer interface  74  for interfacing to printer  10  through bi-directional communication line  76 , floppy disk interface  9  for interfacing to floppy disk  77 , keyboard interface  79  for interfacing to keyboard  5 , and pointing device interface  80  for interfacing to pointing device  6 . Disk  8  includes an operating system section for storing operating system  81 , an applications section for storing applications  82 , and a printer driver section for storing printer driver  84 . 
     A random access main memory (hereinafter “RAM”)  86  interfaces to computer bus  71  to provide CPU  70  with access to memory storage. In, particular, when executing stored application program instruction sequences such as those associated with application programs stored in applications section  82  of disk  8 , CPU  70  loads those application instruction sequences from disk  8  (or other storage media such as media accessed via a network or floppy disk interface  9 ) into random access memory (hereinafter “RAM”)  86  and executes those stored program instruction sequences out of RAM  86 . RAM  86  provides for a print data buffer used by printer driver  84  according to the invention, as described more fully hereinbelow. It should also be recognized that standard disk-swapping techniques available under the windowing operating system allow segments of memory, including the aforementioned print data buffer, to be swapped on and off of disk  8 . Read only memory (hereinafter “ROM”)  87  in host processor  2  stores invariant instruction sequences, such as start-up instruction sequences or basic input/output operating system (BIOS) sequences for operation of keyboard  5 . 
     As shown in FIG. 8, and as previously mentioned, disk  8  stores program instruction sequences for a windowing operating system and for various application programs such as graphics application programs, drawing application programs, desktop publishing application programs, and the like. In addition, disk  8  also stores color image files such as might be displayed by display  4  or printed by printer  10  under control of a designated application program. Disk  8  also stores a color monitor driver in other drivers section  89  which controls how multi-level RGB color primary values are provided to display interface  72 . Printer driver  84  controls printer  10  for both black and color printing and supplies print data for print out according to the configuration of printer  10 . Print data is transferred to printer  10 , and control signals are exchanged between host processor  2  and printer  10 , through printer interface  74  connected to line  76  under control of printer driver  84 . Other device drivers are also stored on disk  8 , for providing appropriate signals to various devices, such as network devices, facsimile devices, and the like, connected to host processor  2 . 
     Ordinarily, application programs and drivers stored on disk  8  need first to be installed by the user onto disk  8  from other computer-readable media on which those programs and drivers are initially stored. For example, it is customary for a user to purchase a floppy disk, or other computer-readable media such as CD-ROM, on which a copy of a printer driver is stored. The user would then install the printer driver onto disk  8  through well-known techniques by which the printer driver is copied onto disk  8 . At the same time, it is also possible for the user, via a modem interface (not shown) or via a network (not shown), to download a printer driver, such as by downloading from a file server or from a computerized bulletin board. 
     Referring again to FIG. 8, printer  10  includes a circuit board  35  on which are mounted CPU  91  such as an 8-bit or a 16-bit microprocessor including programmable timer and interrupt controller, ROM  92 , control logic  94 , and I/O ports unit  96  connected to bus  97 . Also connected to control logic  94  is RAM  99 . Control logic  94  includes controllers for line feed motor  34 , for print image buffer storage in RAM  99 , for heat pulse generation, and for head data. Control logic  94  also provides control signals for nozzles in print heads  100   a  and  100   b  of print engine  101 , carriage motor  39 , line feed motor  34 , and print data for print heads  100   a  and  100   b,  and receives information from print engine  101  for alignment of print heads  100   a  and  100   b  through I/O ports unit  96 . EEPROM  102  is connected to I/O ports unit  96  to provide non-volatile memory for printer information such as print head configuration and print head alignment parameters. EEPROM  102  also stores parameters that identify the printer, the driver, the print heads, alignment of the print heads, the status of ink in the cartridges, etc., which are sent to printer driver  84  of host processor  2  to inform host processor  2  of the operational parameters of printer  10 . 
     I/O ports unit  96  is coupled to print engine  101  in which a pair of print heads  100   a  and  100   b  (which would be stored in cartridge receptacles  37   a  and  37   b,  respectively) perform recording on a recording medium by scanning across the recording medium while printing using print data from a print buffer in RAM  99 . Control logic  94  is also coupled to printer interface  74  of host processor  2  via communication line  76  for exchange of control signals and to receive print data and print data addresses. ROM  92  stores font data, program instruction sequences used to control printer  10 , and other invariant data for printer operation. RAM  99  stores print data in a print buffer defined by printer driver  84  for print heads  100   a  and  100   b  and other information for printer operation. 
     Print heads  100   a  and  100   b  of print engine  101  correspond to ink cartridges that are stored in cartridge receptacles  37   a  and  37   b,  respectively. Sensors, generally indicated as  103 , are arranged in print engine  101  to detect printer status and to measure temperature and other quantities that affect printing, in particular, a temperature sensor  103   a  which is mounted on circuit board  35 , measures ambient environmental temperature. A low precision thermistor, which measures temperature to within plus or minus three degrees Celsius is suitable for temperature sensor  103   a.  A photo sensor (e.g., an automatic alignment sensor) in cartridge receptacles  37   a  and/or  37   b  measures print density and dot locations for automatic alignment. Sensors  103  are also arranged in print engine  101  to detect other conditions such as the open or closed status of access door  12 , presence of recording media, etc. In addition, diode sensors, including a thermistor, are located in print heads  100   a  and  100   b  to measure print head temperature, which is transmitted to I/O ports unit  96 . 
     I/O ports unit  96  also receives input from switches  104  such as power button 24 and resume button  26  and delivers control signals to LEDs  105  to light indicator light  23 , to buzzer  106 , and to line feed motor  34  and carriage motor  39  through line feed motor driver  34   a  and carriage motor driver  39   a,  respectively. As described above, buzzer  106  may comprise a speaker. 
     Although FIG. 8 shows individual components of printer  10  as separate and distinct from one another, it is preferable that some of the components be combined. For example, control logic  94  may be combined with I/O ports  96  in an ASIC to simplify interconnections for the functions of printer  10 . 
     2.2 System Function 
     FIG. 9 shows a high-level functional block diagram that illustrates the interaction between host processor  2  and printer  10 . As illustrated in FIG. 9, when a print instruction is issued from image processing application program  82 a stored in application section  82  of disk  8 , operating system  81  issues graphics device interface calls to printer driver  84 . Printer driver  84  responds by generating print data corresponding to the print instruction and stores the print data in print data store  107 . Print data store  107  may reside in RAM  86  or in disk  8 , or through disk swapping operations of operating system  81  may initially be stored in RAM  86  and swapped in and out of disk  8 . Thereafter, printer driver  84  obtains print data from print data store  107  and transmits the print data through printer interface  74 , to bi-directional communication line  76 , and to print buffer  109  through printer control  110 . Print buffer  109  resides in RAM  99 , and printer control  110  resides in firmware implemented through control logic  94  and CPU  91  of FIG.  8 . Printer control  110  processes the print data in print buffer  109  responsive to commands received from host processor  2  and performs printing tasks under control of instructions stored in ROM  92  (see FIG. 8) to provide appropriate print head and other control signals to print engine  101  for recording images onto recording media. 
     Print buffer  109  has a first section for storing print data to be printed by one of print heads  100   a  and  100   b,  and a second section for storing print data to be printed by the other one of print heads  100   a  and  100   b.  Each print buffer section has storage locations corresponding to the number of print positions of the associated print head. These storage locations are defined by printer driver  84  according to a resolution selected for printing. Each print buffer section also includes additional storage locations for transfer of print data during ramp-up of print heads  100   a  and  100   b  to printing speed. Print data is transferred from print data store  107  in host processor  2  to storage locations of print buffer  109  that are addressed by printer driver  84 . As a result, print data for a next scan may be inserted into vacant storage locations in print buffer  109  both during ramp up and during printing of a current scan. 
     2.3 Control Logic 
     FIG. 10 depicts a block diagram of control logic  94  and I/O ports unit  96  from FIG.  8 . As mentioned above, I/O ports unit  96  may be, alternatively, included within control logic  94 . In FIG. 10, internal bus  112  is connected to printer bus  97  for communication with printer CPU  91 . Bus  112  is coupled to host computer interface  113  which is connected to bi-directional line  76  for carrying out bi-directional such as IEEE-1284 protocol communication. Accordingly, bi-directional communication line  76  is also coupled to printer interface  74  of host processor  2 . Host computer interface  113  is connected to bus  112  and to DRAM bus arbiter/controller  115  for controlling RAM  99  which includes print buffer  109  (see FIGS.  8  and  9 ). Data decompressor  116  is connected between bus  112  and DRAM bus arbiter/controller  115  to decompress print data when processing. Also coupled to bus  112  are line feed motor controller  117  that is connected to line feed motor driver  34   a  of FIG. 8, image buffer controller  118  which provides serial control signals and head data signals for each of print heads  100   a  and  100   b,  and heat pulse generator  119  which provides block control signals and analog heat pulses for each of print heads  100   a  and  100   b.  Carriage motor control is performed by CPU  91  through I/O ports unit  96  and carriage motor driver  39   a  since line feed motor  34  and carriage motor  39  may operate concurrently. 
     Control logic  94  operates to receive commands from host processor  2  for use in CPU  91 , and to send printer status and other response signals to host processor  2  through host computer interface  113  and bi-directional communication line  76 . Print data and print buffer memory addresses for print data received from host processor  2  are sent to print buffer  109  in RAM  99  via DRAM bus arbiter/controller  115 , and the addressed print data from print buffer  109  is transferred through controller  115  to print engine  101  for printing by print heads  100   a  and  100   b.  In this regard, heat pulse generator  119  generates analog heat pulses required for printing the print data. 
     FIG. 11 shows the memory architecture for printer  10 . As shown in FIG. 11, EEPROM  102 , RAM  99 , ROM  92  and temporary storage  121  for control logic  94  form a memory structure with a single addressing arrangement. Referring to FIG. 11, EEPROM  102 , shown as non-volatile memory section  123 , stores a set of parameters that are used by host processor  2  and that identify printer and print heads, print head status, print head alignment, and other print head characteristics. EEPROM  102  also stores another set of parameters, such as clean time, auto-alignment sensor data, etc., which are used by printer  10 . ROM  92 , shown as memory section  124 , stores information for printer operation that is invariant, such as program sequences for printer tasks and print head operation temperature tables that are used to control the generation of nozzle heat pulses, etc. A random access memory section  121  stores temporary operational information for control logic  94 , and memory section  126  corresponding to RAM  99  includes storage for variable operational data for printer tasks and print buffer  109 . 
     2.4 General Operation 
     FIG. 12 is a flowchart illustrating the general operation of the information processing system shown in the block diagram of FIG.  8 . After power is turned on in printer  10  in step S 1201  of FIG. 12, printer  10  is initialized in step S 1202 . In the initialization, as discussed in greater detail in section 3.2 below, CPU  91 , control logic  94  and a system timer are set to an initial state. In addition, ROM  92 , RAM  99  and EEPROM  102  of printer  10  are checked and interrupt request levels in CPU  91  are assigned on application of power to printer  10 . When printer  10  is set to its on state, EEPROM  102  is read by printer driver  84 , controller tasks are started by printer CPU  91  such as resetting the printer, determining if print head cleaning should be performed based on the system timer, etc. Also in the initialization process of step S 1202 , a data compression mode is selected, heat pulses for print heads  100   a  and  100   b  are defined, buffer control is defined, print buffer  109  is cleared, and messages are displayed indicating the status of printer  10 . 
     Next, step S 1203  is performed. In step S 1203 , printer driver  84  calculates printer parameters from data obtained by printer CPU  91  based on printer measurements related to head configuration and alignment if it is determined that the print head configuration has changed. The alignment system is described more fully in U.S. patent application Ser. No. 08/901,560, entitled “Auto-Alignment System For A Printing Device”, filed on Jul. 28, 1997, which is incorporated herein by reference. 
     Following step S 1203 , processing proceeds to step S 1204 , in which it is determined if printer  10  is on-line. Once it is determined that printer  10  is on-line, processing proceeds to step S 1205 , in which the calculated printer parameters are registered in printer EEPROM  102 . 
     Specifically, when printer  10  is determined to be on-line, the printer parameters stored in the EEPROM  102  are registered by printer driver  84  in step S 1205 . The parameters are used to control printer and print head operation. In step S 1205 , the parameters are sent by CPU  91  for storage in host processor  2  so that printer driver  84  can generate appropriate commands for printer operation. Such commands are indicated in the steps of the dashed box of FIG.  12  and take into account the current identification of printer  10 , the print head configuration, print head alignment and cartridge ink status. 
     After registration of the printer parameter information in step S 1205 , the status of each of print head cartridges  43   a  and  43   b  (see FIG. 5) is checked in step S 1206 . This is done by ascertaining whether access door  12  has been opened and closed and detecting whether one or more of the ink cartridges or ink reservoirs has been changed. If a cartridge or reservoir has been changed, a cleaning operation is performed on the corresponding print head, in which the nozzles of the print head are cleaned. 
     Following the cartridge change processing performed in step S 1206 , processing proceeds to step S 1207 . In step S 1207 , it is determined whether an interrupt has been requested by printer  10  for operations such as print head heater control, automatic sheet feed (ASF) control, head cleaning control, and the like. In response to such an interrupt request, the requested printer operation is performed in step S 1208 . Thereafter, processing returns to step S 1206 . 
     If an interrupt has not been requested by the printer in step S 1207 , processing proceeds to step S 1209 . In step S 1209 , it is determined if printer driver  84  has requested a command sequence. In the system of FIG. 8, tasks of printer  10  are controlled by commands from printer driver  84  which have been generated in accordance with parameter and status information received from printer  10 . 
     When a user interface sequence is selected, step S 1213  is entered and user interface processing is performed. Upon completion of user selections by means of keyboard and pointer entry on the user interface display, control is returned to step S 1209  and is directed to use print command sequence step S 1210 . 
     If a print sequence is selected in step S 1209 , processing proceeds to step S 1210 . In step S 1210 , printer driver  84  generates a sequence of commands based on print head configuration, print head alignment, media type and size and target image information stored therein. These commands are sent to printer control  110  (see FIG. 9) in printer  10 . In the printer, printer control  110  receives the commands and the firmware from printer ROM  92  and causes execution of command tasks in print engine  101 . 
     The print command sequence includes transferring print data from print driver  84  to print buffer  109  which is defined for each print job. The print data transfer is performed without a receiving buffer in printer  10 . Print data for a next scan is sent to empty storage locations of the current scan in print buffer  109  during ramp-up of the print heads in the current scan. 
     The command sequence of step S 1210  includes commands to set print resolution of print heads  100   a  and  100   b.  These commands are set by controlling the size of ink droplets based on digital data stored in a print buffer for a print head and the order in which the print data is read out of the print buffer for the print head. Preferably, resolution of the print heads can be controlled independently of each other. For ink jet type print heads which eject ink droplets based on digital data stored in a print buffer, resolution is controlled by controlling ink droplet size and by controlling readout order from the print buffer, with droplet size and readout order preferably being controlled independently for each print head. 
     Further in the print command sequence of step S 1210 , printer driver  84  selects the type of ink that is to be used in printing a target pixel based on an analysis multi-level image data of adjacent pixels. As an example, a dye-based ink may be selected for a black target pixel surrounded by color pixels in an image while a pigment-based ink may be selected for a black target pixel surrounded by black pixels. 
     Upon completion of printing one page, flow proceeds to step S 1211  of FIG. 12, wherein the page is output from printer  10  responsive to a paper eject command. 
     FIG. 13 is a flowchart that illustrates a command sequence generated by printer driver  84  for printing and operating printer  10 . The command sequence in FIG. 13 is simplified to provide a general framework for describing operation of printer  10 . A more detailed command sequence which includes, for example, automatic sheet feed control according to the invention is described in section 4.0 with respect to FIG.  20 . 
     Returning to FIG. 13, the print command sequence is started by a printer initialization command in step S 1301 , which is sent to printer control  110  to reset printer operation. A paper load command (step S 1302 ) is then provided to printer control  110 , which selects a load paper operation in selection step S 1303  and executes a start paper load (step S 1304 ). When a paper load end is detected in printer control  110  in step S 1305 , a signal indicating end paper load is sent to printer driver  84 , and the print data is prepared for a first scan of print heads  100   a  and  100   b  in step S 1306 . Printer control  110  is notified of this scan preparation. The preparation of print data in printer driver  84  is described more fully in U.S. patent application No. 08/901,719, entitled “Print Driver For A Color Printer”, filed Jul. 28, 1997. If no print data for the scan is determined in decision step S 1307 , a virtual skip is performed in printer driver  84  in step S 1308 . Control is returned to step S 1307  when a page finish is not detected in step S 1309 . Until the page finish is detected, steps S 1310  through S 1314  and S 1308  are performed. 
     In step S 1310 , an actual skip command is provided by printer driver  84  to printer control  110  for printing correct print data. Printer control  110  selects the actual skip operation (step S 1303 ) and executes the actual skip (step S 1315 ). Scan setting is then performed (step S 1311 ) in printer driver  84 , and printer control  110  is notified. Next, print data generated in printer driver  84  and print buffer addresses for the print data are transferred to printer control  110  which stores this information in print buffer  109  (step S 1312 ). The next scan is then prepared in printer driver  84 , and printer control  110  is notified (step S 1313 ). Then, a print command generated in printer driver  84  is sent to printer control  110 . In response, printer control  110  selects a print operation in step S 1319  and executes the print task in step S 1314 . A virtual skip is then performed by printer driver  84  in step S 1308  to keep track of the lines of the page being printed. When a page finish is determined in decision step S 1309 , a page eject command is sent by printer driver  84  to printer control  110 , which selects a page eject operation (step S 1316 ) and starts page eject (step S 1317 ). Upon completion of the page eject (step S 1318 ), printer driver  84  is notified of the completion of the page eject and control is passed to step S 1209  of FIG.  12 . 
     An example of the command sequence from the host processor  2  to printer  10  to print a page in color mode with two color print heads is set forth in Table A shown in FIG.  14 . Initially as indicated in Table A, the current time is set by a [UCT] command and printer  10  is reset by a [RESET] command. Data compression is selected to pack the print data by a [COMPRESS] command. The bottom margin size of the printable area is selected by a [BTM_MARGIN] command. Print buffers for print heads  100   a  and  100   b  are defined by [DEFINE_BUF] commands. The print color table is defined by a [DEFINE_COLOR] command. The heat pulse and buffer control tables are defined for the color mode of the print head configuration by [DEFINE_PULSE] and [DEFINE_CONTROL] commands. 
     After the printer tasks are executed for the foregoing initializing commands, a paper load command [LOAD] to load a page or other print medium and a raster skip command [SKIP] to skip to the print position of the first print head scan are sent to printer  10 , and the print direction [DIRECTION] and edges [EDGE] for printing of print heads  100   a  and  100   b  are set for the first scan. A loop of commands is then sent to control printer tasks for printing the lines of the page. In the first portion of the loop for each line, the scanning parameters ([SPEED], [SIZE], [SELECT-PULSE] and [SELECT-CONTROL]) for the line are set. Following completion of the printer tasks for the select buffer control table commands [SELECT_CONTROL], the print data blocks are selected by the [BLOCK] command, and the print colors are selected and transmitted by repeated select color [COLOR] and data transmission [DATA] commands according to the determined print areas for print heads  100   a  and  100   b.    
     The direction of the second scan and the left and right edges of the print areas for the second scan are then set by the [DIRECTION] and [EDGE] commands. The backward direction scan margin for the next scan is set by a [SCAN_MARGIN] command. The auto-trigger delay for the present scan is set by an [AT_DELAY] command. At this time, a [PRINT] command is transferred from host processor  2  to printer  10  to execute printing for the first scan, and a [SKIP] command is sent to skip to the print position of the second scan. When the last line has been printed, a paper eject command [EJECT] is given to printer  10  to execute paper ejection. 
     As can be seen from the command sequences for set scan operations and the example of the printing operations according to the invention, each aspect of printer operation, such as scan setting or printing, is controlled by printer driver  84  taking into account print head configuration and the print mode. The tasks to be performed by printer  10  are thereby defined in detail by printer driver  84  so that the printer architecture is substantially simplified to be less costly. 
     Returning to FIG. 12, when a printer status request is determined in step S 1209 , flow proceeds to step S 1212 . In step S 1212 , a printer status command sequence is performed. The status commands that provide requests for printer status information are described in detail in section 3.6. In general, each of the status commands is sent from host processor  2  to printer  10  to request the information on printer operation or information stored in printer  10 . For example, a base status command [BASE-STATUS] requests the current status of the printer. In response, printer  10  returns one data byte indicating one of the following: printing status, whether print buffer  109  can or cannot receive data, whether printer  10  is busy performing start-up, cartridge replacement, print head cleaning, test printing, etc., and whether an error or alarm has been detected. A [HEAD] command requests return of print head configuration, and a [DATA_SEND] command requests return of EEPROM data to host processor  2 . After return of the requested data in step S 1212 , control is returned to step S 1206 . 
     3.0 Architecture of Printer Software 
     Control over functionality of printer  10  is effected by individual programs executing on CPU  91 . The individual programs include initialization routines such as routines executed on power-on, tasks to interpret commands received from host processors  2 , interrupt handlers such as handlers to process real time hardware interrupts, and cyclic handlers that handle cyclic processes such as handlers for control over bi-directional communications with host processor  2 . 
     Printer CPU  91  further executes an operating system so as to coordinate execution of each of the individual programs (i.e., the initialization routines, the tasks, the interrupt handlers, and the cyclic handlers). The operating system is responsible for inter-program communication through messaging and the like, and inter-program switching so as to switch execution from one program to another when appropriate. Details of the operating system follow. 
     3.1 Operating System 
     The operating system is a real-time operating system (or “kernel” or “monitor”) created to modularize printer control programs and to facilitate maintenance, inheritance, and expansion. The real-time operating system is system software that provides for a preemptive multi-task software environment, in which a currently executing program can be suspended in favor of a switch to another program with a higher priority. 
     The operating system allows for four different types of programs, each of which is executed by the operating system in accordance with its specific type. The types are initialization routines, tasks, interrupt handlers, and cyclic handlers. Initialization routines are routines scheduled by the operating system immediately after printer  10  is reset but after the operating system initializes itself. Tasks are ordinary programs (sometimes called “execution units”) of continuous processing that are executed sequentially. Thus, tasks are one or more sequences of instructions handled by the operating system as units of work executed by CPU  91  in a multiple-programming or multiple-processing environment. An illusion of concurrent processing is created by the operating system by scheduling processing in individual task units. 
     An interrupt handler is a (usually short) program unit that is activated by the operating system immediately upon receipt of a hardware interrupt. Cyclic handlers are similar to interrupt handlers, but rather than being activated by a hardware interrupt, cyclic handlers are activated by a timer interrupt of the operating system. 
     When printer  10  is reset, execution of the operating system is the first software executed by CPU  91 . CPU registers are set according to predefined requirements, and then user-defined initialization routines are executed if any exist. Thereafter, control reverts to the operating system, which activates each of the tasks in the system. One such task is a start task. After the start task begins, the operating system is activated each time a system call is issued or an interrupt occurs. After executing the system call, or handling the interrupt, execution reverts back to the operating system, which schedules tasks so as to execute the executable task with the highest priority. 
     Scheduling of tasks involves a determination of which task is executed if there are several tasks currently eligible for execution. Tasks are scheduled according to an assigned priority in which a higher priority task is executed before all other lower priority tasks. Tasks eligible for execution but not currently being executed because of their lower priority level are placed in a ready queue based on their priorities. 
     As each task becomes newly eligible for execution, it is placed at the end of the ready queue. Scheduling is then performed when returning from a system call issued by a task or when returning from interrupt processing to a task, both of which can cause new tasks to be entered into a queue or can cause a change in priority of tasks already existing in the queue. Scheduling orders the tasks in the task queue based on each task&#39;s priority and makes the task with the highest priority the currently executable run task. If there are two or more tasks in the ready queue of the same priority, the decision as to which task should be selected is made based on which task first entered into the queue. 
     The operating system uses semaphores as one basic means of communication between tasks and for control or synchronization between tasks. Tasks can also communicate and transfer data therebetween using messages. Messages are sent to mailboxes by one task, and a task that needs to receive the message issues a receive request to the mailbox so as to obtain the message. 
     The operating system further uses event flags to synchronize tasks. Any task desiring to be released from a wait state based on a certain event can register an event flag pattern, upon the occurrence of which the operating system will release the task from the wait state. 
     Interrupt management by the operating system is provided by an interrupt handler and by interrupt permission level settings. Time management is provided by the operating system&#39;s actuation of an interrupt handler based on the system timer. 
     Cyclic handlers carry out processing at each of specified time intervals, based on cyclic handlers registered with the operating system. Typically, a cyclic handler is a short program that specifies a task that is performed at each of specified time intervals. 
     Initialization routines, tasks, interrupt handlers, and cyclic handlers that are preferred for printer  10  are described in the following sections. 
     3.2 Initialization 
     During power-up, initialization functions are performed to initialize printer  10 , such as initializing control logic  94 , checking ROM  92 , checking RAM  99 , and checking EEPROM  102 . 
     FIGS. 15 and 16 illustrate a hard power-on sequence and a soft power-on sequence, respectively. In this regard, it is noted that so long as power is supplied to printer  10 , CPU  91  is executing software regardless of the status of power button  24 . Thus, a “hard power-on” refers to initial application of power to printer  10 . Thereafter, user activation of power button  24  simply causes a soft power-on or soft power-off. This arrangement is preferred, since it allows printer  10  to monitor ongoing events (such as elapsed time) even when printer  10  is “off”. 
     Referring to FIG. 15, which shows a hard power-on sequence, upon initial application of power, step S 1501  performs memory checks such as a ROM check, a RAM check, and an EEPROM check. Step S 1502  initializes software tasks, and in step S 1503 , CPU  91  enters an idle loop, awaiting a soft power on. 
     FIG. 16 indicates the soft power-on sequence. Step S 1601  performs mechanical initialization of printer engine  101 , such as a reset to the home position, step S 1602  starts the software control tasks including Centronics communication tasks, and step S 1603  enters the main processing mode. 
     FIG. 17 details a soft power-off sequence. Step S 1701  terminates all software tasks, and step S 1702  enters an idle loop during which, in step S 1703 , printer  10  awaits the next soft power-on sequence. 
     3.3 Tasks 
     FIG. 18 illustrates communication according to the preferred embodiment of the invention between application program  82   a  and other operations running on host processor  2  and various tasks running on printer  10 . In should be noted that the operations and tasks illustrated in FIG. 18 are by no means inclusive. Rather, FIG. 18 provides an overview of the interaction between operations and tasks involved in printing. 
     On the host processor side of a print operation, application program  82   a  communicates with graphical device interface (GDI)  201  of operating system  81 . GDI  201  in turn communicates with printer driver  84  and spooler  202 , which communicates with printer provider  204  through router  203 . Printer provider  204  communicates with printer  10  through language monitor  205 , port monitor  206 , printer (LPT) port  207  and Centronics cable  208 . The function of each of these elements is now described briefly. 
     Application program  82   a  generates a print job in response to user commands, preferably either for an image created on host processor  2  or for an image input from an unshown image input device such as a scanner. This print job is sent to GDI  201 , which preferably provides a device-independent interface to application program  82   a  for outputting graphic images. GDI  201  in turn converts the print job into printer-specific commands through use of printer driver  84 . 
     Printer driver  84  performs various functions on the print data so as to facilitate printing. These functions preferably include input correction  210 , color correction  211 , output correction  212 , binarization and hue/value processing  213 , pre-fire detection  214 , and status-based control  215 . 
     Input correction  210  preferably includes correcting print data based on characteristics of an image input device, for example scanning characteristics of a scanner. Input correction  210  preferably also includes gamma correction and conversion from illuminative color values such as RGB color values to absorptive color values such as CMY or CMYK color values. 
     Color correction  211  preferably includes correction for a type of recording medium, human color perception and lighting under which a printed image is to be viewed. Output correction  212  preferably involves correction based on ink absorption limitations of a recording medium, for example by thinning print data. 
     Binarization and hue/value processing  213  preferably includes selection of different inks and determination of corresponding hue and color value data based on the inks, as explained in more detail below in section  10 . Pre-fire detection  214  concerns detection of various factors that affect pre-firing of ink jet nozzles so as to improve print quality, as explained in more detail below in section 9. Status-based control  215  modifies printing parameters based on printer status, as explained in more detail below in section 7. 
     Print data typically is generated by application program  82   a  and GDI  201  faster than the data can be printed by printer  10 . Spooler  202  stores print data from GDI  201  in print data store  107 , depicted in FIG. 18 as a spool file, as that data is generated. As a result, application program  82   a  can finish sending a print job and can continue with other tasks before the print job is completely printed. 
     Router  203  routes print data from spooler  202  to printer provider  204 , which provides a connection to printer  10  through language monitor  205 , port monitor  206 , LPT port  207 , and a bi-directional communication line such as Centronics cable  208 . Language monitor  205  monitors the language of the print data, for example to determine if the language is supported by the printer. Port monitor  206  controls access to LPT port  207 . 
     Print data from host processor  2  is processed by various tasks running on printer  10 . In the preferred embodiment of the invention, printer tasks are designed to isolate functionality so that each task is responsible for a single cohesive aspect of printer control. These tasks include Centronics task  220 , direct image command task  221 , engine task  222 , and manager task  223 . 
     Centronics task  220  controls communication with host processor  2 . Characters received from host processor  2  are forwarded by GetCharacter operation  225  to direct image command task  221 . Status, communication and command (SCC) information from direct image command task  221  is received by SCC analysis operation  226 . From this SCC information, status information is returned to host processor  2 . 
     Direct image command task  221  receives data from and sends SCC information to Centronics task  220 . Data received from Centronics task  220  is analyzed by analysis operation  231 . If the data is print data, that data is sent to image buffer  233  by print data operation  236 . If the data is control data, engine interface command operation  237  interprets the control data and sends corresponding commands to engine task  222 . 
     Engine task  222  controls actual printing by print heads  100   a  and  100   b  of print data read from image buffer  233 , as well as operation of line feed motor driver  34   a  and carriage motor driver  39   b  to feed sheets of recording media and to purge the recording heads. To this end, engine task  222  includes various other tasks, such as engine control task  241 , engine auto-sheet-feed (ASF) and purge task  242 , engine line feed task  243 , and engine carriage task  244 . 
     Engine task  222  utilizes cyclic timer  251  for controlling cyclic operations, for example as described below with reference to FIG.  19 . Engine ASF and purge task  242 , engine line feed task  243 , and engine carriage task  244  utilize ASF and purge line feed motor handler  252  and carriage motor handler  253  to control line feed motor driver  34   a  and carriage motor driver  39   a,  respectively, to feed sheets of recording media and to purge print heads  100   a  and  100   b.  The sheet feed and purging operations are described in more detail above with respect to FIGS. 5C and 5D. 
     Interface and other communications between tasks in printer  10  are controlled by manager task  223  and preferably are accomplished through use of unshown mailboxes into which messages and semaphores are placed so as to coordinate message communication. 
     3.4 Interrupt Handlers 
     Although the operating system can accommodate interrupt handlers such as handlers for periodic clock interrupts, such cyclic events can also be handled with cyclic handlers. 
     3.5 Cyclic Handlers 
     Cyclic handlers are provided for Centronics communications task  220  and for engine task  222 , as shown and described above in connection with FIG.  18 . In addition, a cyclic handler is provided for controller timer operations. 
     FIG. 19 is a flow diagram illustrating controller timer control according to this cyclic handler. As shown in FIG. 19, upon receipt of a 10 ms interrupt in step S 1901 , head protect control (step S 1902 ) is effected in order to pause printing if print head temperatures exceed 75 degrees Centigrade, thereby preventing damage to the print heads. 
     Next, as further shown in FIG. 19, it is determined whether a 50 ms interrupt has been received (step S 1903 ) and, if so, control is directed to the 50 ms interrupt logic flow (step S 1904 ) in which a head temperature calculation (step S 1905 ) is performed for each head based on the amount of head driving pulses applied at each head. Calculations are based on pre-stored tables in ROM  92  which provide constants for use in calculating temperature increase as well as temperature decrease based on head firings. 
     The 50 ms interrupt logic further executes pulse width modulation control (step S 1906 ) in accordance with pre-stored tables in ROM  92  so as to set the setup time, the pre-heat pulse, the interval time, and the main-heat pulse for each print nozzle. The pulse parameters are then sent to control logic  94 . Next, it is determined if a 500 ms interrupt has been received in step S 1907 . The 500 ms interrupt logic flow (step S 1908 ) thereupon initiates meniscus heater control which is used under low environmental temperatures and before printing in order to maintain good print head temperature (step S 1909 ). Next, it is determined if a one second interrupt has been received in step S 1910 . The one second interrupt logic flow (step S 1911 ) then updates pre-fire timers (step S 1912 ) and then updates real time environmental temperature (step S 1913 ). 
     Next, it is determined if a one minute interrupt has been received in step S 1914 . The one minute interrupt logic flow (step S 1915 ) initiates an update of the long term environmental temperature in step S 1916  after which control is returned from this sequence in step S 1917 . 
     It should be noted that each of the 10 ms, 50 ms, 500 ms, 1 second and 1 minute durations depicted in FIG.  19  and discussed herein are merely illustrative and may be altered. 
     3.6 Commands to and from the Host Processor 
     The following summarizes the commands sent to and from host processor  2  over bi-directional printer interface  74 . Generally speaking, each command will include one or more parameters, with some commands (such as the [DATA] image data transmission command) also including data. 
     The status request command [STATUS] is a generalized command that elicits a response over bi-directional interface  74  from printer  10 . Through use of the status request command, host processor  2  can obtain detailed information concerning printer  10 , such as the contents of EEPROM  102 , alignment and density sensor results, and the like. The status request command is therefore discussed in considerable detail below. 
     In the sections below, a mnemonic for each command is shown enclosed by square brackets (“[ ]”). The mnemonics shown below are simply examples. The actual sequence and combinations of letters used to form the command mnemonics is immaterial, so long as usage is consistent in the printer side and the host processor side such that commands sent by one are understandable to the other. 
     3.6.1 Control Commands 
     Control commands serve to control print operations of printer  10 . The following is a description of the various control commands. 
     [LOAD]—Paper Load 
     The LOAD command causes paper loading, but does not eject the recording medium currently loaded. This command must be sent to printer  10  even when a medium is already loaded manually. The LOAD command includes parameters to allow for specification of the recording media type and size, and for specification of the paper loading mode. The paper loading mode can be one of either: (1) Auto Sheet Feeder—Normal Feed; (2) Auto Sheet Feeder—High Feed; or (3) Manual Feed. 
     [EJECT]—Paper Eject 
     This command prints all data remaining in the print buffer, then ejects the medium currently loaded. This command can provide for various eject speeds. 
     [PRINT]—Print Execution 
     The Print Execution command causes the data in the print buffer to be printed on a currently-loaded recording medium. The printing area extends from the left edge to the right edge of each print buffer specified by the Left and Right parameters of the [EDGE] command described below. 
     [CARRIAGE]—Carriage Movement 
     The Carriage Movement command includes a Position parameter which specifies carriage position in units of column position. This command is used for forward and reverse seeking. 
     [SKIP]—Raster Skip 
     The Raster Skip command is used to advance the vertical print position by the number of raster lines specified by a Skip parameter. A SKIP command with an argument of zero is used to instruct printer  10  to perform a nozzle-number-change prefire operation. 
     [DATA]—Image Data Transmission 
     This command is used to transmit bit image data of yellow (Y), magenta (M), cyan (C) or black (Bk or K) to printer  10  individually in column image format. Multiple sequences of this command may be issued to make a single scan line. Bit image data is stored into the area specified by the block [BLOCK] and color [COLOR] commands described below. Printer  10  will actually start printing when the [PRINT] command is received. 
     3.6.2 Setting Commands 
     Setting commands specify settings for print operations performed by printer  10 . Once these commands are set, they are valid until the settings are changed by another command. If no settings are provided for a page, the settings will be reset to default settings. Setting commands are described in more detail below: 
     [RESET]—Printer Reset 
     The Mode parameter defines the Printer Reset command and specifies the reset mode. Default settings are included for data compression flag, buffer size, droplet size, print speed, pulse control tables, buffer control tables, and the like. 
     [COMPRESS]—Select Data Compression 
     The Mode parameter of the Select Data Compression command specifies whether the image data is compressed or un-compressed, with un-compressed being the default setting. 
     [BTM_MARGIN]—Select Bottom Margin 
     The Select Bottom Margin command is used to specify the bottom margin of the printable area on the recordable medium. The margin parameter of this command provides for the selection of one of multiple bottom margin sizes. 
     [DEFINE_BUF]—Define Print Buffer 
     The Define Print Buffer command is used to define the memory size and configuration of print buffer  109 , for each of heads A and B in common. 
     [DROP]—Select Droplet Size 
     This command is used to specify the ink droplet size (large or small) for each print head. 
     [SPEED]—Select Print Speed 
     This command is used to specify the printing speed. 
     [SPEED_RSKIP]—Select Speed for Raster Skip 
     The Select Speed for Raster Skip command is used to specify the raster skip speed of the line feed. This command allows for the specification of one of multiple allowable raster skip speeds. 
     [DIRECTION]—Set Print Direction 
     The Direction parameter of this command specifies whether printing will be in the forward direction (left to right) or the backward direction (right to left). 
     [EDGE]—Set Print Edge 
     The Set Print Edge command specifies the left edge and the right edge of print position in units of column position; the left edge must be smaller than the right edge. 
     [BLOCK]—Select Print Block 
     This command is used to specify the left edge and the right edge of a data block in units of column position from the top of each print buffer. The [BLOCK] command also specifies where bit images following a [DATA] command (described above) are stored. 
     [DEFINE_COLOR]—Define Print Color 
     The Define Print Color Command is used to define the color table which specifies the location in the printer head where the bit image data that follows the [DATA] command is stored. This command has parameters to specify the color table to be defined, the color start position, the color height, and the color offset. 
     [COLOR]—Select Print Color 
     This command is used to specify the color table which was defined by the DEFINE_COLOR command. 
     [DEFINE_PULSE]—Define Heat Pulse Table 
     The [DEFINE_PULSE] command is used to define up to plural different heat pulse block tables. The pulse block table must be defined before printer  10  receives the [SELECT_PULSE] command which will be defined below. 
     [SELECT_ULSE]—Select Heat Pulse Table 
     The Select Heat Pulse Table command is used to select one heat pulse block table, from among plural tables defined by the [DEFINE_PULSE] command above, that is in common with all heads. 
     [DEFINE_CONTROL]—Define Buffer Control Table 
     This command is used to define up to plural different print buffer control tables. The print buffer control table must be defined before the printer receives [SELECT_CONTROL] command (described below). 
     [SELECT_CONTROL]—Select Buffer Control Table 
     This command is used to select a print buffer control table for each print head  100   a  and  100   b,  from among the plural tables defined in the [DEFINE_CONTROL] command. 
     [SCAN_MARGIN]—Set Scan Margin 
     The Set Scan Margin command is used to set the scan margin. This command is to be received by printer  10  before a line is printed so that the printer can seek the carriage logically. 
     [AT_DELAY]—Set Auto-Trigger Delay 
     This command is used to set the auto-trigger delay by specifying the scan direction as either forward or backward, and by specifying an auto-trigger delay time in units of 10 μsec up to a maximum auto-trigger delay time of 2,550 μsec. 
     3.6.3 Maintenance Commands 
     Maintenance commands serve to maintain print operations of printer  10  and are described in more detail below. 
     [RECOVER]—Head Recover 
     Receiving this command causes printer  10  to go into head recovery mode, such as cleaning and ink suction operations. 
     [HEAD_EXC]—Head Exchange 
     The Head Exchange command places printer  10  in head exchange mode. Upon entering head exchange mode, the carriage moves to the exchange position. This parameters of this command specifies the head and/or ink tank to be exchanged. 
     [PCR]—Change Pulse Control Ratio 
     This command is used to change a ratio of the Pulse Control Table. Each ratio can be set from 1 through 200, which means 1% through 200%. Default setting is 100 which means 100%. 
     [UCT]—Universal Coordinated Time 
     This command is used to set the current time in printer  10 , and must be sent to printer  10  at the onset of a print job start. Printer  10  uses the time to determine whether or not printer  10  should recover the print head. The time value is expressed as the number of seconds elapsed since midnight (00:00:00), Jan. 1, 1970, Universal Coordinated Time (UCT), according to the system clock of host processor  2 . 
     [HEAD_CHECK]—Head Check 
     The head check command is used to check the print head type currently installed in the printer  10 . 
     [AUTO_POWER]—Auto Power Management 
     This command is used to specify whether the auto power management function within printer  10  is enabled or not. 
     [SCAN]—Scan Sensor 
     This command is used to read an auto-alignment sensor value and to send the result back to host processor  2 . Scanning speed, direction, resolution and area are defined by the [SPEED], [DIRECTION], [DEFINE_BUF] and [EDGE] commands, respectively, as described above. 
     [NVRAM]—NV-RAM Control 
     This command is used to read data from EEPROM  102  and send the read data back to host processor  2 . 
     [SMEAR]—Smear Control 
     The Smear Control command is used to prevent the print medium being used from being smeared with undried ink. This command allows a specified time to be set for delay of the printing time of the current page thereby preventing smearing. 
     [IF_CONTROL]—Interface Control 
     The Interface Control command is used to specify whether or not a specific interface mode on printer  10  is enabled. 
     [STATUS]—Status Request 
     This command is used as a prefix command to send status requests to printer  10 . Requests can be made for basic settings, main status, and detailed status. 
     Basic Setting Commands are commands used by host processor  2  to set printer  10  and do not necessarily require a response from printer  10 . 
     Main Status Request/Response commands are commands which are used to obtain status information in regular mode and include Base Status [BASE_STATUS], Echo Command [ECHO], print head configuration [HEAD], Alignment Sensor Results [SENSOR_RESULTS], EEPROM data sending to host [DATA_SEND], and Shift Buffer Size sending to host [BUFFER_SIZE]. For each Main Status Request/Response command issued, a response is automatically returned to host processor  2 . 
     Detailed Status Request/Response commands are used to obtain detailed status information. These commands include Detailed Job Status [JOB_STATUS], Detailed Busy Status [BUSY_STATUS], Detailed Warning Status [WARNING_STATUS], Detailed Operator Call Status [OPERATOR_CALL], and Detailed Service Call Status [SERVICE_CALL]. Like Main Status Request/Response commands, for each Detailed Status Request/Response command issued, a response is automatically returned to host processor  2 . 
     [PREFIRE_EX]—Prefire Execution 
     The Prefire Execution command is used to execute the prefire of ink. The parameters of this command allow for identification of the specific head to be prefired. 
     [PREFIRE_CYC]—Prefire Cycle Set 
     The Prefire Cycle Set command is used to set the auto prefire execution cycle. The parameters of this command allow for the identification of the target head to be prefired and the amount of auto prefire cycle time in increments of seconds up to a maximum of 255 seconds. 
     4.0 Automatic Sheet Feed Control 
     In brief, this section provides a description of the present invention in which an automatic sheet feed control process is provided for a printer whereby the printer is commanded to load a sheet of recording medium into the printer and to prepare said sheet for printing in an efficient and reliable manner. Specifically, a first aspect of the invention provides logic for selecting the speed at which the recording medium is loaded into the printer based upon the type of recording medium being loaded and upon print modes selected by the user and other printing-related conditions. In a related aspect, the line feed speed used to pass the recording medium through the printer during printing and the eject speed used during ejection of the recording medium from the printer after printing can also be selected in a similar manner. In a further aspect, the present invention also provides for an automatic sheet feed control whereby other pre-printing tasks can be carried out prior to completion of the automatic sheet feed sequence. Lastly, the present invention provides an automatic sheet feed sequence whereby a determination is made whether the sheet feed sequence will be successful prior to actual completion of the sheet feed sequence, thereby allowing a printer driver to send print data to the printer prior to completion of the automatic sheet feed sequence. 
     As described in more detail below, the foregoing arrangement provides for increased reliability during the loading of a recording medium into the printer and also reduces the amount of time required to load the recording medium and to complete other pre-printing tasks in preparation for printing on the recording medium. 
     4.1 ASF, Line Feed and Elect Speed Selection 
     Printer  10  includes an automatic feeder  14  for automatically feeding a recording medium into printer  10  prior to printing. A sheet of recording medium is automatically loaded from automatic feeder  14  into printer  10  by automatic feeder rollers  32  which are driven by line feed motor  34  through clutch device  140  as depicted in FIG.  5 A. Movement of cartridge receptacles  37   a  and  37   b  are necessary in order to position clutch device  140  so as to engage automatic feeder rollers  32  with line feed motor  34  for loading the recording medium into printer  10 . The sequence of events necessary to engage and operate automatic feeder rollers  32  via clutch device  140  is depicted in FIG. 5C, as discussed in more detail in Section 1.1, above. 
     The operation of automatic feeder  14  and automatic sheet rollers  32  is controlled by printer  10  in conjunction with printer driver  84  whereby printer driver  84  sends control commands to printer  10  via communication line  76 . In the present aspect of the invention, printer driver  84  preferably sends a command to printer  10  to begin loading the recording medium prior to printing. Upon receipt of the load command from printer driver  84 , printer  10  starts to load the recording medium pursuant to the parameters and conditions specified in the load command. As shown in FIG. 14, the load ([LOAD]) command is utilized during the command sequence from printer driver  84  to printer  10  to instruct printer  10  to load the recording medium. The load ([LOAD]) command provides parameters to printer  10  regarding the type and size of recording medium to be loaded, and informs printer  10  whether the recording medium is to be loaded using automatic feeder  14  or manual feeder  17 . When automatic feeder  14  is to be used, the load ([LOAD]) command also indicates which one of a plurality of speeds, such as high speed or normal speed, is to be used by automatic feeder rollers  32  for loading the recording medium into printer  10 . As discussed earlier in reference to FIG. 14, a skip ([SKIP]) command is used to direct printer  10  to advance the recording medium through printer  10  during printing and an eject ([EJECT]) command is used to eject the recording medium from printer  10  after printing has been completed. 
     FIG. 20 is a flow chart that depicts a sequence of steps that are preferably executed within printer driver  84  for commanding printer  10  to load and print a page of recording medium according to the present invention. In FIG. 20, the sequence is started in step  2000  in which printer driver  84  sends a reset command ([RESET]) to printer  10  in order to initialize printer  10 . Printer driver  84  then determines (step S 2001 ) the print modes and conditions related to the type of recording medium to be loaded, the type of image to be printed on the recording medium and the modes to define the manner in which printer  10  shall print the image. Once the print modes and conditions have been determined, printer driver  84  determines an appropriate automatic sheet feed speed, line feed speed and eject speed for use during the loading, printing and ejection of the recording medium, and then sends a paper load command ([LOAD]), which includes the determined load speed, line feed speed and eject speed, to printer  10  to begin loading the recording medium (S 2002 ). Printer driver  84  then prepares print data for a first scan of printing in step S 2003  and notifies printer  10  of the print data preparation. The preparation of print data by print driver  84  is described more fully in U.S. patent application Ser. No. 08/901,719, entitled “PRINT DRIVER FOR A COLOR PRINTER”, filed Jul. 28, 1997. In step S 2004 , a determination is then made whether printer driver  84  has received an indication of early success of loading the recording medium or an indication that the loading is complete. If either indication is received, then printer  10  is ready to proceed with printing and control passes to step S 2005 . If neither indication is received, control passes to the end of the sequence. If no print data is to be printed for this scan, (step S 2005 ), control proceeds to step S 2016  in which print data for the next scan is prepared. Printer driver  84  then performs a virtual skip in step S 2017  in order to keep track of the total number of scan lines processed for this particular page of recording medium. If it is determined that printing for this page of recording medium has not yet been completed (step S 2013 ), control is returned to step S 2005 . Until it is determined that printing for the current page is finished, steps S 2005  through S 2013  are repeatedly performed. 
     If there is print data to be printed for this scan (step S 2005 ), printer driver  84  determines whether to override the previous selection for line feed speed of printer  10  based upon user input (step S 2006 ). For example, the user may select No_Override, Low_Speed Override, or High_Speed Override which is sent to printer  10  (step S 2006 ) via a line feed speed command ([SPEED_RSKIP]). A skip command ([SKIP]) is then sent to printer  10  (step S 2007 ) to instruct line feed motor  34  to advance the recording medium by a specific number of raster lines in order to position the recording medium for printing the current scan of print data. Printer driver  84  then sets scan settings and sends them to printer  10  (step S 2008 ) to prepare it for printing the current scan of print data ([DIRECTION], [EDGE], [SPEED], [SIZE], [SELECT_PULSE], [SELECT_CONTROL]). After sending the scan setting parameters to printer  10 , printer driver  84  sends the print data for the current scan to printer  10  via an image data transmission command ([DATA]) in step S 2009 . Printer driver  84  then prepares the next scan of print data in step S 2010 . It is then determined whether the loading of recording medium has been completed successfully (step S 2011 ). If the page of recording medium has not been successfully loaded, control is directed to the end of the printer driver process. 
     If the loading of the recording medium has been successfully completed, printer driver  84  begins printing of the current scan of print data by sending a print command ([PRINT]) to printer  10  (step S 2012 ). If printing for the page is finished (step S 2013 ), printer driver  84  sets the selected eject speed override in step S 2014  to either No_Override, Low_Speed Override, or High_Speed Override, and then sends the override selection to printer  10  as part of a paper eject command ([EJECT]) to instruct printer  10  to eject the current page of recording medium (step S 2015 ). If printing for the current page is not finished, control returns to step S 2005 . In this manner, printer driver  84  provides detailed commands and data to printer  10  based upon the type of recording medium being used, the print modes and conditions requested by the user, and other relevant print related conditions. 
     FIG. 24 is a flow chart providing a detailed view of the process steps performed by printer driver  84  during step S 2002  of FIG. 20 in which automatic sheet feed speed, line feed speed and eject speed are determined. First, it is determined whether the user has selected manual feed for the current print job (step S 2401 ) whereby the user manually feeds a sheet of recording medium into manual feeder  17  of printer  10 . If manual feed is selected, printer driver  84  sends a purge check command to printer  10  and waits for the purge check to finish, thereby preventing the user from manually feeding the recording medium during operation of the purge pump (not shown) contained within ink cleaning mechanism  45 . Once it is determined that the purge pump is not currently in operation, a dialog box is displayed on display  4  prompting the user to insert a sheet of recording medium into the manual feeder (step S 2403 ). A determination is then made whether the user acknowledged the dialog box prompt to manually insert paper (step S 2404 ) and, if so, control proceeds to step S 2406  in which a paper load command ([LOAD]) is sent to printer  10  specifying a manual load. If the user did not acknowledge the dialog box prompt displayed on display  4 , the print job is cancelled in step S 2405 . 
     Returning to step S 2406 , after the manual feed load command is sent to printer  10 , a determination is made whether the recording medium was loaded correctly (step S 2408 ). If it was not, the user is asked to remove the recording medium from the printer and re-insert it for another attempt at manual feed (step S 2409 ). If the user acknowledges the request to re-insert the recording medium for another attempt at manual feed (step S 2407 ), then control is directed back to step S 2406  to send another load command specifying manual feed. If the user does not acknowledge the request to re-insert the recording medium for another attempt at manual feed (step S 2407 ), then the print job is cancelled (step S 2405 ). Returning to step S 2408 , if the recording medium is properly fed into printer  10  after receipt of the manual feed load command, then control is directed to return from the sequence (step S 2422 ). 
     Returning to step S 2401 , if the user does not select manual feed, the current time is obtained in step S 2425 . If printer  10  is being used within a specified time period as defined by predetermined thresholds T 1  and T 2  (step S 2423 ), which preferably define daytime business hours, control proceeds to step S 2410 . If printer  10  is not being used within the specified time period (step S 2423 ), then printer driver  84  selects a low speed automatic sheet feed command, a low speed line feed command and a low speed eject speed command and sends them to printer  10  (step S 2416 ), thereby reducing the noise generated by printer  10  during printing. These settings correspond to default settings when a No_Override mode is selected by the user. If printer  10  is being used within the specified time period (step S 2423 ), but the user has not selected draft or standard mode, then printer driver  84  selects a low load speed setting, a low line speed setting and a low eject speed setting and sends the settings to printer  10  via a paper load ([LOAD]) command (step S 2416 ). If, however, the user has selected a draft or standard mode, a determination is made whether the current print job is to be printed using a regular mode (step S 2411 ). If regular mode is not selected, then a high resolution color mode is in use for the current print job and therefore the printer driver  84  selects low speed settings for the load speed, line feed speed and eject speed and sends them to printer  10  via paper load ([LOAD]) command (step S 2416 ). 
     If, however, regular mode is being used for the current print job (step S 2411 ), then a determination is made in printer driver  84  regarding what type of recording medium is being used for the current print job (step S 2412 ). If plain paper is being used (step S 2412 ), then a high speed is selected for the load speed, line speed and eject speed and these selections are sent to printer  10  via a paper load ([LOAD]) command (step S 2414 ). However, if instead bubble jet paper is being used for the current print job (step S 2413 ), then a low speed setting is selected for the load speed, a high speed setting is selected for the line feed speed, and a low speed setting is selected for the eject speed, and these selections are sent to printer  10  via a paper load ([LOAD]) command (step S 2415 ). If neither plain paper nor bubble jet paper is being used for the current print job, then printer driver  84  selects a low speed setting for the load speed, a low speed setting for the line feed speed and a low speed setting for the eject speed and these selections are sent to printer  10  via a paper load ([LOAD]) command (step S 2416 ). After a paper load command is sent to printer  10  from one of steps S 2414 , S 2415  or S 2416 , a determination is made whether the recording medium was properly fed into printer  10  (step S 2417 ). If the recording medium was not properly fed, a dialog box is displayed on display  4  asking the user to correct the problem and retry the paper load (step S 2418 ). If the user then chooses to retry the paper load from display  4  (step S 2419 ), control is directed to step S 2416  in which low speed settings are set for the load speed, line feed speed and eject speed and another paper load ([LOAD]) command is sent to printer  10  (step S 2416 ). If the user did not select a retry from display  4 , then a determination is made whether the user selected retry from resume button  26  on printer  10  (step S 2420 ), and if so, control is directed to step S 2416 . If the user did not select retry from display  4  or from printer  10 , then the printing job is cancelled (step S 2421 ). Returning to step S 2417 , if the recording medium was loaded properly into printer  10 , flow is directed to step S 2422  which returns control from the entire sequence. 
     In this manner, the present invention provides logic within printer driver  84  to select from one of multiple speeds for loading recording medium from automatic sheet feeder  14  and for similar selection of line feed speed and eject speed based upon the conditions and requirements of a given print job such as the type and size of recording medium, print modes, previous unsuccessful load attempts, and other modes and conditions. As a result, the fastest speeds that are appropriate for a given print job are utilized during loading of the recording medium, and during printing and ejection of the recording medium, thereby reducing the overall time required for a particular print job while still providing reliable performance. 
     FIG. 25 is a flow chart depicting logic used within CPU  91  of printer  10  for setting eject speed based on an override command provided from printer driver  84 . Control begins in step S 2501  in which a determination is made whether a No_Override command was received from printer driver  84 . If the No_Override setting was selected, a determination is made whether the load speed is currently set to a high speed setting (step S 2504 ). If the load speed is currently set to a high speed, then the line feed speed to be used during eject is also set to a high speed selection (step S 2505 ). If the load speed is not set to a high speed, then the line feed speed to be utilized during eject is set to a low speed (step S 2506 ). Returning to step S 2501 , if a No_Override was not sent by driver  84 , then it is determined whether a Low_Speed Override was sent (step S 2502 ). If a Low_Speed Override command was sent, then the line feed speed to be used for ejection is set to a low speed (step S 2507 ). On the other hand, if a Low_Speed Override was not sent, then a determination is made whether a High_Speed Override command was sent (step S 2503 ), and if so, a high speed line feed speed is selected for ejection (step S 2508 ). If neither a No_Override, a Low_Speed Override or a High_Speed Override has been sent, then a default value, preferably low speed, for line feed speed is set for ejection (step S 2509 ). In this manner, printer driver  84  can select an ejection speed override to change a previously set ejection speed command from printer driver  84 . 
     In a similar manner, FIG. 26 provides a flow chart for operation of logic in CPU  91  of printer  10  whereby a prior setting for line feed speed can be overridden at a subsequent time by printer driver  84 . Control begins in step S 2601  in which it is determined whether the resolution for printing has been set to 1440 dpi. If the resolution of 1440 dpi has been selected by printer driver  84 , then a 1440 dpi speed is selected for the line feed speed (step S 2605 ). If, however, a resolution of 1440 dpi has not been selected, then a determination is made whether printer driver  84  has sent a Low_Speed Override (step S 2602 ) and if so, a low speed is selected for the line feed speed (step S 2606 ). If a Low_Speed Override has not been selected, a determination is made whether a High_Speed Override has been selected (step S 2603 ), and if so, a high speed is selected for the line feed speed (step S 2607 ). If a High_Speed Override has not been received, then a determination is made whether the load speed is currently set to a high speed (step S 2604 ) and, if so, a high speed is set for the line feed speed (step S 2608 ). If a high speed has not been set for the load speed, then a default speed of a low speed is selected for the line feed speed (step S 2609 ). In this manner, printer driver  84  can select an override setting for line feed speed after a previous line feed speed setting has been provided by printer driver  84 . 
     4.2 Early Determination of Paper Load Success 
     In a preferred embodiment of the present invention, a determination is made within CPU  91  of printer  10 , prior to completion of the loading of the recording medium, whether the loading will probably be successful. If the loading will probably be successful, printer  10  notifies printer driver  84  of the early success indication so that printer driver  84  can begin sending print data to printer  10  as soon as possible. In this manner, the printer can begin printing more quickly after a successful completion of the loading of the recording medium. 
     FIG. 21A is a flow chart which illustrates the steps performed in CPU  91  of printer  10  during the loading of a page of recording medium by automatic feeder  14  in printer  10 , including steps necessary to obtain an early success indication regarding the loading of the recording medium. Control begins in step S 2101  in which cartridge receptacles  37   a  and  37   b  are commanded to move to home location  46  and then to wait once they arrive there. The cartridge receptacles are driven by carriage motor driver  39   a.  Next, it is determined whether a previous recording medium was ejected immediately prior to this loading sequence (step S 2102 ). If there was an ejection, then a process wait is entered into (step S 2103 ) until line feed motor  34  has ramped from the ejection line feed speed to the automatic sheet feed pickup speed at which speed automatic feeder rollers  32  can be engaged. This wait is performed so that adjustment of clutch unit  140  for engaging automatic feeder rollers  32  is not attempted until line feed motor  34  is at an appropriate speed. Once the line feed motor is at the appropriate speed, a determination is made whether automatic feeder rollers  32  are currently at their initial home position (step S 2104 ). If so, a flag is set to indicate that automatic feeder rollers  32  were in their home position at the beginning of the automatic sheet feed sequence (step S 2106 ). 
     If the automatic feeder rollers were not initially in the home position, then the flag is set to false (step S 2105 ). Next, cartridge receptacles  37   a  and  37   b  are commanded to move to clutch unit  140  for engaging automatic feeder rollers  32  (step S 2107 ). A Retry_Load Flag is set to false in step S 2108  to indicate that a retry has not yet been attempted for loading of the recording medium. Next, a determination is again made whether there was an ejection of a previous recording medium prior to the beginning of this load sequence (step S 2109 ). If there was an ejection, then control is directed to step S 2111  and, if there was not an ejection, the load speed is selected based upon various conditions as described in further detail in FIG. 21D, after which the start of automatic feeder rollers  32  is commanded (step S 2110 ). Control flow then proceeds to step S 2111  in which it is determined whether automatic feeder rollers  32  are currently at their home position. If they are currently at their home position, control is again returned to step S 2111  to keep checking their position until they are no longer at the home position. If automatic feeder rollers  32  are not currently at the home position and it is also determined that automatic feeder rollers  32  did, in fact, start in the home position (step S 2112 ), then clutch unit  140  is properly engaged for driving automatic feeder rollers  32  and, therefore, cartridge receptacles  37   a  and  37   b  are no longer required to be positioned near clutch unit  140 . Cartridge receptacles  37   a  and  37   b  are then commanded to move back to home location  46  for the cleaning of print heads  100   a  and  100   b  (step S 2113 ). 
     Returning to step S 2112 , if automatic feeder rollers  32  were not initially in the home position, then cartridge receptacles  37   a  and  37   b  should remain positioned against clutch unit  140  so as to engage automatic feeder rollers  32  to provide enough time for them to complete their motion. In this case, cartridge receptacles  37   a  and  37   b  are not commanded to move back to the home position but, instead, control is directed to step S 2114  in which it is determined whether automatic feeder rollers  32  are currently moving. If they are moving, then a determination is made whether the leading edge of the recording medium has been detected within printer  10  (step S 2115 ). If the leading edge has not yet been detected, control is returned to step S 2114  to again determine if automatic feeder rollers  32  are moving. If it is determined in step S 2114  that automatic feeder rollers  32  are not moving, such as upon completion of their required motion for loading the recording medium, then control is directed to step S 2117 . Returning to step S 2115 , if the leading edge of the recording medium is detected, then early success logic is performed (step S 2116 ) to determine whether the loading process will probably be successful even though it has not yet been completed. A more detailed description of the early success logic is discussed further in reference to FIG.  21 C. After execution of the early success logic (step S 2116 ), a determination is made in step S 2117  whether automatic feeder rollers  32  began in their initial home position and, if so, a process wait is entered into (step S 2118 ) to wait for carriage receptacles  37   a  and  37   b  to stop at home location  46 . Print heads  100   a  and  100   b  are then commanded to perform a pre-fire in order to maintain them in at least a good printing condition step S 2118 ). 
     The wait in step S 2118  also allows for cartridge receptacles  37   a  and  37   b  to move past wipers  44   a  and  44   b  for wiping on the way to home location  46 . Step S 2118  is circumvented if automatic feeder rollers  32  were not initially in their home position (step S 2117 ) at the beginning of the automatic sheet feed sequence. Control is continued at step  2119  in FIG. 21B wherein a determination is made whether automatic feeder rollers  32  are currently moving. If rollers  32  are moving, control is returned to step S 2119  until it is determined that rollers  32  are no longer moving. Once rollers  32  have stopped moving, control is directed to step S 2120  to determined whether rollers  32  were initially in their home position at the beginning of the automatic sheet feed sequence, If rollers  32  were not initially at their home position, then cartridge receptacles  37   a  and  37   b  are commanded then proceeds to a determined of whether rollers (step S 2122 ). If rollers  32  are not returned to their home position after they have stopped moving (step  2122 ) then there has been a fatal error and appropriate action is taken to restart all tasks and log the error (step S 2123 ). If rollers  32  did return to their home position, a determination is made (step S 2124 ) whether the leading edge of the recording medium was detected by the paper edge sensor (not shown). 
     If the leading edge of the recording medium was determination is made (step S 2125 ) whether the detection of the edge was made within the specified number of motor steps, e.g. whether the recording medium took too long to load because it was slipping on automatic feeder rollers  32 . If the leading edge was detected within the leading edge of the recording medium was loaded past the paper edge sensor by a sufficient amount (step S 2126 ). If the recoding medium was loaded by a sufficient amount, then the recording medium was loaded successfully and a Return Load Status flag is set to SUCCESS (step S 2128 ). Control is then returned from the automatic sheet feed sequence. 
     If, however, the recording medium took too long to be detected (step S 2125 ) or was not loaded past the paper edge sensor by a sufficient amount (step S 2126 ) the attempt to load the recording medium was unsuccessful and control id then directed to step S 2127  in which a determination is made whether the recording medium allows for the use of a recovery sequence to place the recording medium in the proper position. The recovery sequence is preferably not allowed for recording media that are less than six inches or that are glossy paper, glossy photo card, or high gloss film. If the type of recording medium does not allow for the use of a recovery sequence, the Return Load Status is set to ERROR and control is returned form the entire automatic sheet feed sequence (step S 2131 ). If the type of recording medium allows for utilization of a recovery sequence, then control is directed to the recovery sequence in step S 2129 . The recovery sequence is discussed in greater detailed below in reference to FIG.  21 E. Upon recovery, the Return Load Status is set to SUCCESS and control is returned from the entire automatic sheet feed sequence (step S 2128 ). 
     returning to step  2124 , if the leading edge of the recording medium has not been detected by the paper edge sensor, the type of recording medium is checked to determined whether it supports the use if a recovery sequence (step S 2132 ). If the type of recording medium does not allow for the use of a recovery sequence, the Return Load Status is set to ERROR (step S 2131 ) and control is then returned form the entire automatic sheet feed sequence. If the type of recording medium supports the use of a recovery sequence, a the Retry_Load flag is tested (step S 2133 ) to determined whether this is the second attempt to retry loading of the recording medium. If this is the second retry attempt, the Return Load Status is set to ERROR and control is returned from the entire automatic sheet feed sequence (step S 2131 ). 
     If this of the first retry attempt, the Retry_Load flag is set (step S 2134 ) and rollers  32  are checked to determine if they are currently at their home position (step S 2135 ). The Start_At_Home flag is set according in step S 2136  or step S 2137  in accordance with the current position of rollers  32 . The process then waits for cartridge receptacles  37   a  and  37   b  to stop moving, and then commands cartridge receptacles  37   a  and  37   b  to move to clutch unit  140  to engage automatic feeder rollers  32  with line feed motor  34  (step S 2138 ). Control then returns to step S 2110  in FIG. 21A to repeat the automatic sheet feed sequence steps previously described. 
     The early success logic referenced earlier in step  2116  of FIG. 21B allows an Early Success flag to be sent to printer driver  84  so that printer driver  84  can begin sending print data to printer  10  prior to completion of the loading of the recording medium. FIG. 21C provides a detailed flow diagram of the steps comprising the early success logic. In step S 2139 , a determination is made whether the leading edge of the recording medium was detected within the specified number of motor steps, e.g. whether the recovering medium took too long to load because it was slipping on automatic feeder rollers  32 . If the leading edge of the recording medium was not detected within the specified number of motor steps, then control is returned because there is a probability that the load will not be successful. 
     If the leading edge of the recording medium was detected within the specified number of motor steps, then the type of recording medium is checked to determined whether if supports the use of a recovery sequence as discussed above (step S 2140 ). If the type of recording medium does not allow for the use of a recovery sequence, control is returned because there is a probability that the load will not be successful. Alternatively, if the type of recording medium for the use of a recovery sequence, an Early Success flag is set and the process gives up control of CPU  91  for 10 milliseconds (step S 2141 ) to allow another process to send a SUCCESS indication in the Return Load Status to printer driver  84 . In this manner, the automatic sheet feed sequence performed in CPU  91  of printer medium from automatic feeder  14  in an efficient manner while also providing reliable performance by allowing printer driver  84  to begin sending print data prior to completion of the loading process based upon an early success indication, This arrangement therefore reduces the time required between the completion of loading the recording medium and the beginning of printing image data on the recording medium. 
     FIG. 21F is a flow diagram that illustrates the process steps referenced in the reference in FIG. 21D to step S 2110  in which CPU  91  of printer  10  sets the load speed based upon the automatic sheet feed speed provided by printer driver  84  and by current conditions and parameters related to the automatic sheet feed sequence. In step S 2142 , the length of the recording medium is checked to determine if it is less than six inches. If it is, the recording medium is treated similar to an envelope and a two-part load sequence is initiated whereby the first part of the motion for automatic feeder rollers  32  is started (step S 2146 ). After a 250 millisecond wait (step S 2147 ), the second part of the motion for automatic feeder rollers  32  is started (step S 2148 ). Control is then returned from this process. This two-part motion provides reliability when attempting to load smaller size recording medium, such as bulky, heavier envelopes. 
     If the recording medium is not less than six inches, the currently set load speed is checked to determine if it is set to low speed, the Start_at_Home flag is checked to determine if automatic feeder rollers  32  were not initially at their home position, and the Retry_Load flag is checked to determine if a prior attempt to load the recording medium was unsuccessful (step S 2143 ). If any of the aforementioned checks are answered in the affirmative, line feed motor  34  is commanded to drive automatic feeder rollers  32  at low speed (step S 2144 ). If none of the aforementioned checks are answered in the affirmative, line feed motor  34  is commanded to drive automatic feeder rollers  32  at high speed (step S 2145 ). Control is then returned from this process. 
     FIG. 21E is a flow diagram that provides a detailed view of the process steps comprising the recovery sequence represented by step S 2129  in FIG.  21 B. The recovery sequence begins in FIG. 21E by first determining if the recording medium slipped too much while being loaded by automatic feeder rollers  32  (step S 2149 ). If so, the recovery sequence waits for cartridge receptacles  37   a  and  37   b  to stop moving (step S 2150 ) and then commands cartridge receptacles  37   a  and  37   b  to move to clutch unit  140  to engage automatic feeder rollers  32  with line feed motor  34  (step S 2151 ). If the paper has not slipped too much, control is directed to step S 2155  which is discussed in more detail below. Automatic feeder rollers  32  are then started at a low speed (step S 2152 ) and the recovery sequence then waits until automatic feeder rollers  32  complete the loading motion. Next, it is determined whether automatic feeder rollers  32  have stopped at their home position (step S 2153 ). If they have stopped at their home position, then the recovery sequence continues to step S 2155 . If they have not stopped at their home position, then all tasks are restarted and a fatal error is logged (step S 2154 ). 
     The recovery sequence continues at step S 2155  wherein cartridge receptacles  37   a  and  37   b  are commanded to move to home location  46  thereby disengaging automatic feeder rollers  32  from line feed motor  34  via clutch unit  140 . Line feed motor  34  is then commanded to rotate line feed roller  165  in the reverse direction (step S 2156 ) to feed the recording medium behind a pinch roller (not shown). Cartridge receptacles  37   a  and  37   b  are then commanded to move to clutch unit  140  to engage automatic feeder rollers  32  with line feed motor  34  (step S 2157 ) via clutch unit  140 . The recording medium is then clamped by moving automatic feeder rollers  32  from their home position (step S 2158 ). 
     Cartridge receptacles  37   a  and  37   b  are then commanded to move to home location  46  thereby disengaging automatic feeder rollers  32  from line feed motor  34  (step S 2159 ). The recording medium is then curled behind the pinch roller (not shown) by driving line feed motor  34  (step S 2160 ). Cartridge receptacles  37   a  and  37   b  are then commanded to move to clutch unit  140  to engage automatic feeder rollers  32  with line feed motor  34  (step S 2161 ). Automatic feeder rollers  32  are started at a low speed in step S 2162  and the recovery sequence then waits until automatic feeder rollers  32  complete the loading motion. Cartridge receptacles  37   a  and  37   b  are then commanded to move to home location  46  thereby disengaging automatic feeder rollers  32  from line feed motor  34  (step S 2163 ). The recording medium is then positioned such that the leading edge of the recording medium is loaded {fraction (70/720)}th of an inch past the location of the first nozzle of print heads  100   a  and  100   b  (step S 2164 ). At this point, the recording medium is positioned for printing and control is returned from this recovery process. 
     4.3 Print Head Maintenance During Paper Load 
     As discussed above and depicted in FIGS. 5A,  5 B and  5 C, the movement of cartridge receptacles  37   a  and  37   b  is necessary in order to adjust clutch unit  140  so as to engage automatic feeder rollers  32  with line feed motor  34  thereby driving automatic feeder rollers  32  to load recording medium into printer  10 . Conventional printers typically wait until loading of the recording medium is successfully completed before performing other pre-printing tasks such as cleaning the print heads. In such an arrangement, cartridge receptacles  37   a  and  37   b  are kept near clutch unit  140  during loading of the recording medium in the event that there is a loading problem that requires the use of cartridge receptacles  37   a  and  37   b  to engage or disengage automatic feeder rollers  32  from line feed motor  34 . 
     In the preferred embodiment of the present invention, it is determined whether automatic feeder rollers  32  began an automatic sheet feed sequence in the proper position and whether the automatic sheet feed sequence is progressing properly. Therefore, in the event that the automatic loading of a recording medium is proceeding properly, cartridge receptacles  37   a  and  37   b  can be utilized for other pre-printing tasks such as print head cleaning and maintenance prior to the completion of the automatic sheet feed sequence. 
     The specific steps performed by printer  10  to achieve this function are shown in steps S 2111  through S 2118  of the automatic sheet feed sequence depicted in FIG.  21 A and have been thoroughly discussed above. Specifically, when it is determined that the automatic sheet feed sequence is proceeding properly, cartridge receptacles  37   a  and  37   b  are sent to home location  46  in step S 2113  of FIG.  21 A. The process waits for cartridge receptacles  37   a  and  37   b  to stop at home location  46 , thereby providing time for wiping print heads  100   a  and  100   b  on the way to home location  46 , after which print heads  100   a  and  100   b  are commanded to perform a pre-fire of ink to maintain them in a good printing condition (step S 2118  of FIG.  21 A). 
     FIG. 22 is a flow diagram that functionally depicts the relationships among automatic feeder rollers  32 , cartridge receptacles  37   a  and  37   b,  print heads  100   a  and  100   b  and printer driver  84  during execution of an automatic sheet feed sequence in printer  10  for loading a first page of recording medium during a print job. Starting with step S 2201 , the line feed motor speed is selected as described earlier in reference to FIG.  21 D. Then, the status of print head connection caps  47   a  and  47   b  are checked to determine if they are closed (step S 2202 ). If caps  47   a  and  47   b  are closed, they are commanded to open (step S 2203 ), after which cartridge receptacles  37   a  and  37   b  are commanded to home location  46  (step S 2204 ), and print heads  100   a  and  100   b  are commanded to pre-fire (step S 2205 ). Control is then returned to step  2206  in which carriage motor  39  is commanded to move cartridge receptacles  37   a  and  37   b  to clutch unit  140  to engage automatic feeder rollers  32  with line feed motor  34 . The motion of cartridge receptacles  37   a  and  37   b  thereupon continues under the supervision of an interrupt background process as shown in step S 2209 . 
     Control continues to step S 2207  in which line feed motor  34  is started to begin the loading of the recording medium via automatic feeder rollers  32 . The paper loading thereupon continues under the supervision of an interrupt background process as shown in step S 2210 . In step S 2208 , a process wait is entered until the interrupt background process of step S 2209  returns an interrupt indicating that cartridge receptacles  37   a  and  37   b  have moved to clutch unit  140 . Then, control proceeds to step S 2211  whereupon carriage motor  39  is commanded to move cartridge receptacles  37   a  and  37   b  to home location  46 , thereby initiating an interrupt background process to supervise the wiping of print heads  100   a  and  100   b  as shown in step S 2212 . A determination is then made whether an Early Success flag has been set for the automatic sheet feed sequence (step S 2220 ). If the Early Success flag is set to FALSE, control is directed to step S 2213 . However, if the Early Success flag is set to TRUE, control of CPU  91  is given up in order to transmit the Return Load Status to printer driver  84  as depicted in steps S 2221  through S 2223 . Control is then directed to proceed immediately prior to step S 2213 . 
     The interrupt background process that moves cartridge receptacles  37   a  and  37   b  to home location  46  (step S 2212 ), during which the wiping of print heads  100   a  and  100   b  is performed, returns an interrupt prior to step S 2213  indicating that cartridge receptacles  37   a  and  37   b  have arrived at home location  46 . Pre-fire of print heads  100   a  and  100   b  is then performed in step S 2213 . A process wait is entered in step S 2214  until the interrupt background process that monitors the loading of the recording medium (step S 2210 ) returns an interrupt indicating that the loading of the recording medium is complete. 
     Upon receipt of an indication that the loading of the recording medium is complete (step S 2214 ), a determination is made whether Early Success was previously detected for the automatic sheet feed sequence (step  2215 ). If there was an Early Success detection, control is returned from this sequence (step S 2219 ). If there was not an Early Success detection, control is given up to CPU  91  of printer  10  (step S 2216 ) in order to transmit the Return Load Status to printer driver  84  as depicted in steps S 2217  and S 2218 . Control is then returned from this sequence in step S 2219 . In this manner, cartridge receptacles  37   a  and  37   b  are allowed to perform other pre-printing tasks, such as wiping and pre-firing of print heads  100   a  and  100   b,  concurrently with the loading of the recording medium if the loading process is proceeding properly. Thus, the overall time required between completion of loading the recording medium and the start of printing is reduced without adversely affecting reliability and performance of the automatic recording medium load sequence. 
     FIG. 23 is a flow diagram that functionally depicts execution of an automatic sheet feed sequence in printer  10  for ejection of a page of recording medium followed by loading of a new page of recording medium First, the speed of line feed motor  34  is selected for ejection of the previous page of recording medium (step S 2301 ). Then, line feed motor  34  is commanded to begin the ejection of the previous page of recording medium (step S 2302 ). This initiates an interrupt background process to monitor the ejection of the previous page of recording medium as shown in step S 2307 . Next, carriage motor  39  is commanded to move cartridge receptacles  37   a  and  37   b  to home location  46  (step S 2203 ), thereby initiating an interrupt background process to monitor the movement of cartridge receptacles  37   a  and  37   b  (step S 2306 ). Control then waits until an interrupt is returned from the interrupt background process monitoring the ejection of the previous page of recording medium (step S 2307 ) indicating that the ejection is complete, whereupon control proceeds to step S 2304 . A determination is made whether the ejection is to be followed by the loading of a new page of recording medium (step S 2304 ), and if not, then control is returned from the process in step S 2305 . 
     If the ejection is to be followed by the loading of a new page of recording medium, then a determination is made whether the current line feed speed is equal to the speed required for engaging clutch unit  140  for driving automatic feeder rollers  32  (step  2308 ). If the line feed speed is not the same, then an interrupt background process is initiated to monitor the ramping of the current line feed speed to the speed required to engage clutch unit  140  for driving automatic feeder rollers  32  (step S 2309 ). Control then continues at step S 2308  until the required speed is obtained, after which control is directed to step S 2310 . 
     Carriage motor  39  is then commanded to move cartridge receptacles  37   a  and  37   b  to clutch unit  140  in step S 2310  in order to engage automatic feeder rollers  32  with line feed motor  34 . The motion of cartridge receptacles  37   a  and  37   b  thereupon continues under the control of an interrupt background process as shown in step S 2311 . Upon the return of an interrupt from the background process of step S 2311 , the loading of the recording medium then proceeds under the monitoring of an interrupt background process as shown in step S 2314 . In step S 2312 , a process wait is entered until the interrupt background process of step S 2311  returns an interrupt indicating that cartridge receptacles  37   a  and  37   b  have moved to clutch unit  140  and thereby engaged automatic feeder rollers  32  to line feed motor  34 . Then, control proceeds to step S 2313  whereupon carriage motor  39  is commanded to move cartridge receptacles  37   a  and  37   b  to home location  46 , thereby initiating an interrupt background process (step S 2316 ) to monitor cartridge receptacles  37   a  and  37   b  as they move to home location  46 , during which wiping of print heads  100   a  and  100   b  is performed. A determination is then made whether an Early Success flag has been set for the automatic sheet feed sequence (step S 2315 ). If the Early Success flag is FALSE, control is directed to step S 2318 , but if the Early Success flag is TRUE, control is given up from CPU  91  of printer  10  in order to transmit the load status to printer driver  84  as depicted in steps S 2317 ,  2320  and S 2321 . Control is then directed to step S 2318 . 
     The interrupt background process of step S 2316  returns an interrupt prior to step S 2318  indicating that cartridge receptacles  37   a  and  37   b  have arrived at home location  46 . Pre-fire of print heads  100   a  and  100   b  is then performed in step S 2318 . A process wait is entered in step S 2319  until the interrupt background process that monitors the loading of the recording medium (step S 2314 ) returns an interrupt indicating that the loading of the recording medium is complete. Upon receipt of an indication that the loading of the recording medium is complete (step S 2319 ), a determination is made whether Early Success was previously detected for the automatic sheet feed sequence (step  2322 ). If there was an Early Success detection, control is returned from this sequence (step S 2326 ). If there was not an Early Success detection, control is given up to CPU  91  of printer  10  (step  2323 ) in order to transmit the Return Load Status to printer driver  84  as depicted in steps S 2324  and S 2325 . Control is then returned from this sequence in step S 2326 . 
     Thus, similar to the loading of a first page of recording medium depicted in FIG. 22 as described above, the overall time required after the completion of loading the recording medium to perform other pre-printing tasks, such as wiping and pre-firing, is reduced without adversely affecting reliability and performance. 
     5.0 Carriage Control 
     This Section describes carriage motor control according to the invention so as to accommodate a faster carriage motor. 
     5.1 Margin And Direction Control 
     Carriage motor  39  of printer  10  preferably is a high-speed motor so as to increase overall printing speed by scanning print heads  100   a  and  100   b  more rapidly across a recording medium than in a conventional printer. However, high-speed motors tend to exhibit non-uniform speeds when they start. These speed non-uniformities can result in rippled or otherwise degraded image formation. The ripples tend to be most apparent in continuous images, for example non-color graphics such as charts or tables, and color images. The impact of the non-uniformities can be alleviated, however, by appropriate carriage motor control. 
     Briefly, the invention addresses speed non-uniformity by determining content of print data, and then printing the print data either with a first lateral scan process using a critical zone at edges in a lateral scan of the print head for printing, or with a second lateral scan process that does not use the critical zone for printing. The first or second lateral scan process is selected based on the print data. The critical zone is an unstable zone for moving the print head in a lateral scan. 
     Preferably, the critical zone is sized in correspondence with ramp up non-uniformities of a print carriage on which the print head is mounted, so as to accommodate a distance between a point where print degradation due to speed non-uniformities are noticeable to a point where print degradation due to speed non-uniformities are no longer noticeable. 
     Preferably, it is determined whether or not print data for a current scan and print data for a previous scan, in at least the critical zone, are continuous print data. The current scan is printed in a direction opposite to that of the previous scan by the first lateral scan process in a case that the print data for the current scan and the print data for the previous scan are not continuous print data. The current scan is printed in a same direction as that of the previous scan by the second lateral scan process in a case that the print data for the current scan and the print data for the previous scan are continuous print data. 
     In more detail, FIG. 27A is a representative view for describing carriage control for standard mode (i.e., not draft or best mode) printing of isolated scan lines  300 , continuous images  301 , and color images  302  on plain-paper recording medium  303 . Isolated scan lines  300  are separated by whitespaces  305  and typically comprise text having a height less than a printable height of print head  100   a  or  100   b.  According to the invention, isolated scan lines  300  are printed using bi-directional printing  304  without additional scan margins. Because these scan lines typically are text, ripples and other distortions caused by speed non-uniformity of carriage motor  39  tend not to be noticed. Accordingly, the faster bi-directional printing without scan margins produces satisfactory image quality at high speed. 
     Continuous images  301  are non-color images that require multiple scan lines to print, without any whitespaces between scan lines. Examples of continuous images  301  are large-font text that has a height greater than a print height of print head  100   a  or  100   b,  and black-and-white or grey-scale graphics including tables and charts. 
     If continuous images  301  are printed using bi-directional printing without margins, speed non-uniformity occurs on opposite sides of recording medium  303  from scan line to scan line. As a result, distortions caused by speed non-uniformity at a start of each scan line become more noticeable by proximity to vertically-adjacent non-distorted ends of previous and subsequent scan lines. In order to address this problem, printer  10  according to the invention prints continuous images using unidirectional printing  306 . Furthermore, scan margin  307  is inserted before each scan line so as to allow motor non-uniformities to dissipate before ink is ejected onto recording medium  303 . Because unidirectional printing is preformed, only left scan margin  307  needs to be inserted on a left side of the scan lines. 
     By virtue of the foregoing, bi-directional printing that includes printing in the critical zone is used for isolated (e.g., text) scan lines, where distortion from speed non-uniformity is less noticeable, thereby improving printing speed. Unidirectional printing that does not include printing in the critical zone is used for scan lines of continuous images, thereby alleviating image distortion from speed non-uniformity where such distortion is most noticeable. 
     With respect to color images  302 , each scan line recorded by a color print head such as print head  62  in FIG. 7 is 23 pixels high, as opposed to 127 pixels for a black print head or 63 pixels for black nozzles of a color print head. As a result, more scans of print heads  100   a  and  100   b  are required to print a given sized color image with a color print head as compared to printing isolated or continuous images. Unidirectional printing might unacceptably slow such a printing operation, unless extremely high quality output is desired. Accordingly, bi-directional printing  309  is used to print color images  302 . Because bi-directional printing is used, left scan margin  307  is inserted before forward (left-to-right) scans of print heads  100   a  and  100   b,  and right scan margin  308  is inserted before reverse (right-to-left) scans of print heads  100   a  and  100   b.    
     As noted above, the foregoing combinations of scan margins and scan directions illustrated in FIG. 27A are applicable to standard mode printing on plain-paper recording medium  303 . Other printing directions can result from different combinations of print mode, recording media type, print head configuration, and error diffusion mode. These different combinations and the resulting printing directions are explained in more detail below with reference to FIGS. 27C to  27 G. If continuous or color images are recorded using these different combinations, scan margin  307  preferably is inserted before each scan line for unidirectional printing, and scan margins  307  and  310  preferably are inserted before scan lines for bi-directional printing (margin  307  is inserted before forward scan lines, and margin  310  is inserted before reverse scan lines). 
     FIG. 27B is a representative view for describing carriage direction control for scan lines which include both non-color continuous and color image portions. As shown in FIG. 27B, non-color continuous portions preferably are printed unidirectionally, and color portions preferably are printed bi-directionally. By printing the continuous portions unidirectionally, noticeable image distortion caused by carriage motor speed non-uniformities is reduced. 
     If a print head such as print head  62  shown in FIG. 7 is used, then 23 color nozzles are used for each pass of the print head for color portions, while 46 black nozzles are used for each pass of the print head for non-color portions. As a result, less passes are needed for the non-color portions, thereby compensating for the loss in speed that results from printing the non-color portions unidirectionally. 
     FIGS. 27C to  27 G provide a series of print mode tables containing printing schemes for printing an image using different combinations of print mode, recording media type, print head configuration, and error diffusion mode. More specifically, FIG. 27C shows a Print Mode With High Speed Error Diffusion table, which contains multiple printing schemes for use by printer  10  when printing an image with print heads  100   a  and  100   b.  FIG. 27C contains six Factors for each particular printing scheme provided; they are: (1) Raster Resolution of the image to be printed; (2) Print Resolution of the image to be printed; (3) number of Passes and Direction for print heads  100   a  and  100   b  to scan over each scan line; (4) automatic sheet feed (ASFU) speed; (5) line feed (“LF”) speed; and (6) cartridge receptacle (“CR”) speed. 
     These six Factors vary from one printing scheme to the next depending on the print mode for image resolution, which can be Draft mode, Standard mode or High mode. The six Factors also very depending on the mode of image quality which can be Regular mode or a Photo quality mode, and depending on the type of recording medium being used which can be Plain, Special  1  or Special  2 . The Speed Identifications table shown in FIG. 27D defines the speed in pulses per second for each particular mode of automatic sheet feed (“ASF”) speed, line feed (“LF”) speed, and cartridge receptacle (“CR”) speed. The Recording Media Types table shown in FIG. 27E provides the types of recording media that fall into the categories of Plain, Special  1  and Special  2 . For example, the Plain category includes plain paper, bubble jet paper, brochure paper, and greeting cards. The Special  1  category includes high resolution paper (“HR-101”), and Special  2  category includes all other recording media types. 
     Returning to the Print Mode With High Speed Error Diffusion table shown in FIG. 27C, the various combinations of print modes and recording media types result in eighteen separate printing schemes for printing with alignment. For example, when a print job with alignment is requested that requires use of the Standard print mode and the Regular image quality mode and the use of high resolution paper in the Special  1  category, a printing scheme is defined for the six Factors as follows: (1) Raster Resolution is 360 by 360 dpi; (2) Print Resolution is 720 by 720 dpi; (3) two Passes are required for print heads  100   a  and  100   b  to scan over each scan line and scanning is to take place in both directions; (4) ASF speed is set to normal; (5) LF speed is set to normal; and (6) cartridge receptacle CR speed is set to slow. Some of the printing schemes in FIG. 27C are not applicable by definition, such as an attempt to print a Photo quality image in Draft mode, or use of Special recording media in Draft mode. 
     Certain printing schemes require the use of a sub-printing scheme (“1pass_U/B*1”) shown in FIG. 27F, in which only one scan pass is utilized for printing each scan and in which the scan direction and nozzle pattern to be utilized is determined by the type of print heads  100   a  and  100   b  installed in printer  10  and by the type of image to be printed on the current scan line. As discussed earlier, the type of print heads  100   a  and  100   b  installed in printer  10  can include any two print heads of from a selection of color ink print heads (“BC-21e”) and/or black ink print heads (“BC-23”). The type of image to be printed on a scan line can be either Isolated Black which is used during printing of lines of text, Continuous Black which is used during a continuous section of black image such as a graphic, and In Color which is used during color printing. 
     For example, the printing scheme for a print request in Standard resolution mode and Regular image quality mode using Plain paper refers to the 1pass_U/B*1 sub-printing scheme. If printer  10  contains one color ink print head and one black print head for print heads  100   a  and  100   b,  and if the image to be printed on the current scan line is a continuous black graphic, then only one scan is required by print heads  100   a  and  100   b  to print the scan line. In addition, the color nozzles of the color ink print head are not utilized at all, 63 nozzles of black ink from the color ink print head are utilized for printing in only one direction, and 127 nozzles of the black ink print head are utilized for printing in only the forward direction (unidirectional scanning can occur in the forward or backward direction). Thus, the number of scan passes, printing direction, and nozzle selection is selected as part of the printing scheme in order to provide reliable printing of a quality image based upon the types of print heads  100   a  and  100   b  installed in printer  10 , the type of image being printed on the current scan line, and upon the print modes and recording media type requested for the current print job. 
     The present invention is particularly reflected in the table shown in FIG.  27 F. Specifically, Isolated Black is printed bi-directionally, Continuous Black is printed unidirectionally (with a BC-21e and BC-23 print head combination), and Color is printed bi-directionally. 
     Print Mode With Normal Error Diffusion table is shown in FIG.  27 G. This table also provides six Factors for each particular printing scheme; they are: (1) Raster Resolution of the image to be printed; (2) Print Resolution of the image to be printed; (3) number of Passes and Direction for one of print heads  100   a  and  100   b  to scan over each scan line; (4) automatic sheet feed (“ASF”) speed; (5) line feed (“LF”) speed; and (6) cartridge receptacle (“CR”) speed. 
     These six Factors vary from one printing scheme to the next depending on the print mode for image resolution, which can be Draft mode, Standard mode or High mode. The six Factors also very depending on the mode of image quality which can be Regular mode or a Photo quality mode, and depending on the type of recording medium being used which can be Plain, Special  1  or Special  2 . The printing schemes for printing without alignment are therefore determined by the various combinations of print modes and recording media types. 
     FIG. 28 is a representative view for explaining movement of print heads according to the invention for a print operation. Shown in FIG. 28 are carriage positions and scan margins for three scan lines  311 ,  312  and  313 . For explanation purposes hereinbelow, scan line  311  is defined as a previous scan line, scan line  312  is defined as a current scan line, and scan line  313  is defined as a next scan line. 
     Shown for previous scan line  311  are LeftPos[A] (A_L 1 )  314  and RightPos[A] (A_R 1 )  315  for print area  316  of print head  100   a,  LeftPos[B] (B_L 1 )  317  and RightPos[B] (B_R 1 )  318  for print area  319  of print head  100   b,  RangeLeft  320  and RangeRight  321  for the combined print area, RampUp  322 , and RampDown  323 . Shown for current scan line  312  are LeftPos[A] (A_L 2 )  324  and RightPos[A] (A_R 2 )  325  for print area  326  of print head  100   a,  LeftPos[B] (B_L 2 )  327  and RightPos[B] (B_R 2 )  328  for print area  329  of print head  100   b,  RangeLeft  330  and RangeRight  331  for the combined print area, RampUp  332 , and RampDown  333 . Shown for next scan line  313  are LeftPos[A] (A_L 3 )  334  and RightPos[A] (A_R 3 )  335  for print area  336  of print head  100   a,  LeftPos[B] (B_L 3 )  337  and RightPos[B] (B_R 3 )  338  for print area  339  of print head  100   b,  RangeLeft  340  and RangeRight  341  for the combined print area, and RampUp  342 . 
     The position values shown in FIG. 28 run from left to right. Thus, a lower-valued position is to the left of a higher-valued position. 
     The ramp ups and ramp downs are distances travelled by print heads  100   a  and  100   b  while carriage motor  39  accelerates to or decelerates from scanning speed. These distances preferably are represented by a constant value such as 25 steps of carriage motor  39 , or 16 millimeters. 
     The print operation illustrated in FIG. 28 is representative of bi-directional printing with scan margins. In more detail, after print heads  100   a  and  100   b  complete printing previous scan line  311 , the print heads are at RangeRight  321 . The print heads are then moved from RangeRight  321  to a right of RangeRight  331  for current scan line  312  by a distance equal to scan margin  310  plus RampUp  332 , so as to be ready to begin printing current scan line  312 . After printing current scan line  312 , the print heads are at RangeLeft  330 . The print heads are then moved from RangeLeft  330  to a left of RangeLeft  340  for next scan line  313  by a distance equal to scan margin  307  plus RampUp  342 , so as to be ready to begin printing next scan line  313 . 
     If printing in FIG. 28 were unidirectional, print heads  100   a  and  100   b  would move from RangeRight  321  at the end of printing previous scan line  311  to the left of RangeLeft  330  for current scan line  312  by scan margin  307  plus RampUp  332  (which would be on the left of the Figure). 
     The operation of printer driver  84  and printer control  110  (i.e., printer firmware) in moving print heads  100   a  and  100   b  is described next. 
     5.1.1 Printer Driver Initiated Operation 
     FIG. 29 is a flowchart for describing a SKIP command issued by a printer driver according to the invention. This function is called from step S 2008  in FIG.  20  and is used to feed a recording medium so as to advance a vertical print position by a number of raster lines specified by a Skip parameter. A SKIP command with an argument of zero is used to instruct printer  10  to perform a nozzle-number-change prefire operation, as described below in Section 8.0. In order to advance from one isolated scan line to another isolated scan line, for example across whitespaces  305  in FIG. 27A, the Skip argument corresponds to a distance greater than a height of print heads  100   a  or  100   b.    
     In more detail, step S 2901  determines if the Skip argument indicates a feed of zero lines. If the Skip argument is zero, flow proceeds to step S 2902 , where a nozzle-number-change-prefire request is sent to printer control  110 , as described in more detail below in Section 9.0. Otherwise, any pending nozzle-number-prefire request is resent in step S 2903 , and the recording medium is feed by Skip raster lines in step S 2904 . 
     After an appropriate SKIP operation, a PRINT command is issued by printer driver  84  (See FIG.  20 ). FIG. 30 is a flowchart for describing the PRINT command according to the invention. 
     In step S 3001 , RangeLeft  330  for current scan line  312  is determined from LeftPos [A]  324  and LeftPos[B]  327 , by setting RangeLeft  330  equal to the lesser of LeftPos[A]  324  and LeftPos[B]  327 . Likewise, in step S 3002 , RangeRight  331  for current scan line  312  is determined from RightPos[A]  325  and RightPos[B]  328 , by setting RangeRight  331  equal to the greater of RightPos[A]  325  and RightPos[B]  328 . 
     In step S 3003 , RangeLeft  340  for next scan line  313  is determined from LeftPos[A]  334  and LeftPos[B]  337 , by setting RangeLeft  340  equal to the lesser of LeftPos[A]  334  and LeftPos[B]  337 . Likewise, in step S 3004 , RangeRight  341  for next scan line  313  is determined from RightPos[A]  335  and RightPos[B]  338 , by setting RangeRight  341  equal to the greater of RightPos[A]  335  and RightPos[B]  338 . 
     The LeftPos and RightPos values used in steps S 3001  through S 3004  are defined through the EDGE command, explained in more detail below with reference to FIG.  32 . 
     In step S 3005 , print information such as print direction, speed, scan margin, automatic trigger delay, and the like are stored for performance of a print operation. Setting of print direction is described below with reference to FIG. 31, of scan margin with reference to FIGS. 33 and 34, and of automatic trigger delay with reference to FIG.  35 . 
     In step S 3006 , printer driver  84  instructs printer control  110  to initiate the carriage task, which is shown in more detail in FIGS. 36 to  38 . The carriage task is responsible for positioning and scanning the print heads across a recording medium, during which time ink is ejected from the print heads. After the carriage task is initiated, step S 3007  provides a two millisecond wait to allow printer control  110  to perform any necessary processing and communication with printer driver  84 . Then, flow returns to FIG.  20 . 
     FIG. 31 is a flowchart for describing a DIRECTION command issued by a printer driver according to the invention. In steps S 3101 , it is determined if the DIRECTION command is being called for current scan  312 , in which case direction information for the current scan is set as Direction. Otherwise, in step S 3103 , it is determined if the DIRECTION command is being called for next scan  313 , in which case direction information for the next scan is set as NextDirection. 
     Direction and NextDirection can store values for forward and reverse scanning. In order to determine values for Direction and NextDirection, printer driver  84  first determines if unidirectional or bi-directional printing is being performed. Unidirectional or bi-directional printing is determined based on print mode, recording media type, image type, print head configuration, and alignment status for the print heads, as discussed in section 6.0. Briefly, for standard-quality print mode with plain paper, unidirectional printing is used for continuous image types, and bi-directional printing is used for text and color image types, as explained above with reference to FIG.  27 A. 
     If unidirectional printing is under way, Direction and NextDirection are set to forward printing. If bi-directional printing is under way, Direction and NextDirection are set opposite to their values for previous scan  311 . 
     FIG. 32 is a flowchart for describing an EDGE command issued by a printer driver according to the invention. The Edge command specifies the left edge and the right edge of print position in units of column position, for both the current and the next scan line. Printer driver  84  preferably calculates these values based on input print data. 
     In step S 3201 , it is determined if EDGE is being called for print head  100   a  (print head A) or print head  100   b  (print head B). If EDGE is called for print head A, flow proceeds to step S 3202 , where it is determined if EDGE is being called for current scan line  312 , in which case step S 3203  sets LeftPos[A]  324  and RightPos[A]  325  for current scan line  312 . Otherwise, it is determined in step S 3204  that EDGE is being called for next scan line  313 . In that case, step S 3205  sets LeftPos[A]  334  and RightPos[A]  335  for next scan line  313 . 
     If EDGE is called for print head B, similar processing in steps S 3206  through S 3209  sets LeftPos[B]  327  and RightPos[B]  328  for current scan line  312  and sets LeftPos[B]  337  and RightPos[B]  338  for next scan line  313 . The LeftPos and RightPos values are used by printer control  110  to control movement of print heads  100   a  and  100   b,  as described in more detail below in Section 5.1.2. 
     FIG. 33 is a flowchart for describing determination of a scan margin by a printer driver according to the invention. In step S 3301 , a print mode is checked. In step S 3302 , it is determined that a scan margin is needed if the print mode is continuous or color. If Direction from FIG. 31 for current scan  312  is forward, step S 3303  directs flow to step S 3304 , where scan margin  307  for a forward scan is set. If Direction is reverse, step S 3303  directs flow to step S 3305 , where scan margin  310  for a reverse scan is set. 
     If it is determined in step S 3302  that no scan margin is needed, which occurs for isolated scan line printing, flow proceeds from step S 3302  to step S 3306 . If Direction from FIG. 31 for current scan  312  is forward, step S 3306  directs flow to step S 3307 , where a scan margin of zero (no-margin) is set for the forward scan. If Direction is reverse, step S 3306  directs flow to step S 3308 , where a scan margin of zero (no-margin) is set for the reverse scan. 
     FIG. 34 is a flowchart for describing a NEXT_MARGIN command issued by a printer driver according to the invention. The NEXT_MARGIN command stores a value for a next scan margin in an appropriate one of ScanMarginLeft or ScanMarginRight. ScanMarginLeft is used if the next scan margin inserted into a scan line is a left scan margin for a forward scan, and ScanMarginRight is used if the next scan margin is a right scan margin for a reverse scan. Step S 3401  determines if next scan line  313  is forward or reverse, and steps S 3402  and S 3403  store a margin value in ScanMarginLeft or ScanMarginRight, accordingly. 
     FIG. 35 is a flowchart for describing an AT_DELAY (automatic delay) command issued by a printer driver according to the invention. The automatic delay is used to alleviate satelliting that can occur when printing in a reverse direction, as explained below with respect to FIGS. 39 a,    39   b,  and  40  to  42 . This command sets the auto-trigger delay by specifying the scan direction as either forward or backward, and by specifying an auto-trigger delay time in units of 10 μsec up to a maximum auto-trigger delay time of 2,550 μsec. Step S 3501  determines if a next scan margin is for a forward or a reverse scan, and the value for the automatic delay is stored in AutoTriggerDelayLeft or AutoTriggerDelayRight, respectively, in steps S 3502  and S 3503 . 
     5.1.2 Print Control Operation 
     FIG. 36 is a flowchart for describing carriage task  244  performed by a printer control according to the invention. Communication between carriage task  244  and other tasks in printer  10  is explained above with reference to FIG.  18 . In printer  10 , carriage task  244  controls scanning of print heads  100   a  and  100   b  across carriage  41  as printing occurs in printer  10 . 
     In step S 3601 , carriage task  244  determines if printer driver  84  has sent a move or a print command to printer  10 . If no move or print command has been sent, flow returns to engine control task  241  in FIG.  18 . If a move command is received, carriage task  244  in step S 3602  executes a move process according to the arguments of the move command, and control again returns to engine control task  241  in FIG.  18 . If a print command is received, flow proceeds to step S 3604  for a print process, which starts with step S 3605 . 
     In step S 3605 , carriage task  244  waits until movement of print heads  100   a  and  100   b  ceases at then end of a scan line. Flow then proceeds to step S 3607  for scan prefire processing, as explained in detail below in Section 9.0 
     After scan prefire processing, flow proceeds to step S 3608 , where a scan direction for current scan  312  is determined by examining Direction set by print driver  84  through the DIRECTION command shown in FIG.  31 . If the scan direction is forward, flow proceeds to step S 3609 ; if the scan direction is reverse, flow proceeds to step S 3612 . 
     If the scan direction is forward, CrStartPosL is calculated in step S 3609  from RangeLeft  330  for current scan  312 . CrStartPosL is a start position for the print heads for a next forward scan across a recording medium. In step S 3610 , carriage task  244  determines if the current position of the print heads, CrPosition, is less than or equal to CrStartPosL minus RampUp, a ramp up distance for carriage motor  39 . If CrPosition is not less than or equal to CrStartPosL minus RampUp, then the print heads are to the right of CrStartPosL minus RampUp. Accordingly, carriage task  244  in step S 3611  moves the print heads left to CrStartPosL minus RampUp. Furthermore, because the print heads are moving to a start of the scan line, flow returns to step S 3607  so as to perform any needed prefire processing before the scan line is started. Steps S 3607  through S 3611  are repeated until CrPosition is less than or equal to CrStartPosL minus RampUp, at which point the print heads are at the start of the forward scan line. Flow then proceeds to step S 3615 . 
     If the print direction is reverse, CrStartPosR is calculated in step S 3612  from RangeRight  331  for current scan  312 . CrStartPosR is a start position for the print heads for a next reverse scan across a recording medium. In step S 3613 , carriage task  244  determines if the current position of the print heads, CrPosition, is greater than or equal to CrStartPosR plus RampUp, a ramp up distance for carriage motor  39 . If CrPosition is not greater than or equal to CrStartPosR plus RampUp, then the print heads are to the left of CrStartPosR plus RampUp. Accordingly, carriage task  244  in step S 3614  moves the print heads right to CrStartPosR plus RampUp. Furthermore, because the print heads are moving to a start of a scan line, flow returns to step S 3607  so as to perform any needed prefire processing before the scan line is started. Steps S 3607 , S 3608  and S 3612  through S 3614  are repeated until CrPosition is greater than or equal to CrStartPosR plus RampUp, at which point the print heads are at the start of the reverse scan line. Flow then proceeds to step S 3615 . 
     In step S 3615 , print information is retrieved. This print information was stored by printer control  110  in response to a PRINT command from printer driver  84 , as shown in FIG.  30 . Relevant parts of the print information, such as automatic trigger delay, droplet size, heat pulse control and buffer control, are sent in step S 3616  to other tasks running on printer control  110 , such as heat control handler  254 . 
     In step S 3617 , carriage control parameters are prepared. This control parameters are used to control carriage motor driver  39   a,  which in turn controls carriage motor  39 . Examples of the control parameters include control method (half/full/quarter), RampUp Table, RampDown Table, RampUpSteps, ConstantSteps, RampDownSteps, CrHeatStartPosition, CrHeatEndCount, CrScanEndPosition, CrStopPosition, etc. 
     The carriage motor is started in step S 3618 , and an automatic triggering mechanism controlled by printer control  110  causes print heads  100   a  and  100   b  to eject ink as the print heads are scanned across a recording medium by carriage motor  29 . This triggering mechanism is explained in more detail below with respect to FIGS. 40 through 42. 
     After the carriage motor is started, step S 3619  determines if bi-directional or unidirectional printing is being used. For standard mode printing, the type of printing is determined based on print mode (e.g., isolated, continuous, or color). As discussed above with respect to FIGS. 27C to  27 G, the type of printing also can depend on recording media type, print head configuration, error diffusion mode, and the like. If bi-directional printing is being used, flow proceeds to step S 3620  for carriage scan control  1  illustrated in FIG.  37 . If unidirectional printing is being used, flow proceeds to step S 3621  for carriage scan control  2  illustrated in FIG.  38 . 
     FIG. 37 is a flowchart for describing a first carriage scan control called by carriage task  244  of FIG. 36 for bi-directional printing. 
     Carriage task  244  in step S 3701  determines if Direction for the current scan is forward (left) and NextDirection for the next scan is reverse (right), in which case steps S 3702  through S 3707  are performed. Otherwise, carriage task  244  in step S 3708  determines if Direction for the current scan is reverse (right) and NextDirection for the next scan is forward (left), in which case steps S 3709  through S 3714  are performed. 
     For a forward current scan line, CrStartPosR for next scan  313  is calculated in step S 3702  from RangeRight  341  for next scan  313 . Then, in step S 3703 , TempNewPos is calculated from CrStartPosR plus ScanMarginRight plus RampUp. ScanMarginRight preferably is part of the information calculated by printer control  110  in response to a NEXT_MARGIN command (see FIG.  34 ). If a margin is to be inserted before the reverse next scan, ScanMarginRight contains the size of the margin. If a margin is not to be inserted, ScanMarginRight contains no-margin (zero). 
     Carriage task  244  in step S 3704  determines if TempNewPos is less than MaxPos, the right-most position possible for print heads  100   a  and  100   b.  If TempNewPos is not less than MaxPos, then TempNewPos is an invalid position to the right of MaxPos. Accordingly, TempNewPos is set equal to MaxPos in step S 3705 . After steps S 3704  and S 3705 , TempNewPos is equal to the start of the next (reverse) scan line, accounting for scan margin and motor ramp up. 
     It is determined in step S 3706  if CrScanEndPos is less than TempNewPos. CrScanEndPos is the position for print heads  100   a  and  100   b  after printing the current (forward) scan line. Thus, if CrScanEndPos is less than TempNewPos, the current forward scan line ends before the next reverse scan line begins. In that case, step S 3707  updates CrScanEndPos with TempNewPos, thereby extending the current scan line to the start of the next scan line. 
     For a reverse current scan line, CrStartPosL for next scan  313  is calculated in step S 3709  from RangeLeft  340  for next scan  313 . Then, in step S 3710 , TempNewPos is calculated from CrStartPosL minus ScanMarginLeft minus RampUp. ScanMarginLeft preferably is part of the information calculated by printer control  110  in response to a NEXT_MARGIN command (see FIG.  34 ). If a margin is to be inserted before the forward next scan, ScanMarginLeft contains the size of the margin. If a margin is not to be inserted, ScanMarginLeft contains no-margin (zero). 
     Carriage task  244  in step S 3711  determines if TempNewPos is greater than MinPos, the left-most position possible for print heads  100   a  and  100   b.  If TempNewPos is not greater than MinPos, then TempNewPos an invalid position to the left of MinPos. Accordingly, TempNewPos is set equal to MinPos in step S 3712 . After steps S 3711  and S 3712 , TempNewPos is equal to the start of the next (forward) scan line, accounting for scan margin and motor ramp up. 
     It is determined in step S 3713  if CrScanEndPos is greater than TempNewPos. CrScanEndPos is the position for print heads  100   a  and  100   b  after printing the current (reverse) scan line. Thus, if CrScanEndPos is greater than TempNewPos, the current reverse scan line ends before the next forward scan line begins. In that case, step S 3714  updates CrScanEndPos with TempNewPos, thereby extending the current scan line to the start of the next scan line. 
     FIG. 38 is a flowchart for describing a second carriage scan control called by the carriage task of FIG. 36 for unidirectional printing. 
     Carriage task  244  in step S 3801  determines if Direction for the current scan is forward (left) and NextDirection for the next scan is forward (left), in which case steps S 3802  through S 3807  are performed. Otherwise, carriage task  244  in step S 3808  determines if Direction for the current scan is reverse (right) and NextDirection for the next scan is reverse (right), in which case steps S 3809  through S 3814  are performed. 
     For forward scanning, CrStartPosL for next scan  313  is calculated in step S 3802  from RangeLeft  340  for next scan  313 . Then, in step S 3803 , TempNewPos is calculated from CrStartPosL minus ScanMarginLeft minus RampUp. ScanMarginLeft preferably is calculated by printer control  110  in response to a NEXT_MARGIN command (see FIG.  34 ). If a margin is to be inserted before the next scan, ScanMarginLeft contains the size of the margin. If a margin is not to be inserted, ScanMarginLeft contains no-margin (zero). 
     Carriage task  244  in step S 3804  determines if TempNewPos is greater then MinPos, the left-most position possible for print heads  100   a  and  100   b.  If TempNewPos is not greater than MinPos, then TempNewPos is an invalid position to the left of MinPos. Accordingly, TempNewPos is set equal to MinPos in step S 3805 . After steps S 3804  and S 3805 , TempNewPos is equal to the start of the next (forward) scan line, accounting for scan margin and motor ramp up. 
     In step S 3806 , the carriage control waits until the current scan line is finished. Then, in step S 3807 , the carriage control moves print heads  100   a  and  100   b  to TempNewPos for the start of a next forward scan line. Control is then returned to FIG.  36 . 
     For reverse scanning, CrStartPosR for next scan  313  is calculated in step S 3809  from RangeRight  341  for next scan  313 . Then, in step S 3810 , TempNewPos is calculated from CrStartPosR plus ScanMarginRight plus RampUp. ScanMarginRight preferably is calculated by printer control  110  in response to a NEXT_MARGIN command (see FIG.  34 ). If a margin is to be inserted before the forward next scan, ScanMarginRight contains the size of the margin. If a margin is not to be inserted, ScanMarginRight contains no-margin (zero). 
     Carriage task  244  in step S 3811  determines if TempNewPos is less than MaxPos, the right-most position possible for print heads  100   a  and  100   b.  If TempNewPos is not less than MaxPos, then TempNewPos an invalid position to the right of MaxPos. Accordingly, TempNewPos is set equal to MaxPos in step S 3812 . After steps S 3811  and S 3812 , TempNewPos is equal to the start of the next (reverse) scan line, accounting for scan margin and motor ramp up. 
     In step S 3813 , the carriage control waits until the current scan line is finished. Then, in step S 3814 , the carriage control moves print heads  100   a  and  100   b  to TempNewPos for the start of a next reverse scan line. Control is then returned to FIG.  36 . 
     5.2 Automatic Ink Election and Satelliting Control 
     FIGS. 39 a  and  39   b  are representative views for describing satelliting control according to the invention. FIG. 39 a  illustrates image degradation that can occur due to satelliting, particularly with high-speed scanning of print heads across a recording medium as ink is ejected from those print heads. When a main droplet of ink is ejected from an ink jet print head so as to record a pixel, a small satellite droplet often is also ejected. Ink jet print heads typically are angled slightly with respect to a recording medium so that the satellite droplet overlaps the main droplet when the print head is scanned across a recording medium in a forward direction. However, in the reverse direction, this angling tends to cause the satellite droplet to land near an edge of or even outside of the main droplet, resulting in a small satellite being recorded next to each recorded pixel during a reverse scan. 
     Accordingly, FIG. 39 a  shows pixels  351  printed by ejecting ink during forward scans and pixels  352  printed by ejecting ink during reverse scans. Pixels  352  are accompanied by satellites  353 , forming jagged side  355  for the column of pixels. Jagged left side  355  can noticeably degrade image quality, particularly in a case of continuous images (i.e., non-color graphics). 
     FIG. 39 b  shows pixels printed according to the invention so as to reduce image degradation due to satelliting. 
     Briefly, image degradation due to satelliting can be addressed for forward and reverse printing on a recording medium by reciprocal forward and reverse scans of a print head in accordance with print data. According to this invention, print data is printed in one direction of the reciprocal forward and reverse scans of the print head, and print data is printed in another direction of the reciprocal forward and reverse scans so that the printed data in the other direction is laterally shifted a predetermined distance as compared to printing where each pixel printed in the other direction vertically matches each pixel printed in the one direction. Preferably, the predetermined distance is a distance corresponding one fourth of a printed pixel. This lateral shift tends to mask satelliting effects, particularly in the case of printing continuous image data. 
     In FIG. 39 b,  pixels  362  printed during reverse scans have been offset by AT_DELAY  360 , shown as a one fourth pixel delay, from pixels  361  printed during forward scans. As a result, any unevenness in the printed column of pixels is split between left side  365  and right side  366 . The offset tends to mask the satellites, rendering them far less noticeable. 
     As mentioned above, satelliting is more noticeable in continuous image data. Accordingly, in the preferred embodiment of the invention, the foregoing pixel shifts are applied only to reverse scans for recording continuous images. The pixel shifts preferably are not applied to isolated (e.g., text) or color images. 
     FIGS. 40 through 42 explain automatic ink ejection while a print head is scanned across a recording medium, wherein the automatic ink ejection adds a delay to pixels printed in a reverse direction. Briefly, an AT_DELAY command from printer driver  84  sets an automatic trigger delay corresponding to one fourth of a pixel for reverse scan lines, and an automatic trigger delay of zero for forward scan lines. 
     FIG. 40 is a flowchart for describing carriage motor start performed by printer control according to the invention. CR MOTOR START is received from step S 3618  of the carriage task operation illustrated in FIG.  36 . In response, a hardware timer for the carriage motor interrupts is initiated in step S 4001 . This hardware timer is used to perform carriage motor control, as explained with reference to FIGS. 41 and 42 below. Carriage motor driver  39   a  is initiated in step S 4002 , and a look-up table is updated in step S 4003 . The look up table is used during carriage motor control, such as to define times and to set phase current mode for driving the carriage motor. Control then returns to FIG.  36 . 
     FIG. 41 a flowchart for describing a carriage interrupt process performed by a printer control according to the invention. This process is initiated by step S 4001  in FIG.  40 . In step S 4101 , an interrupt occurs, activating carriage interrupt process S 4102 . 
     The interrupt process of FIG. 41 determines in step S 4103  if motor  39  is ramping up. If motor  39  is ramping up, the motor is driven so as to reach its target speed in step S 4104 . CrPosition, the current position of print heads  100   a  and  100   b,  is updated in step S 4105 , and a counter and look up table for carriage motor control are updated in step S 4106 . 
     If motor  39  is not ramping up, step S 4107  determines if the motor is operating in a constant-speed (i.e., printing) region. If the motor is operating in a constant-speed region, steps S 4108  and S 4109  drive the motor and update CrPosition. Step S 4110  then initiates automatic trigger control, as explained in more detail below with respect to FIG. 42, so as to eject ink from the print heads as the motor scans the print heads across a recording medium. Then, a counter and look up table for carriage motor control are updated in step S 4111 . 
     In step S 4112 , it is determined if motor  39  is ramping down, in which case flow proceeds to step S 4113 . The motor is driven is step S 4113 , CrPosition for the motor is updated in step S 4114 , and a counter and look up table for carriage motor control are updated in step S 4115 . 
     If motor  39  is not ramping down at step S 4112 , then motor  39  has stopped. Accordingly, motor control is stopped in S 4116 , and the hardware timer for motor interrupts is stopped. 
     FIG. 42 is a flowchart for describing automatic triggering of nozzles of print heads, including use of automatic trigger delay by printer control so as to mask satelliting according to the invention. The automatic triggering preferably is performed by printer control  110 , and the automatic trigger delay preferably is supplied to printer control  110  from printer driver  84  through the AT_DELAY command described above with reference to FIG.  35 . According to the invention, printer driver  84  sets the automatic trigger delay for forward scans to zero, and printer driver  84  sets the automatic trigger delay for reverse scans to a time for print heads  100   a  and  100   b  to traverse one fourth of a pixel. 
     Turning to FIG. 42, in step S 4201 , printer control  110  determines if heating for nozzles of a print head is on. If heating is on, printer control  110  automatically drives print head nozzles to eject ink while the print heads are scanned across a recording medium. Flow proceeds to step S 4202 , where it is determined if CrHeatEndCount[A] equals zero. If CrHeatEndCount[A] is not equal to zero, it is decremented in step S 4203 . Likewise, it is determined if CrHeadEndCount[B] equals zero in step S 4204 , and if CrHeatEndCount[B] is not equal to zero, it is decremented in step S 4205 . 
     In step S 4206 , it is determined if both CrHeatEndCount[A] and CrHeatEndCount[B] are equal to zero, in which case heat control registers in printer control  110  are reset and heating is turned off. When heating is off, ink is not ejected from the print heads. 
     Returning to step S 4201 , if heating is off when automatic trigger control is called from the carriage interrupt process, flow proceeds to step S 4209 . In step S 4209 , printer control  110  determines if the current scan line direction is forward (left), based on a DIRECTION command from printer driver  84 . If the direction is forward, steps S 4210  and S 4211  determine if CrPosition, the current print head position, is greater than or equal to CrHeatStartPos for print head A or B, in which case flow proceeds to step S 4212  through S 4214 . 
     In step S 4212 , a software loop introduces an automatic trigger delay into the automatic trigger control. The duration of the delay is set by printer driver  84  through the AT_DELAY command. However, because S 4212  is reached only if the current scan direction is forward, the delay set by AT_DELAY according to the invention preferably is zero (no-margin). Therefore, flow proceeds immediately to steps S 4213  and S 4214 , were AutoTrigger and heating are turned on so as to allow for automatic ejection of ink for print heads scanned across a recording medium. 
     Returning to step S 4209 , if the current scan direction is not forward, flow proceeds to step S 4215 , where it is determined if the current scan line direction is reverse (right). If the direction is reverse, steps S 4216  and S 4217  determine if CrPosition, the current print head position, is less than or equal to CrHeatStartPos for print head A or B, in which case flow proceeds to step S 4218  through S 4219 . 
     In step S 4218 , a software loop introduces an automatic trigger delay into the automatic trigger control. The duration of the delay is set by printer driver  84  through the AT_DELAY command. In order to offset pixels printed in the reverse direction, printer driver  84  preferably sets the delay equal to a time require for print heads  100   a  and  100   b  to traverse one fourth of a pixel. After the delay, flow proceeds immediately to steps S 4219  and S 4220 , where AutoTrigger and heating are turned on so as to allow for automatic ejection of ink for print heads scanned across a recording medium. 
     By virtue of the foregoing, a shift is introduced into pixels printed in the reverse direction, thereby tending to mask satellites that accompany those pixels. 
     6.0 Printer Control Based On Head Alignment 
     In brief, this section is a description of the present invention whereby a printing system is provided for a multiple print head printer in which it is determined whether the print heads are effectively aligned and in which one of multiple different printing schemes for controlling the printing of print data is then selected based upon the aforementioned alignment determination. Specifically, the present invention relates to a printer driver  84  that notifies the user if print heads  100   a  and  100   b  need to be aligned once a print job is requested by the user. If the user chooses to continue the print request without performing the alignment process, printer driver  84  directs printer  10  to print the requested image by using only one of print heads  100   a  and  100   b,  thereby reducing the adverse effects caused by misalignment of print heads  100   a  and  100   b.    
     In a related aspect, when the user has chosen not to perform the alignment process after being prompted to do so by printer driver  84 , printer driver  84  also directs printer  10  to print the requested image by laterally scanning print heads  100   a  and  100   b  in one direction only. In this manner, the image quality is improved when printing in a no-alignment mode because unidirectional, rather than bi-directional, scanning of print heads  100   a  and  100   b  results in a higher quality printed image when print heads  100   a  and  100   b  are not aligned. 
     As described above, printer  10  includes cartridge receptacles  37   a  and  37   b  which hold ink cartridges  43   a  and  43   b  having print heads  100   a  and  100   b.  Printer  10  prints an image on a recording medium by laterally scanning print heads  100   a  and  100   b  across the recording medium while directing print heads  100   a  and  100   b  to print image data. The manner in which printer driver  84  directs print heads  100   a  and  100   b  to scan the recording medium for printing the image depends upon several factors including the type of image being printed, the desired resolution, and the type of recording medium being used. For example, printer driver  84  may command printer  10  to print an image according to a printing scheme whereby print heads  100   a  and  100   b  are scanned across the same scan line of the recording medium several times in succession in order to improve the image quality. The same printing scheme may also direct printer  10  to print the current scan line first in one direction and then in the other direction; e.g. bidirectional printing. The printing scheme may also direct a speed for carriage motor  39  to control print head speed during printing and may direct the use of a particular pattern of print head nozzles on print heads  100   a  and  100   b  to achieve the printed image desired by the user. Various printing schemes can be utilized based upon combinations of the aforementioned factors. Printer driver  84  selects a particular printing scheme to achieve the desired image quality according to the type of recording media, print modes and other print-related conditions being utilized for a given job print request. 
     An alignment process (not shown) is provided by printer driver  84  for directing printer  10  to align print heads  100   a  and  100   b  when printer driver  84  detects that print heads  100   a  and  100   b  are not known to be aligned. Print heads  100   a  and  100   b  may be misaligned either because they are not aligned with respect to each other, or because their individual positions within printer  10  are not in their proper aligned positions, respectively. If printer driver  84  determines that print heads  100   a  and  100   b  may be in a misaligned state, printer driver  84  prompts the user to initiate the alignment process when the user initiates a print job request. If the user chooses to initiate the alignment process, printer driver  84  performs the alignment process after which print heads  100   a  and  100   b  are presumed to be sufficiently aligned by printer driver  84 . If the user chooses not to perform the alignment process, printer driver selects only one of print heads  100   a  and  100   b  for printing the image, and also selects a particular printing scheme to control the selected print head during printing such that the selected print head is directed to print the image while scanning the recording medium in only one direction. As a result of the present invention, the user is allowed to proceed with a print request when print heads  100   a  and  100   b  are in a misaligned state by utilizing a predetermined printing scheme for directing printer  10  to print the requested image using only one of print heads  100   a  and  100   b,  thereby improving the quality of the printed image when print heads  100   a  and  100   b  are in a misaligned state. 
     In a preferred embodiment of the present invention, a print request is denied by the printer driver if it is determined that print heads  100   a  and  100   b  may be misaligned and if the user&#39;s print request requires the use of a particular print mode that cannot be supported by using only one print head in a no-alignment situation. 
     FIG. 43 is a flow diagram which depicts a software alignment process for execution within printer driver  84  of the present invention. The process begins in step S 4301  in which printer driver  84  receives a print request job from the user via an application software module  82 . Printer driver  84  first determines whether print heads  100   a  and  100   b  are aligned in step S 4302 . Printer driver  84  determines whether print heads  100   a  and  100   b  may be misaligned based upon the status of the printer and other conditions, such as: (1) an indication from printer  10  that the user has changed one or both of ink cartridges  43   a  and  43   b  in the printer; (2) an indication that a specified amount of time or a specified number of print jobs has elapsed since the last time the alignment process was performed, or (3) an indication from printer  10  that print heads  100   a  and  100   b  are misaligned. 
     If it is determined that print heads  100   a  and  100   b  are sufficiently aligned in step S 4302 , printer  10  is directed by print driver  84  to print the requested print job pursuant to commands and data provided to printer  10  by print driver  84  (step S 4303 ). Therefore, in the case when print heads  100   a  and  100   b  do not need further alignment, a particular printing scheme is selected by printer driver  84  to provide for reliable printing of a quality image in accordance with the print modes and print-related conditions of the current print job request (step S 4303 ). The selection of a particular printing scheme by printer driver  84  for printing with alignment is discussed in more detail below in reference to FIG.  44 . 
     If printer driver  84  determines that print heads  100   a  and  100   b  are not sufficiently aligned in step S 4302 , a determination is then made whether the user has requested the use of a photo-quality mode to print the current print job (step  4304 ). If a photo-quality mode is selected for the current print job, a dialog box is then displayed on display  4  (step S 4305 ) asking if the user would like to initiate the alignment process to align print heads  100   a  and  100   b.  If the user indicates via keyboard  5  or pointing device  6  to not perform the alignment process (step S 4307 ), the print job is cancelled (step S 4308 ) because the image cannot be printed in a photo-quality mode without using two aligned print heads  100   a  and  100   b.    
     If the user decides to perform the alignment process (step S 4307 ), control passes to step S 4312  in which printer driver  84  initiates the alignment process. After the alignment process is complete, printer driver  84  directs printer  10  to print the requested print job pursuant to the commands and data provided to printer  10  by print driver  84  in accordance with a particular printing scheme for printing with alignment (step S 4303 ). 
     If a photo-quality mode is not selected for this print job (step  4304 ), printer driver  84  next asks the user, via a dialog box on display  4 , if the user would like to see a message regarding misalignment of print heads  100   a  and  100   b  (step S 4306 ). If the user does not want to see the misalignment message, control is directed to step S 4316  in which printer driver  84  directs printer  10  to print the requested print job pursuant to the commands and data provided by printer driver  84  in accordance with a particular printing scheme for printing without alignment (step S 4316 ). The selection of a particular printing scheme by printer driver  84  for printing without alignment is depicted in more detail below in reference to FIG.  44 . 
     If the user wants to see the misalignment message, control is directed to step S 4309  in which printer driver  84  displays a dialog box on display  4  (step S 4309 ) asking if the user would like to initiate the alignment process to align print heads  100   a  and  100   b.  If the user decides to cancel the print request after reading the dialog box, (step S 4310 ), the print job is cancelled (step S 4311 ). If the user decides after reading the dialog box to initiate the alignment process, (step S 4310 ), control passes to step S 4312  in which printer driver  84  initiates the alignment process. After the alignment process is complete, printer driver  84  directs printer  10  to print the requested print job pursuant to the commands and data provided to printer  10  by print driver  84  in accordance with a particular printing scheme for printing with alignment (step S 4303 ). If the user decides after reading the dialog box not to initiate the alignment process, (step S 4310 ), the user is asked via a dialog box on display  4  whether the user would like to be notified in the future of the misalignment of print heads  100   a  and  100   b  whenever another print job is requested (step S 4313 ). If the user decides to not see the misalignment message in the future (step S 4314 ), the message is turned off and prevented from being displayed in the future until the user changes one or both of ink cartridges  43   a  and  43   b  (step S 4315 ). Control is then directed to step S 4316  to print the requested print job as dicussed in further detail below. If the user decides to continue seeing the misalignment message in the future (step S 4314 ), control is directed to step S 4316  in which printer driver  84  directs printer  10  to print the print job pursuant to commands and data provided by printer driver  84  according to a printing scheme for printing without alignment (step S 4316 ). 
     Upon starting the printing without alignment in step S 4316 , control is directed to step S 4317  in which printer driver  84  determines whether print heads  100   a  and  100   b  comprise a particular combination wherein one print head is capable of printing color ink, including black ink, and the other print head is capable of printing black ink only (step S 4317 ). In the preferred mode, if printer  10  contains a print head that is capable of printing both color ink and black ink, that print head is print head  100   a  and must be positioned in carriage receptacle  37   a  and the other print head is print head  100   b  and must be positioned in carriage receptacle  37   b  regardless of the type of the other print head. If printer  10  contains a color ink print head and a black ink print head (step  4317 ), printer driver  84  next determines whether the print job requires the image to be printed in black ink only (step S 4318 ). If the print job is to be printed using black ink only (step S 4318 ), printer driver  84  directs printer  10  to print the print job using only the black ink print head, which is print head  100   b  in the preferred embodiment (step  4319 ). If, in the alternative, the print job requires the use of color ink, (step S 4318 ), printer driver  84  directs printer  10  to print the print job using only the color ink print head, which is print head  100   a  in the preferred embodiment (step  4320 ). 
     For all other possible combinations of print heads  100   a  and  100   b  in step S 4317 , such as two black ink print heads or two color ink print heads, printer driver  84  directs printer  10  to print the print job using only the color ink print head, which is print head  100   a  in the preferred embodiment (step  4320 ). The above arrangement therefore allows the user to proceed with a print job request whenever possible, even if print heads  100   a  and  100   b  are not sufficiently aligned and the user does not wish to initiate the alignment process. Moreover, in such a situation, printer driver  84  selects only one print head to use in conjunction with a particular printing scheme so as to provide reliable printing of a quality image when print heads  100   a  and  100   b  are not sufficiently aligned. 
     FIG. 44 provides a series of print mode tables containing printing schemes for printing an image with alignment, e.g. when the alignment process has been performed, and for printing an image without alignment pursuant to the printer driver software alignment process of FIG.  43 . More specifically, Print Mode With Alignment table  385  contains multiple printing schemes for use by printer  10  when printing an image with aligned print heads  100   a  and  100   b  as referenced in step S 4303  of FIG.  43 . Table  385  generally contains two attributes for each particular printing scheme provided; they are: (1) Print Resolution; and (2) (3) the number of scan Passes and print Direction during which print heads  100   a  and  100   b  are to print the image. 
     These attributes vary from one printing scheme to the next depending on the print mode for image resolution, which can be Draft mode, Standard mode or High mode. The attributes also vary depending on the mode of image quality which can be either Regular mode or a Photo quality mode, and depending on the type of recording medium being used which can be Plain Paper, High Resolution or Glossy. Returning to Print Mode With Alignment table  385 , the various combinations of print modes and recording media types result in twelve separate printing schemes for printing with alignment. For example, when printing with alignment requires use of the Standard print mode, the Regular image quality mode and High Resolution paper, a printing scheme is defined by the attributes in table  385  as follows: (1) Print Resolution is 720 by 720 dpi; and (2) two Passes are required for print heads  100   a  and  100   b  to scan over each printed scan line and printing is to be performed in both directions (bidirectional). Some of the printing schemes in table  385  are not applicable by definition, such as an attempt to print a Photo quality image in Draft mode, or the use of Glossy recording medium in Draft mode. 
     Certain printing schemes depicted in table  385  require the use of a sub-printing scheme, “1pass_U/B*1”, as shown in table  386  of FIG.  44 . The “1pass_U/B*1” sub-printing scheme provides printing schemes in which only one scan pass is utilized for printing each scan and in which the scan direction and nozzle pattern to be utilized are determined by the type of print heads  100   a  and  100   b  that are installed in printer  10  and by the type of image to be printed on the current scan line. As discussed earlier, the type of print heads  100   a  and  100   b  installed in printer  10  can include any two print heads from a selection of color ink print heads (“BC-21e”) and black ink print heads (“BC-23”). The type of image to be printed on a scan line can be Isolated Black, which refers to successive lines of text, Continuous Black, which is a continuous section of black or grey-scale image such as a graphic, or In Color, which is color text and/or image. 
     Pursuant to the Print Mode with Alignment table  385 , it is seen that the printing scheme corresponding to a print request in Standard resolution mode and Regular image quality mode using Plain paper refers to the 1pass_U/B*1sub-printing scheme. Turning to table  386 , if the image to be printed on the current scan line is a continuous black graphic, then only one scan pass is required for print heads  100   a  and  100   b  to print the scan line. In addition, the color nozzles of the color ink print head are not utilized at all, 63 nozzles of black ink from the color ink print head are utilized for printing in only one direction (unidirectional), and 127 nozzles of the black ink print head are utilized for printing in only the forward direction (unidirectional scanning can occur in the forward or backward direction). Thus, the number of scan passes, printing direction, and nozzle selection are selected as part of the printing scheme in order to provide reliable printing of a quality image based upon the types of print heads  100   a  and  100   b  installed in printer  10 , the type of image being printed on the current scan line, and upon the print modes and recording medium type requested for the current print job. 
     Print Mode Without Alignment table  387  contains multiple printing schemes for use by printer  10  when printing an image without aligned print heads  100   a  and  100   b  as referenced in step S 4316  of FIG.  43 . Table  387  generally contains two attributes for each particular printing scheme provided; they are: (1) Print Resolution; and (2) (3) the number of scan Passes and print Direction during which print heads  100   a  and  100   b  are to print the image. 
     These attributes vary from one printing scheme to the next depending on the print mode for image resolution, which can be Draft mode, Standard mode or High mode. The attributes also vary depending on the mode of image quality which can be either Regular mode or a Photo quality mode, and depending on the type of recording medium being used which can be Plain Paper, High Resolution or Glossy. Returning to Print Mode Without Alignment table  387 , the various combinations of print modes and recording media types result in twelve separate printing schemes for printing with alignment. For example, when printing with alignment requires use of the Standard print mode, the Regular image quality mode and High Resolution paper, a printing scheme is defined by the attributes in table  387  as follows: (1) Print Resolution is 720 by 720 dpi; and (2) two Passes are required for print heads  100   a  and  100   b  to scan over each printed scan line and printing is to be performed in only one direction (unidirectional). Some of the printing schemes in table  387  are not applicable by definition, such as an attempt to print a Photo quality image in Draft mode, or the use of Glossy recording medium in Draft mode. 
     Certain printing schemes depicted in table  387  require the use of a sub-printing scheme, “1pass_U/B*2”, as shown in table  388  of FIG.  44 . The “1pass_U/B*2” sub-printing scheme provides printing schemes in which only one scan pass is utilized for printing each scan and in which the scan direction and nozzle pattern to be utilized are determined by the type of print heads  100   a  and  100   b  that are installed in printer  10  and by the type of image to be printed on the current scan line. As discussed earlier, the type of print heads  100   a  and  100   b  installed in printer  10  can include any two print heads from a selection of color ink print heads (“BC-21e”) and black ink print heads (“BC-23”). As discussed above in reference to FIG. 43, only one of print heads  100   a  and  100   b  is selected for use during printing without alignment. The type of image to be printed on a scan line can be Isolated Black, which refers to successive lines of text, Continuous Black, which is a continuous section of black or grey-scale image such as a graphic, or In Color, which is color text and/or image. 
     Pursuant to the Print Mode without Alignment table  387 , it is seen that the printing scheme corresponding to a print request in Standard resolution mode and Regular image quality mode using Plain paper refers to the 1pass_U/B*2sub-printing scheme. Turning to table  388 , if the image to be printed on the current scan line is a continuous black graphic, then only one scan pass is required for print heads  100   a  and  100   b  to print the scan line. In addition, if the color ink print head is selected for use during printing without alignment, the color nozzles of the color ink print head are not utilized at all, but 63 nozzles of black ink from the color ink print head are utilized for printing in only one direction (unidirectional). If, however, the black ink print head is selected for use during printing without alignment, then 127 nozzles of the black ink print head are utilized for printing in only the forward direction (unidirectional scanning can occur in the forward or backward direction). Thus, the number of scan passes, printing direction, and nozzle selection are selected as part of the printing scheme in order to provide reliable printing of a quality image based upon the types of print heads  100   a  and  100   b  installed in printer  10 , the type of image being printed on the current scan line, and upon the print modes and recording medium type requested for the current print job. 
     7.0 Dual Head Multicolor Printing 
     FIG. 45 is a flow diagram illustrating computer-executable process steps used to print color data onto a recording medium. As shown, these steps are preferably included in language monitor  205  and executed by CPU  70  of host processor  2 . It should be noted that these steps may also be executed by CPU  91  of printer  10 . 
     Briefly, the FIG. 45 process steps include steps to print print data other than black print data included in the bands of print data using bidirectional printing and a step to print black print data included in the bands of print data using unidirectional printing. 
     More specifically, flow begins at step S 4501 , in which a band of print data is received from driver  84 . Using the configuration illustrated in FIG. 18, the band is actually received printer provider  204 . The received print data preferably includes binarized data indicating whether or not droplets of yellow, magenta, cyan or black ink are to be placed on particular pixel locations of the recording medium. The particular pixel locations are those which can be printed upon during a single scan of receptacles  37   a  and  37   b  using ink cartridges  43   a  and  43   b.  In the foregoing example, cartridge  43   a  utilizes print head  62  of FIG. 7, and ink jet cartridge  43   b  utilizes print head  64  of FIG.  7 . In addition, ink cartridge  43   a  preferably stores yellow, magenta, cyan and black high-penetration inks, while ink cartridge  43   b  stores low penetration black ink. 
     Turning to FIG. 46, FIG. 46 illustrates a sequence of printing according to the FIG. 45 process steps. As shown, a color region exists above dashed line  390  and a black region exists below dashed line  390 . Also shown in FIG. 46 are relative positions of ink nozzles of print head  62  during several passes of print head  62  over the recording medium during printing. Nozzles illustrated in each pass are those nozzles which perform printing during the pass according to the present example. Moreover, gaps shown between nozzle groupings are to illustrate the different groupings; these gaps are not to scale. 
     Returning to the FIG. 45 flow, a band of print data corresponding to pass  1  of FIG. 46 is received in step S 4501 . In step S 4502 , it is determined whether the received band includes color data. In this regard, a band is determined to include color data if any pixel location in the band is to be, or has previously been, printed upon using either a yellow, magenta, or cyan ink droplet. Accordingly, the received band of print data is determined to include color data in step S 4502 . Flow therefore proceeds to step S 4504 , where it is determined whether the current pass is in a backward direction. 
     In the present example, this first pass will be in a forward direction, therefore flow proceeds from step S 4504  to step S 4505 . In step S 4505 , it is determined whether unprinted black data exists. Such unprinted black data will be described below with reference to FIG.  45 . In the present instance, no such unprinted data exists and flow continues to step S 4506 , wherein the received band is sent to printer  10  for printing. 
     Pass  1  of FIG. 46 shows nozzles used during printing of the received band in step S 4506 . Preferably, 23 nozzles are used to print each of the inks during a single scan of print head  62 . It should be noted that, after step S 4506  of the present example, ink cartridge  43   a  is at an end of printer  10  opposite from the end at which the first pass began. 
     Flow continues from step S 4506  to step S 4508 , wherein it is determined whether the previously-received band is a last band of print data. Since more bands of data exist in the present example, flow returns to step S 4501 . A band of print data for a second pass is received in step S 4501  and, since, as shown in FIG. 46, the band includes color data, flow proceeds from step S 4502  to step S 4504 . Since pass  1  was in a forward direction, pass  2  will be in a backward direction. Accordingly, flow continues to step S 4509 , wherein black print data of the received band is saved, preferably in print buffer  109 . The remaining data of the band is then sent to printer  10  in step S 4510 . FIG. 46 shows that, in pass  2 , only yellow, magenta and cyan droplets are printed. 
     It should be noted that, after pass  1  was completed, the recording medium was advanced a distance corresponding to 23 nozzles, and therefore pixels printed using magenta and yellow nozzles in pass  1  may be printed using cyan and magenta nozzles, respectively, in pass  2 . 
     Flow continues from step S 4508  to step S 4501 , wherein a next band of print data is received. Accordingly, flow proceeds from step S 4502  to step S 4504 , wherein, since pass  3  is in a forward direction, flow continues to step S 4505 . Since the black print data of pass  2  was saved in step S 4509  as described above, flow continues from S 4505  to step S 4512 , wherein the saved data is retrieved from print buffer  109 . Next, in step S 4514 , both the band of print data received in step S 4501  and the retrieved saved black data are sent to printer  10  for printing. As shown in FIG. 46, the lower-most black nozzles of print head  62  are used, along with the cyan, magenta and yellow nozzles, to print black print data of the received band of data while the upper-most black nozzles are used to print the saved black data of the band printed in pass  2 . Advantageously, the black data is printed only in a forward direction. Accordingly, image degradation caused by backward printing of black ink is avoided. 
     Flow continues as described above with regard to pass  2  and pass  3  for each of passes  4  and  5 , respectively, as illustrated in FIG.  46 . However, as shown in FIG. 46, yellow nozzles of print head  62  are not used during pass  4  nor are magenta or yellow nozzles of print head  62  used during pass  5  because no data for those nozzles is present in the bands printed during either pass. 
     With regard to pass  6 , a band of print data corresponding to pass  6  is received in step S 4501 . Although the received band does not contain any data corresponding to yellow, magenta or cyan ink, pixel locations of the band have previously been printed upon, in passes  3 ,  4  and  5 , using yellow, magenta and cyan ink, respectively. Accordingly, flow proceeds to step S 4504 . Since pass  6  would be in a backward direction, flow continues to step S 4509 , wherein black print data of the received band is saved in buffer  109 . In step S 4510 , data other than black data of pass  6  is sent to printer  10  for printing. In this case, the received band of print data includes only black print data, therefore head  62  merely scans across the recording medium in a backward direction without printing during step S 4510  of pass  6 . Flow then continues from step S 4508  to step S 4501 , wherein a next band of print data is received. 
     In the present example, the received band corresponds to the black region shown in FIG. 46, therefore flow proceeds from step S 4502  to step S 4515 . In step S 4515 , it is determined whether a previously printed band included color data. Since the band of print data analyzed with respect to pass  6  was determined to include color data, flow continues to step S 4516 , wherein it is determined whether a last pass was in a backward direction. Again, since pass  6  was in a backward direction, flow continues to step S 4517 . In step S 4517 , saved black data is retrieved from print buffer  109 . In this regard, since step S 4517  can be reached only if a previously-printed band included color data and a last pass was backward, it is assumed that black data of the previously-printed band was saved and not printed. Accordingly, next, in step S 4519 , the retrieved black data is sent to printer  10 . 
     It should be noted that, after pass  5 , the recording medium was advanced 23 nozzles and after pass  6 , the recording was again advanced 23 nozzles. Accordingly, the retrieved black data is printed during pass  7  using nozzles  24  to  46  of print head  62 . Flow then proceeds to step S 4520 , wherein the retrieved band of black data is sent to printer  10  for printing during pass  8  using print head  64  and ink jet cartridge  43   b  which, as described above, includes low-penetration black ink. It should be noted that pass  8  is performed in a forward direction to avoid image degradation caused by printing black ink in a reverse direction. 
     Flow proceeds from steps S 4520  to S 4508  and then, if another band is to be printed, to step S 4501 . If the next band includes no color data, flow proceeds from step S 4515  directly to step S 4520  as described above. 
     Flow continues as described above until, in step S 4508 , it is determined that a last band has been printed. In this case, flow then proceeds to step S 4522 , wherein it is determined whether the last pass was in a backward direction. If not, flow terminates. If so, saved black data yet to be printed is sent to printer  10  to be printed, in a forward direction, in step S 4524 . Flow then terminates. 
     By virtue of the foregoing process, printing of certain data in a backward direction can be avoided if it is determined that it is not desirable to print the data in the backward direction. In this regard, it should be noted that the foregoing process steps are not limited to forward-direction printing of black print data only, but can be applied to print other types of print data exclusively in a backward direction. 
     8.0 Prefiring and Pulse Width Modulation 
     This Section describes prefiring and pulse width modulation control according to the invention. 
     8.1 Prefire Control 
     Prefiring is performed in an ink jet printer so as to clear drying or coagulating ink from print head nozzles. Prefire timing according to the invention is described in Section 8.1.1. An embodiment of a system for control of prefire timing according to the invention is described in Section 8.1.2. 
     8.1.1 Prefire Timing 
     FIG. 47 is a diagram for describing prefire control in which a prefire operation is performed at a predetermined interval. Shown in FIG. 47 is recording medium  401  with image  402  printed thereon. In FIG. 47, image  402  includes smaller-font text  403  and larger-font text  404 . 
     Also shown in FIG. 47 is cartridge receptacle  405  at various times during printing of image  402 . Cartridge receptacle  405  is one of cartridge receptacles  37   a  and  37   b  of printer  10  described above with reference to FIG. 5 in Section 1.0. Cartridge receptacle  405  preferably carries an ink jet cartridge such as ink jet cartridge  43   a  shown in FIG. 6 above. The ink jet cartridge preferably has a print head such as print head  61  or print head  62  shown in FIG. 7 above. 
     Arrows  409  to  433  indicate movement of cartridge receptacle  405 , and therefore of a print head carried by cartridge receptacle  405 , across recording medium  401  before, during and after multiple scans for printing image  402 . Circled numbers are located next to starts of those of arrows  409  to  433  that represent scans during which parts of image  402  are printed. The circled numbers are in order of the scans used to print image  402 . Thus, in FIG. 47, a first scan occurs at the top of image  402 , and a last scan occurs at the bottom of image  402 . 
     FIG. 47 also shows ASF position  437 , wiping area  438 , and prefire area  439  for cartridge receptacle  405 . Cartridge receptacle  405  moves to ASF position  437  so as to initiate an automatic sheet feed operation, as discussed in more detail above in Sections 1.0 and 4.0. 
     In the preferred embodiment, wiping area  438  and prefire area  439  are located at home position  46  shown in FIG.  5 . wiping area  438  includes wipers  44   a  and  44   b.  At wiping area  438 , a print head held by cartridge receptacle  405  is wiped by a wiping mechanism so as to wipe excess ink, dust, paper particles and other debris from the print head. 
     Prefire area  439  is also located and at home position  46  and includes prefire receptacles  42   a  and  42   b.  A print head ejects ink from its nozzles into one of these receptacles so as to clear drying or coagulating ink from the nozzles. 
     Positioning of cartridge receptacle  405  at one of ASF position  437 , wiping area  438 , or prefire area  439  is indicated in FIG. 47 by showing cartridge receptacle  405  or an arrow representing movement of cartridge receptacle  405  below the position or area. 
     Event list  441  is shown to the left of recording medium  401 . Circled symbols in event list  441  represent events that occur as image  402  is printed. In FIG. 47, start of printing  443  is represented by circled symbol St. Automatic sheet feed  444  is represented by circled symbol ASF, and initial load wipe/prefire  445  is represented by circled symbol LP. Automatic prefire events  447  to  451 , which are represented by circled symbols AP″, AP 1 , AP 2 , AP 3  and AP 4 , respectively, also are shown in event list  441 . 
     Timeline  453  is shown to the right of recording medium  401 . The timeline runs from top to bottom in FIG.  47  and illustrates the timing relationship between scans of cartridge receptacle  405  for printing image  402  and events shown in event list  441 . Accordingly, starts of each scan of cartridge receptacle  405  for printing image  402  are represented in timeline  453  by circled numbers corresponding to the circled numbers shown at the starts of the ones of arrows  409  to  433  that represent scan movement of cartridge receptacle  405 . Likewise, events shown in event list  441  are represented in timeline  453  by symbols identical to those used in event list  441 , and common reference numerals are used in both event list  441  and timeline  453  for identical symbols corresponding to a single event. For example, circled symbol St in event list  441  and circled symbol St in timeline  453  both represent start of printing  443 . 
     In the prefire control illustrated by FIG. 47, an automatic prefire operation is preformed based on a two second interval. In more detail, event list  441  and timeline  453  show start of printing  443  followed by automatic sheet feed  444  and initial load wipe/prefire  445 . Accordingly, arrow  409  shows cartridge receptacle  405  moving from circled symbol St at start of printing  443  to circled symbol ASF for automatic sheet feed  444  of recording medium  401 . Arrow  410  shows cartridge receptacle  405  then moving past wiping area  438  for initial wiping to prefire area  439  for initial prefire, completing initial load wipe/prefire  445 . 
     Following load wipe/prefire  445 , a first automatic prefire  447  represented by circled symbol AP″ optionally is performed. In particular, if a sufficient delay (e.g., two seconds) occurs between load wipe/prefire  445  and a start of printing, automatic prefire  447  is performed to maintain clear ink nozzles. Such a delay can occur, for example, while data is processed by a host processor or sent to the printer. In addition, the delay can occur while a user manually feeds a recording medium to the printer. 
     In order to perform automatic prefire  447 , cartridge receptacle  405  is positioned at prefire area  439 , as illustrated by the position of cartridge receptacle  405  next to circled symbol AP″ below prefire area  439 . Then, the print head nozzles are prefired to clear them of drying or coagulating ink. 
     Three scans of cartridge receptacle  405  are performed and a fourth scan is started before two second interval  459  elapses. This interval is measured from initial load wipe/prefire  445  (or automatic prefire  447 , if applicable). The movement of cartridge receptacle  405  for these four scans is represented by arrows  411  to  414 , and the starts of the four scans are represented by circled numbers  1  to  4 . 
     Once two second interval  459  elapses, cartridge receptacle  405  completes a current scan and then moves to prefire area  439  for an automatic prefire operation. Accordingly, after the fourth scan, cartridge receptacle  405  moves to prefire area  439  for automatic prefire  448 , as illustrated by arrow  415 . After automatic prefire  448 , cartridge receptacle  405  resumes scanning across recording medium  401 . 
     The foregoing process continues until image  402  is printed onto recording medium  401 . In particular, an automatic prefire operation occurs whenever a two second interval from a previous prefire elapses during a given scan. Whenever the interval elapses, the current scan preferably is completed, and then cartridge receptacle  405  is moved to prefire area  439  for a prefire operation. If the scan during which the interval elapses is a scan in which cartridge receptacle  405  is moving away from prefire area  439 , then after the current scan is completed, a next scan is completed as cartridge receptacle  405  moves to prefire area  439 . 
     Thus, in FIG. 47, cartridge receptacle  405  performs fifth through eighth scans corresponding to arrows  416  to  419 ; moves to prefire area  439  for automatic prefire  449  as illustrated by arrow  420 ; performs ninth through eleventh scans corresponding to arrows  421  to  423 ; performs a twelfth scan and then moves to prefire area  439  for automatic prefire  450  as illustrated by arrows  424  and  425  (the twelfth scan is performed because the eleventh scan is moving away from prefire area  439 ); performs thirteenth through sixteenth scans corresponding to arrows  426  to  429 ; moves to prefire area  439  for automatic prefire  451  as illustrated by arrow  430 ; and performs seventeenth and eighteenth scans corresponding to arrows  431  and  432  to complete printing image  402 . 
     After image  402  is printed, cartridge receptacle  405  moves off of recording medium  401  for ejection of the recording medium, as shown by arrow  433 . The ejection process is described in more detail above with respect to Section 3.0. 
     The foregoing prefire control results in frequent prefire operations to ensure proper ink ejection from nozzles of the ink jet head, thereby tending to ensure image quality. However, some of the prefire operations are unnecessary. In particular, when text of a single font size is printed during successive scans, one block of nozzles of a print head tends to be re-used for each scan. As long as the same block of nozzles is used from scan to scan, the act of printing the text ensures that the nozzles in the block remain free of drying or coagulating ink. 
     Thus, for example, automatic prefire  448  (corresponding to circled symbol AP 1 ) between scans for printing smaller-font text  403  is at least partly unnecessary for maintaining image formation quality for the fifth through eighth scans in FIG. 47 (corresponding to arrows  416  to  419 ). The previous scans have already kept the block of nozzles used for those scans free of drying or coagulating ink. Likewise, automatic prefire  451  (corresponding to circled symbol AP 4 ) between scans for printing larger-font text  404  is at least partly unnecessary. These unnecessary prefire operations unacceptably slow the image formation process, particularly in a case where high speed image formation is desired. 
     One technique for increasing image formation speed is to increase the time interval between automatic prefire operations. However, increasing the time interval between all prefire operations can unacceptably degrade image quality. 
     FIGS. 48 and 49A to  49 C are diagrams for describing image degradation that can result from use of overly-long intervals between prefire operations. Shown in FIG. 48 is recording medium  461  with image  462  printed thereon. In FIG. 48, image  462  includes smaller-font text  463  and larger-font text  464 . 
     Also shown in FIG. 48 is cartridge receptacle  405  at various times during printing of image  462 . Examples of cartridge receptacle  405  are cartridge receptacles  37   a  and  37   b  described above with reference to FIG. 5 in Section 1.0. Cartridge receptacle  405  preferably carries an ink jet cartridge such as ink jet cartridge  43   a  shown in FIG. 6 above. The ink jet cartridge preferably has a print head such as print head  61  or print head  62  shown in FIG. 7 above. 
     Arrows  469  to  491  indicate movement of cartridge receptacle  405 , and therefore of a print head carried by cartridge receptacle  405 , across recording medium  461  before, during and after multiple scans for printing image  462 . Circled numbers are located next to starts of those of arrows  469  to  491  that represent scans during which parts of image  462  are printed. The circled numbers are in order of the scans used to print image  462 . Thus, in FIG. 48, a first scan occurs at the top of image  462 , and a last scan occurs at the bottom of image  462 . 
     FIG. 48 also shows ASF position  437 , wiping area  438 , and prefire area  439  for cartridge receptacle  405 . Cartridge receptacle  405  moves to ASF position  437  so as to initiate an automatic sheet feed operation, as discussed in more detail above in Sections 1.0 and 4.0. 
     Wiping area  438  and prefire area  439  preferably are located at home position  46  shown in FIG.  5 . At wiping area  438 , a print head held by cartridge receptacle  405  is wiped by a wiping mechanism so as to wipe excess ink, dust, paper particles and other debris from the print head. The print head ejects ink from its nozzles into prefire area  439  so as to clear drying or coagulating ink from the nozzles. The position of cartridge receptacle  405  at one of ASF position  437 , wiping area  438 , or prefire area  439  is indicated in FIG. 48 by showing cartridge receptacle  405  or an arrow representing movement of cartridge receptacle  405  below the position or area. 
     Event list  501  is shown to the left of recording medium  461 . Circled symbols in event list  501  represent events that occur as image  462  is printed. In FIG. 48, start of printing  503  is represented by circled symbol St. Automatic sheet feed  504  is represented by circled symbol ASF, and initial load wipe/prefire  505  is represented by circled symbol LP. Automatic prefire events  507 ,  508  and  510 , which are represented by circled symbols AP″, AP 1 , and AP 2 , respectively, also are shown in event list  501 , along with data wait  509  represented by circled symbol DW. The data wait event represents a pause in printing as host processor  2  spools print data to printer  10 . 
     Timeline  513  is shown to the right of recording medium  461 . The timeline runs from top to bottom in FIG.  48  and illustrates the timing relationship between scans of cartridge receptacle  505  for printing image  462  and events shown in event list  501 . Accordingly, starts of each scan of cartridge receptacle  405  for printing image  462  are represented in timeline  513  by circled numbers corresponding to the circled numbers shown at the starts of the ones of arrows  469  to  491  that represent scan movement of cartridge receptacle  405 . Likewise, events shown in event list  501  are represented in timeline  513  by symbols identical to those used in event list  501 , and common reference numerals are used in both event list  501  and timeline  513  for identical symbols corresponding to a single event. For example, circled symbol St in event list  501  and circled symbol St in timeline  513  both represent start of printing  503 . 
     In the prefire control illustrated by FIG. 48, an automatic prefire operation is preformed based on a six second interval. In more detail, event list  501  and timeline  513  show start of printing  503  followed by automatic sheet feed  504  and initial load wipe/prefire  505 . Accordingly, arrow  469  shows cartridge receptacle  405  moving from circled symbol St at start of printing  503  to circled symbol ASF for automatic sheet feed  504  of recording medium  461 . Arrow  470  shows cartridge receptacle  405  then moving past wiping area  438  for initial wiping to prefire area  439  for initial prefire, completing initial load wipe/prefire  505 . 
     Following load wipe/prefire  505 , a first automatic prefire  507  represented by circled symbol AP″ optionally is performed. In particular, automatic prefire  507  is performed if six second delay  514  elapses before actual printing begins. Such a delay can occur, for example, while data is processed by a host processor or sent to the printer. In addition, the delay can occur while a user manually feeds a recording medium to the printer. 
     Such a delay also can occur while data is processed or loaded into the printer, particularly if data is being processed by a low-end host processor connected to the printer. In addition, such a delay can occur if the printing operation must await user intervention, for example to load recording medium  461  or to initiate actual printing of image  462 . If six second delay  514  elapses, cartridge receptacle  405  is positioned at prefire area  439  so that automatic prefire  507  can be performed, as illustrated by the position of cartridge receptacle  405  next to circled symbol AP″ below prefire area  439 . Then, the print head nozzles are prefired to clear them of drying or coagulating ink. 
     In a case that the delay is insufficient to trigger automatic prefire  507 , the delay still can be sufficient to adversely affect image quality. In particular, a delay of just under six seconds easily can lead to image degradation such as that illustrated in FIG.  49 A. This image degradation can appear as jagged or offset pixels for a left side of printed text for the first scan line. The pixels are offset or distorted by partially dried or coagulated ink in the print head nozzles. 
     In any event, after printing starts in FIG. 48, eleven scans of cartridge receptacle  405  are performed and a twelfth scan is started before six second interval  515  elapses. This interval is measured from automatic prefire  507  (or initial load wipe/prefire  505 , if applicable). The movement of cartridge receptacle  405  for these twelve scans is represented by arrows  471  to  482 , and the starts of the twelve scans are represented by circled numbers  1  to  12 . 
     Image degradation can occur during the first twelve scans due to the long interval for prefiring. In particular, a block of print head nozzles are unused while smaller-font text  463  is printed. During this time, ink in nozzles in this block can begin to dry or to coagulate. Then, when a line of larger-font text  464  is started at the tenth scan, these nozzles can misfire for several pixels. One example of image degradation that can result from this misfiring is illustrated in FIG.  49 B. 
     Returning to FIG. 48, once six second interval  515  elapses, cartridge receptacle  405  moves to prefire area  439  for automatic prefire at the end of the current scan. Accordingly, after the twelfth scan, cartridge receptacle  405  moves to prefire area  439  for automatic prefire  508 , as illustrated by arrow  483 . After automatic prefire  508 , cartridge receptacle  405  resumes scanning across recording medium  461 . 
     The foregoing process continues until image  462  is printed onto recording medium  461 . In particular, an automatic prefire operation occurs whenever a six second interval from a previous prefire elapses during a given scan. Whenever the interval elapses, the current scan preferably is completed, and then cartridge receptacle  405  is moved to prefire area  439  for a prefire operation. If the scan during which the interval elapses is a scan in which cartridge receptacle  405  is moving away from prefire area  439 , then after the current scan is completed, a next scan is completed as moving cartridge receptacle  405  moves to prefire area  439 . 
     Thus, in FIG. 48, cartridge receptacle  405  performs thirteenth through sixteenth scans corresponding to arrows  484  to  487 . Then, data wait event  509  occurs. If this data wait event is sufficiently slow that six second interval  516  elapses before the seventeenth scan, then automatic prefire  510  occurs. In that case, cartridge receptacle  405  moves to prefire area  439 , as illustrated by arrow  488 , so that the prefire operation can be performed. Otherwise, the seventeenth scan is performed without a prefire operation. 
     In the case that the seventeenth scan is performed without a prefire operation, image degradation such as that shown in FIG. 49C can occur. Because all print head nozzles were idle during data wait event  509 , ink in the nozzles can begin to dry or to coagulate, adversely affecting the first few pixels of the seventeenth scan. An example of the resulting image degradation that can occur is shown in FIG. 49C in the form of a jagged or offset left edge for the first letter of the printed text. 
     Returning to FIG. 48, once again, cartridge receptacle  405  next performs seventeenth and eighteenth scans corresponding to arrows  489  and  490  to complete printing image  462 . 
     After image  462  is printed, cartridge receptacle  405  moves off of recording medium  461  for ejection of the recording medium, as shown by arrow  491 . The ejection process is described in more detail above with respect to Section 3.0. 
     In the printing operation discussed above, the longer interval between prefiring operations can result in image degradation such as that shown in FIGS. 49A to  49 C. Significantly, the image degradation illustrated in FIGS. 49A and 49C can occur if a delay in printing caused by a data wait event is long enough for ink to start drying or coagulating, but not long enough to trigger automatic prefire. 
     Data wait events for low-end host processors tend to be long enough to trigger automatic prefire. Thus, a user printing images from a slow low-end host processor would be less likely to experience the problems illustrated in FIGS. 49A and 49C, although these problems still could occur. Users printing from more expensive and faster high-end host processors would be more likely to experience these problems. Therefore, in order for a printer to be suitable for use with high-end host processors, the problems with prefiring detailed above should be addressed. 
     While the foregoing has illustrated image degradation for printing an image composed of text having different font sizes, such degradation also can occur when printing color or non-color graphics. For example, image degradation can occur when long intervals between automatic prefire operations are used while printing graphics with a color print head such as print head  62  shown in FIG.  7 . When a part of an image is printed in color using such a print head, the recording medium is advanced between each scan by a distance corresponding to the number of nozzles for a single color. For print head  62 , the recording medium is advanced each scan by a distance corresponding to 24 nozzles. As explained above in Section 5.0, only 48 of the available 64 black nozzles are used for each scan; a block of 16 nozzles are unused. Then, if printing transitions to all black printing, all 64 black nozzles are used, including the previously unused block of 16 black nozzles. These previously unused nozzles can misfire due to dried or coagulated ink in the nozzles, resulting in image degradation along the lines shown in FIG.  49 B. Therefore, the foregoing problems of image degradation also should be addressed in the context of a color printing apparatus. 
     FIG. 50 is a diagram for describing prefire control according to the invention which addresses the problems discussed above with respect to use of fixed time intervals for automatic prefire operations. 
     Briefly, in an ink jet printing apparatus which performs printing by using a print head with at least a predetermined number of nozzles to eject ink, a prefiring operation is performed to eject ink from nozzles of the print head for maintaining printing quality after a first time interval during a printing operation. Nozzles of the print head are driven based on data to be printed and the prefiring operation is performed in a case where a number of the nozzles to be driven is changed. Preferably, the prefiring operation can be delayed to a second time interval longer than the first time interval. After the second time interval, the prefiring operation is performed. 
     In more detail, FIG. 50 shows recording medium  521  with image  522  printed thereon. In FIG. 50, image  522  includes smaller-font text  523  and larger-font text  524 . Also shown in FIG. 50 is cartridge receptacle  405  at various times during printing of image  402 . Examples of cartridge receptacle  405  are cartridge receptacles  37   a  and  37   b  of printer  10  described above with reference to FIG. 5 in Section 1.0. Cartridge receptacle  405  preferably carries an ink jet cartridge such as ink jet cartridge  43   a  shown in FIG. 6 above. The ink jet cartridge preferably has a print head such as print head  61  or print head  62  shown in FIG. 7 above. 
     Arrows  529  to  551  indicate movement of cartridge receptacle  405 , and therefore of a print head carried by cartridge receptacle  405 , across recording medium  521  before, during and after multiple scans for printing image  522 . Circled numbers are located next to starts of those of arrows  529  to  552  that represent scans during which parts of image  522  are printed. The circled numbers are in order of the scans used to print image  122 . Thus, in FIG. 50, a first scan occurs at the top of image  522 , and a last scan occurs at the bottom of image  522 . 
     FIG. 50 also shows ASF position  437 , wiping area  438 , and prefire area  439  for cartridge receptacle  405 . Cartridge receptacle  405  moves to ASF position  437  so as to initiate an automatic sheet feed operation, as discussed in more detail above in Sections 1.0 and 4.0. 
     Wiping area  438  and prefire area  439  preferably are located at home position  46  shown in FIG.  5 . At wiping area  438 , a print head held by cartridge receptacle  405  is wiped by a wiping mechanism so as to wipe excess ink, dust, paper particles and other debris from the print head. The print head ejects ink from its nozzles into prefire area  439  so as to clear drying or coagulating ink from the nozzles. The position of cartridge receptacle  405  at one of ASF position  437 , wiping area  438 , or prefire area  439  is indicated in FIG. 50 by showing cartridge receptacle  405  or an arrow representing movement of cartridge receptacle  405  below the position or area. 
     Event list  561  is shown to the left of recording medium  521 . Circled symbols in event list  561  represent events that occur as image  522  is printed. In FIG. 50, start of printing  563  is represented by circled symbol St. Automatic sheet feed  564  is represented by circled symbol ASF, and initial load wipe/prefire  565  is represented by circled symbol LP. Automatic prefire events  567 ,  570  and  572 , which are represented by circled symbols AP″, AP 1  and AP 2 , respectively, also are shown in event list  561 . In addition, just-before-scan prefire (JBSP) events  568  and  573  are represented in FIG. 50 by circled symbols JBSP, nozzle-number-change prefire (NNCP) event  569  is represented by circled symbol NNCP, and data wait (DW) event  571  is represented by circled symbol DW. These events are explained ink more detail hereinbelow. 
     Briefly, according to the invention, nozzle-number-change prefire occurs when data to be printed requires driving nozzles that have not been driven for a first time interval since a previous prefiring operation. Just-before-scan prefire occurs when none of the nozzles of a print head have be driven for a second time interval. Automatic prefire occurs when a third time interval has elapsed since a previous prefiring operation. The third time interval is longer than the first and second time intervals. As a result, prefire operations are delayed until the longer third time interval unless a prefire operation is triggered by a nozzle number change or a pause before scanning a line, which can result from a data wait event. 
     Returning to FIG. 50, timeline  574  is shown to the right of recording medium  521 . The timeline runs from top to bottom in FIG.  50  and illustrates the timing relationship between scans of cartridge receptacle  405  for printing image  5122  and events shown in event list  561 . Accordingly, starts of each scan of cartridge receptacle  405  for printing image  522  are represented in timeline  574  by circled numbers corresponding to the circled numbers shown at the starts of the ones of arrows  529  to  552  that represent scan movement of cartridge receptacle  405 . Likewise, events shown in event list  561  are represented in timeline  574  by symbols identical to those used in event list  561 , and common reference numerals are used in both event list  561  and timeline  574  for identical symbols corresponding to a single event. For example, circled symbol St in event list  561  and circled symbol St in timeline  574  both represent start of printing  563 . 
     In the prefire control illustrated by FIG. 50, an automatic prefire operation is preformed based on a six second interval. However, certain events can trigger an earlier prefire operation, including a change in a number of nozzles used in a scan across recording medium  521  or a pause in use of all nozzles. 
     In more detail, event list  561  and timeline  574  show start of printing  563  followed by automatic sheet feed  564  and initial load wipe/prefire  565 . Accordingly, arrow  529  shows cartridge receptacle  405  moving from circled symbol St at start of printing  563  to circled symbol ASF for automatic sheet feed  564  of recording medium  521 . Arrow  530  shows cartridge receptacle  405  then moving past wiping area  438  for initial wiping to prefire area  439  for initial prefire, completing initial load wipe/prefire  565 . 
     Following load wipe/prefire  565 , a first automatic prefire  567  represented by circled symbol AP″ optionally is performed. In particular, automatic prefire  567  is performed if a predetermined interval elapses between load wipe/prefire  165  and a start of printing. The predetermined interval can elapse, for example, while data is processed by a host processor or sent to the printer. In addition, the interval can elapse while a user manually feeds a recording medium to the printer. 
     In FIG. 50, the predetermined interval is six second interval  575 . After the six second interval has elapsed, the nozzles are in a “danger region” of operation in which ink ejection errors are more likely to occur. Thus, a prefiring operation should be performed before printing occurs. In order to perform automatic prefire  567 , cartridge receptacle  405  is positioned at prefire area  439 , as illustrated by the position of cartridge receptacle  405  next to circled symbol AP″ below prefire area  439 . Then, the print head nozzles are prefired to clear them of drying or coagulating ink. 
     If a further delay occurs before printing starts, then nozzles of the print head might remain idle long enough for ink to begin drying or coagulating. Accordingly, the invention determines if no printing (including prefiring) has occurred for a predetermined interval, which in FIG. 50 is three seconds. If no printing has occurred for this interval, just-before-scan prefire  568  is performed, thereby tending to ensure that the nozzles remain clear of drying or coagulating ink. This operation tends to prevent image degradation along the lines discussed above with respect to FIG.  49 A. 
     Once printing starts, elapsed time is measured from a previous prefire operation. In the example illustrated in FIG. 50, the previous prefire operation is just-before-scan prefire  568 , and the interval for performing an automatic prefire is six seconds. However, before this interval elapses, nine scans of cartridge receptacle  405  are performed, as shown by arrows  531  to  539 . These nine scans print all of smaller-font text  523 . In order to print larger-font text  524  for the tenth scan represented by arrow  540 , previously unused nozzles must be driven to eject ink. According to the invention, this change in a number of used nozzles is detected, as explained in more detail below with reference to FIG.  54 . 
     In FIG. 50, the nozzle number change occurs after a first time interval of three seconds has elapsed since a last prefiring operation. Thus, the nozzles are operating in a “sensitive region” in which a change in the number of driven nozzles can lead to image degradation such as that illustrated in FIG. 49B discussed above. Accordingly, nozzle-number-change prefire  569  is performed. However, if the change had occurred before the first three second time interval had elapsed, the nozzles would have been operating in a “safe region” in which image degradation is less likely. In that case, no prefiring would have been performed. 
     Preferably, it is determined if a scan will have a nozzle number change before that scan is performed. Carriage receptacle  405  is moved to prefire area  439  before the scan is performed so that unused print head nozzles can be cleared before further printing occurs. Then, after the nozzle-number-change prefire is performed, printing continues. This situation is illustrated in FIG. 50, where cartridge receptacle  405  is shown moving to prefire area  439  after the ninth scan, and prefiring occurs before cartridge receptacle  405  begins the tenth scan at circled number  10  for larger-font text  524 . This operation is in contrast to the prefire control discussed above with respect to FIGS. 47 and 48, in which cartridge receptacle  405  completes a current scan and possibly performs a next scan in order to move to prefire area  439 . 
     Returning to FIG. 50, six seconds elapse from just-before-scan prefire  568  to after a start of the twelfth scan represented by arrow  542 . However, automatic prefire  570  is not performed because nozzle-number-change prefire  569  occurs during the elapsed time. Instead, the prefire is postponed until automatic prefire  572 , which occurs after the thirteenth through sixteenth scans represented by arrows  543  through  546 . Automatic prefire  572  is triggered by the elapse during the sixteenth scan of six second interval  576  from nozzle-number-change prefire  569 . 
     In order to perform the automatic prefire operation, cartridge receptacle  505  moves to prefire area  439 , as shown by arrow  147 . If the sixteenth scan had moved cartridge receptacle  405  away from prefire area  439  (i.e., arrow  546  had been pointed away from prefire area  439 ), a next scan line preferably would have been printed while moving cartridge receptacle  405  to prefire area  439 . This operation is in contrast to the operation of a nozzle-number-change prefire operation discussed above, in which a next scan line preferably would not be printed. 
     Also illustrated in FIG. 50 is a case where data wait  571  is sufficiently long so that no nozzles are driven for a predetermined interval (e.g., three seconds) after automatic prefire  572 . As a result, just-before-scan prefire  573  is performed before the seventeenth scan begins, thereby tending to avoid image degradation of the type shown in FIG.  49 C. 
     After just-before-scan prefire  573 , the seventeenth and eighteenth scans are performed so as to complete printing image  522 . After image  522  is printed, cartridge receptacle  405  moves off of recording medium  521  for ejection of the recording medium, as shown by arrow  551 . The ejection process is described in more detail above with respect to Section 3.0. 
     FIG. 51 is a flowchart for describing prefire control timing according to the invention. 
     In step S 5101 , printer  10  loads a recording medium. A timer is then set equal to zero seconds in step S 5102 . 
     Line feed and printing operations occur in step S 5103 . In step S 5104 , it is determined if the timer is less than Threshold  1 . Threshold  1  represents a safe time interval during which prefire operations are generally unnecessary. However, if the timer is not less than Threshold  1 , flow proceeds to step S 5105 . 
     In step S 5105 , it is determined if printer  10  is operating in a “sensitive region” or a “danger region”. In particular, step S 5105  determines if the timer is less than Threshold  2 . If the timer is not less than Threshold  2 , then printer  10  is operating in a “danger region”, and flow proceeds to step S 5106  for performance of a support operation such as a prefire operation. 
     On the other hand, if the timer is less than Threshold  2 , printer  10  is operating in a “sensitive region”. In that case, flow proceeds to step S 5107 , where it is determined if support is needed. For example, support would be needed if a number of nozzles that were driven to print on the recording medium were changed. If support is needed, flow proceeds to step S 5108  for performance of the support operation. After either step S 5106  or step S 5108 , the timer is reset to zero in step S 5109 . 
     In step S 5110 , it is determined if printer  10  has reached an end of a page. If printer  10  has reached the end of a page, step S 5111  ejects the recording medium. Otherwise, flow returns to step S 5103  for continued printing. 
     8.1.2 Embodiment 
     FIGS. 52 through 56 are flowcharts for describing a preferred embodiment for implementing the timing of prefire control described above with respect to FIGS. 50 and 51. In this embodiment, certain functions preferably are executed by printer control  110  discussed above with reference to FIG. 8, for example in printer firmware. Other functions preferably are executed by printer driver  84  running on host processor  2 . 
     FIG. 52 is a flowchart for describing a prefire-timer-update function that preferably is executed by printer control  110 . This function is called every second from step S 1912  shown in FIG. 19, which also preferably is executed by printer control  110 . Accordingly, the prefire timers are updated every second by printer control  110 . 
     In more detail, when the prefire-timer-update function is called, in step S 5201  it is first determined if automatic prefire is enabled. Automatic prefire preferably can be enabled or disabled by a user, for example through printer driver  84 . In addition, in a high-speed printing mode, automatic prefire can be disabled so as to improve print speed. Likewise, in a high-quality printing mode, automatic prefire can be enabled so as to improve print quality. Certain print heads such as the Canon BC-21(e) also are less sensitive to long intervals between prefiring operations, and automatic prefiring can be disabled for those print heads. 
     If automatic prefire is enabled, flow proceeds to step S 5202 . If automatic prefire is disabled, flow skips steps to step S 5206 . 
     In step S 5202 , it is determined if print head A (reference numeral  100   a  above) is present. For example, it is determined if a cartridge with a usable print head is properly installed in cartridge receptacle  37   a.  If print head A is present, prefire timer PFT_A for print head A is incremented in step S 5203 . Likewise, in step S 5204 , it is determined if print head B (reference numeral  100   a  above) is present, in which case step S 5205  increments prefire time PFT_B for print head B. PFT_A and PFT_B are used according to the invention to control automatic prefire operations such as automatic prefire operation  567 ,  570  and  572  explained above. 
     In step S 5206 , it is determined if printing or prefiring has occurred since a last invocation of the prefire-timer-update function. If printing or prefiring has occurred, flow proceeds to step S 5207 , and no-printing timer NPT is set to zero. Otherwise, flow proceeds to step S 5208 , and no-printing timer NPT is incremented. Thus, no-printing timer NPT stores a time since a last printing or prefiring operation. 
     No-printing timer NPT is used according to the invention to control just-before-scan prefire operations such as just-before-scan prefire operations  568  and  573  described above. It should be noted that no-printing timer NPT is updated regardless of whether automatic prefiring is enabled. 
     In step S 5209 , a PFCHECK command is executed. This command preferably invokes a prefire check function executed by printer control  110 . The prefire check function is described below with reference to FIG.  53 . After step S 5209 , flow returns to the flowchart of FIG.  19 . 
     FIG. 53 is a flowchart for describing a prefire check operation preferably executed by printer control  110  according to the invention. In step S 5301 , it is determined if cartridge receptacle  405  is moving in the correct direction, which is toward prefire area  439 . If cartridge receptacle  405  is not moving in the correct direction, flow skips to the end of the function and returns to FIG. 52, where flow then returns to FIG.  19 . When step S 1912  of FIG. 19 is called at succeeding one second interrupts, this process is repeated until the cartridge receptacle is moving in the correct direction. Once the cartridge receptacle is moving in the correct direction, flow proceeds to step S 5302 . 
     The foregoing operation of step S 5301  ensures that in a case where an interval for an automatic prefire operation elapses during a scan that moves cartridge receptacle  405  away from prefire area  439 , printing is performed for a next scan while returning cartridge receptacle  405  to prefire area  439 . 
     In step S 5302 , it is determined if PFT_A is greater than a prefire set time for print head A. Likewise, in step S 5303 , it is determined if PFT_B is greater than a prefire set time for print head B. In the example described above with respect to FIG. 50, these set times are both six seconds. It should be noted, however, that these set times do not need to be equal, but rather can be different so as to accommodate use of different print heads for print head A and print head B. 
     If either prefire timer PFT_A or prefire timer PFT_B is greater than its respective set time, the corresponding print head is operating in the “danger region” discussed above with reference to FIG. 50, and a prefire operation should be performed. Accordingly, flow proceeds to step S 5304  where a prefire (print) function is called, thereby performing an automatic prefire operation. The prefire (print) function is described in more detail below with reference to FIG.  56 . 
     FIG. 54 is a flowchart for describing generation of a nozzle-number-change prefire request by printer driver  84  according to the invention. 
     In step S 5401 , printer driver  84  sets PREVIOUS FEED and CURRENT FEED to zero at a start of a page for a print job. In step S 5402 , printer driver  84  sends a LOAD command to printer  10  so as to cause printer  10  to load a recording medium, as described above in Section 3.6.1. 
     Printer driver  84  determines scan height X in raster lines for a next scan line to be printed. In step S 5403 , printer driver  84  instructs printer  10  to advance the recording medium by X scan lines using the SKIP command. In step S 5404 , CURRENT FEED is set equal to X. 
     Step S 5405  determines that a nozzle-number-change has occurred if CURRENT FEED is less than or equal to THRESHOLD_ 1  and PREVIOUS FEED (the CURRENT FEED for a previous scan) is greater than THRESHOLD_ 1 . In the preferred embodiment, THRESHOLD_ 1  is one less than a height of a print head that is being used for printing in raster lines. For example, for print head  61  shown in FIG. 7, THRESHOLD_ 1  preferably is 127 raster lines. 
     In more detail, if PREVIOUS FEED is greater than THRESHOLD_ 1 , printer  10  fed the recording medium for the previous scan by more than the height of the print head. As a result, a whitespace exists between the previous scan and the current scan, indicating that the data being printed for the previous scan was so-called isolated data in which scan lines are separated from other scan lines by horizontal whitespaces. Typically, less than all of the nozzles of a print head are used to print isolated data. In particular, at least some of the top or bottom nozzles of the print head typically are unused. 
     If CURRENT_FEED is less than or equal to THRESHOLD_ 1 , no whitespace separates the current scan line from the previous scan line. Accordingly, the current scan data is continuous scan data such as data for a table or chart, in which all nozzles of the print head typically are used. Thus, testing if CURRENT FEED is less than or equal to THRESHOLD_ 1  and PREVIOUS FEED is greater than THRESHOLD_ 1  detects a nozzle number change that occurs when transitioning from printing isolated data to printing continuous data. 
     Step S 5406  determines that a nozzle number change has occurred if CURRENT FEED is greater than THRESHOLD_ 2  and PREVIOUS FEED is less than or equal to THRESHOLD_ 2 . In the preferred embodiment, THRESHOLD_ 2  is equal to a number of color nozzles used to eject ink of one color (e.g., cyan, magenta or yellow), which preferably is one less than a number of nozzles of a part of a color print head for ejecting ink of one color. For example, for print head  62  shown in FIG. 7, THRESHOLD_ 2  preferably is 23. 
     In more detail, if CURRENT FEED is greater than THRESHOLD_ 2 , then the data for the current scan most likely is not color data, because the number of raster lines printed for the current scan is greater than the number of raster lines of color ink that can be recorded using the color print head. If PREVIOUS FEED is less than or equal to THRESHOLD_ 2 , then the previous scan most likely was color data. Thus, this test determines that printing has transitioned from printing color data to printing non-color data. 
     During color printing, a number of black nozzles used for one scan typically equals the number of color nozzles for a single color. For example, as explained in Section 8.0, only 46 black nozzles of print head  62  typically are used for each scan during color printing, leaving 18 nozzles unused. However, during non-color printing, all of the black nozzles typically are used. Therefore, after a transition from color printing to non-color printing, a nozzle number change typically occurs for the black nozzles being used. 
     If either step S 5405  or S 5406  determines that a nozzle number change has occurred, a nozzle-number-change prefire request is sent to printer  10  in step S 5407 . In the case that the command set available to printer driver  10  does not include a nozzle-number-change prefire request, the instruction can be sent by sending an existing command with an out-of-range argument. Then, firmware in the printer can be modified to recognize the command with the out-of-range argument as a nozzle-number-change prefire request. For example, in the preferred embodiment, a raster SKIP command with an argument of zero lines is used as a nozzle-number-change prefire request. 
     In any event, the scan line is printed in step S 5408  using the PRINT command. In step S 5409 , PREVIOUS FEED is set equal to CURRENT FEED. If the end of the page has not been reached, step S 5410  returns flow to step S 5403  for processing the next scan line. Otherwise, processing for the page ends. 
     FIG. 55 is a flowchart for describing scan prefire processing preferably executed by printer control  110  according to the invention. This processing occurs every time printer  10  receives a PRINT command to print a scan line. 
     In step S 5501 , it is determined if a nozzle-number-change prefire request has been received. As discussed above with respect to step S 5407  of FIG. 54, in the preferred embodiment this request takes the form of a SKIP command with an argument of zero lines. If such a request has been received, flow proceeds to step S 5502 . Otherwise, flow skips to step S 5505 . 
     In step S 5502 , it is determined if a prefire timer, namely PFT_A or PFT_B discussed above with respect to FIG. 52, is greater than a threshold T 1 . If the prefire timer is less than this threshold, the print head is operating in a “safe region” as explained above with reference to FIG.  50 . Accordingly, a prefire operation is not necessary and would only serve to delay printing, and flow skips to step S 5505 . 
     If the prefire timer is greater than this threshold, the print head is operating in a “sensitive region” (or a “danger region”), and a prefire operation should be performed. Accordingly, flow proceeds to step S 5503  where a prefire (print) function is called, thereby performing a nozzle-number-change prefire (NNCP) operation. This prefire (print) function is discussed in more detail below with reference to FIG.  56 . In step S 5504 , the nozzle-number-change prefire request is reset. 
     In step S 5505 , it is determined if no-printing timer NPT has exceeded a no-printing threshold T 2 . If no-printing timer NPT has exceeded this threshold, flow proceeds to step S 5506  where the prefire (print) function is called, thereby performing a just-before-scan prefire (JBSP) operation. 
     FIG. 56 is a flowchart for describing a prefire (print) function according to the invention. This function preferably is executed by printer control  110 . 
     A prefire lookup table pointer is retrieved in step S 5601 . In step S 5602 , it is determined if cartridge receptacle  405  is at prefire area  439 . If cartridge receptacle  405  is not at prefire area  439 , the cartridge receptacle is moved to prefire area  439  in step S 5603 . 
     As explained above with reference to FIG. 53, in a case that the prefire (printing) function is called from step S 5304  for an automatic prefire operation, cartridge receptacle  405  is on the same side of printer  10  as prefire area  439 . Likewise, in a case that the prefire (printing) operation is called from step S 5506  in FIG. 55 for a just-before-scan prefire operation, no printing has occurred for at least time interval T 2 . Accordingly, cartridge receptacle  405  again is on the same side of printer  10  as prefire area  439 . Preferably, only in the case of a nozzle-number-change prefire is cartridge receptacle  405  moved across a recording medium in step S 5603  without printing. As a result, delay due to prefire operations tends to be further reduced, thereby increasing overall printing speed. In either of these cases, only a short time is needed for step S 5603  to move cartridge receptacle  405  to prefire area  439 . 
     Print head configuration is checked in step S 5604 . Based on the print head configuration, prefire count pattern frequency and pulse width modulation are determined in step S 5605  as explained below in Section 8.2. The determined frequency and modulation are sent to control logic  94  in step S 5606 , which initiates prefiring in step S 5607 . 
     In steps S 5608 , S 5609  and S 5610 , the prefire timers are all reset. In particular, PFT_A, PFT_B and NPT are all reset to zero. Then, flow returns from the prefire (printing) operation. 
     8.2 Pulse Width Modulation Control 
     FIG. 57 is a diagram for describing a relationship between ink jet nozzle heat pulse width and output density. Shown in FIG. 57 is printing density  601  across scan line  602  for printing by ejection of ink from nozzles of an ink jet print head using fixed-width heat pulse  604 . 
     As the print head is scanned across scan line  602 , print head temperature  603  increases due to repeated firing of ink jet nozzles. As the print head heats up, more ink is ejected from the nozzles for a given heat pulse width. As a result, printing density can disadvantageously increase along printing direction  605  independent of print data. 
     FIG. 58 is a diagram for describing heat pulse width modulation. As shown in FIG. 58, different heat pulse widths are used as a print head moves across a scan line. The heat pulses are modulated so as to stabilize print head temperature  609  around best quality temperature  610 , thereby stabilizing printing density  611 . 
     FIG. 59 is a flowchart for explaining control of nozzle heat pulse driving times. In step S 5901 , printer  10  receives a command to set a control ratio for driving a print head pulse width sequence. The command is sent by host processor  2  (step S 5902 ), and in the absence of receiving any such command, printer  10  maintains a default value of 100%. The control ratio for driving that is received in step S 5901  is a factor applied to look-up values from a pre-stored table in ROM  92 , as described more fully below in step S 5912 . 
     In step S 5903 , printer  10  receives a command for a control ratio for head temperature calculations. The command is received from host processor  2  (step S 5904 ), and in the absence of receipt of such a command, printer  10  maintains a default value of 100%. The control ratio for head temperature calculations is applied as a multiplication factor against pre-stored values of heat-up coefficients used for calculating head temperature, as described more fully below in connection with step S 5915 . 
     Preferably, steps S 5901  through S 5904  are effected through use of the change pulse ratio command ([PCR]) defined above in Section 3.6. As described above, the [PCR] command is used to change a ratio of pulse control tables such as a ratio of heat-up coefficients used for calculating head temperature, and such as changing a ratio of pulse widths for a pulse width driving sequence for each individual nozzle of print heads  100   a  and  100   b  when ejecting an ink droplet from the nozzle. 
     Flow continues in printer  10  with steps S 5906  through S 5915  which are executed repeatedly at cyclic intervals of, for example, 50 msec so as to maintain in real time the most current values for print head driving parameters. More specifically, as described above in connection with FIG. 19, steps S 5906  through S 5915  are executed at 50 msec cyclic intervals, for example, so as to calculate head temperature and to derive pulse width timings for a pulse width sequence applied to eject an ink droplet from a nozzle, together with other tasks also executed at 50 msec intervals. 
     Referring again to FIG. 59, step S 5906  reads current environmental temperature (T env ) from temperature sensor  103   a  in printer  10 , preferably in real time as explained in FIG. 61 below. The current environmental temperature may be the most current value read from the thermistor, or more preferably the actual value read from the thermistor is subjected to low pass filtering so as to smooth any irregularities, discount bad readings of the thermistor, remove noise such as analog-to-digital sampling noise, and the like. 
     Based on the environmental temperature T env  read in step S 5906 , a target temperature (T tgt ) is calculated in step S 5907 . The target temperature is the preferred operational temperature for printer  10  based on the current environmental temperature. Generally speaking, printer  10  is controlled through unshown heaters in print heads  100   a  and  100   b  so as to reach the target temperature, as explained above in connection with FIG. 19 at the 500 msec interrupt level. The target temperature is the most preferred temperature for print head operation based on the current environmental temperature. The relationship between target temperature and environmental temperature is inverse, meaning that low environmental temperatures result in relatively higher target temperatures, whereas high environmental temperatures result in relatively lower target temperatures. For example, at extremely low environmental temperatures such as T env =5° C., a preferred target temperature might be T tgt =35° C., whereas at extremely high environmental temperatures such as T env =35° C., a preferred target temperature might be T tgt =15° C. 
     Step S 5909  calculates the effect on print head temperature caused by actual ink droplet ejection from print heads  100   a  and  100   b.  More particularly, the environmental temperature read in step S 5906  is based on an environmental temperature read by a thermistor mounted exteriorly of print heads  100   a  and  100   b.  Proper control over print head driving parameters, on the other hand, is more directly affected by the internal temperature of ink adjacent the print head nozzles. It is not generally considered practicable to mount a thermistor within such a small area. At the same time, it is known that active ink droplet ejection will cause a rise in ink temperature and that in the absence of any ink ejection, ink temperature will generally fall. It is the purpose of step S 5909  to calculate the effect of print head temperature caused by ink droplet ejection to make this calculation. 
     The calculation of print head temperature in step S 5909  is made based in part on the number of ink droplets actually ejected over a previous time interval such as 50 msec. Each ejection of an ink droplet within the predetermined time interval is assigned a heat coefficient weight. Based on the number of ink droplet ejections within the predetermined time period, it is possible to calculate the effect of ink droplet ejection on print head temperature. 
     At the same time, it is known that such heat-up coefficients vary in dependence on the particular type of print head used, the ink characteristics used in the read, the resolution of printout by the head, and the like. Each different combination of head/ink/resolution changes heat-up coefficient values corresponding to the number of dots printed. Accordingly, ROM  92  is pre-stored with tables for heat-up coefficients. This situation is illustrated in FIG.  60 . 
     As shown in FIG. 60, one portion of ROM  92  includes pre-stored tables  621  for heat-up coefficients. The tables include plural tables  622   a,    622   b,  etc., one table for each different combination of printer head, ink characteristics, and resolution. Each of the plural tables includes tabularly accessed coefficients such as the coefficients labelled 1, 2 and 3 (reference numerals  623 ,  624  and  625 ), which are accessed through look-up operations based on the number of ink drops ejected in any one particular interval, for example, 50 msec. Printer  10  selects one heat-up table from the tables stored at  621 , based on a default selection or based on a commanded selection, and then selects heat-up coefficients from the selected table based on the number of droplets ejected in a 50 msec period. 
     The coefficients obtained through look-up operation in tables  621  are used to calculate the effect on print head temperature by ink droplet ejection. One suitable calculation is as follows: 
     
       
         Δ T   main =(coeff1(# black droplets ejected))+(coeff2(# color droplets ejected))+(coeff3(heater duty cycle))−coeff4 
       
     
     where coeff1 is a heat-up coefficient based on the number of black ink droplets ejected, coeff2 is a heat-up coefficient based on the number of color droplets ejected, coeff3 is a heat-up coefficient based on the current duty cycle of the heater, and coeff4 is a heat-up coefficient which actually shows cool down of the print head based on inactivity. Of course, the actual coefficients and calculations used depend on the head/ink/resolution combination. For example, the calculation given above is suitable for a four-color print head whereas an all-black print head would use a different calculation that excludes, for example, dependence on the number of color droplets ejected. 
     Armed with the environmental temperature T env , the target temperature T tgt  and the print head temperature effect ΔT main , step S 5910  calculates the difference ΔT diff , as follows: 
     
       
         
           T 
           diff 
           =T 
           tgt 
           −T 
           env 
           −ΔT 
           main 
         
       
     
     Step S 5911  accesses a look-up table in ROM  92  that stores pulse width times for a pulse width driving sequence, based on the temperature difference T diff . Suitable tables are illustrated diagrammatically in FIG. 60 as described below. 
     Specifically, as shown in FIG. 60, ROM  92  includes look-up table  630  for storing driving times. The driving times are pulse widths for a pulse sequence used to drive nozzle heaters to eject an ink droplet. A typical pulse sequence is shown at  640  in FIG. 59, and includes a pre-heat pulse of width T pre , a quiescent period of width T int , and a main heating pulse of width T main . Such a pulse sequence is applied to nozzle heaters in each nozzle of print heads  100   a  and  100   b  so as to eject a droplet of ink for printing. It is the purpose of table  630  to calculate each of T rep , T int  and T main  based in part on the temperature difference calculated in step S 5910 . 
     At the same time, it is recognized that the pulse widths of the pulse driving sequence differ based on particular combinations of print head, ink characteristics, resolution, and the like. Accordingly, as shown in FIG. 60, tables  630  include individual tables such as  632   a,    632   b,  etc. Each table  632   a,    632   b,  etc. is tailored for a particular combination of print head, ink type and resolution. As shown at  630 , each table includes entries  634  for the width of the pre-heat pulse T pre , entries  635  for the width of the quiescent interval T int , and entries  636  for the width of the main heating pulse T main . Any one particular entry is accessed through look-up operation based on the temperature difference T diff  calculated at step S 5910 . 
     Printer  10  selects one table of driving time from the tables stored at  630 , based on a default selection or based on a commanded selection. Printer  10  thereafter accesses the entries in the selected table, and looks up appropriate times for the pre-heat pulse, the quiescent interval, and the main heat pulse, all based on the temperature difference calculated in step S 5910 , and in a particular combination of print head/ink/resolution. 
     Reverting to FIG. 59, step S 5912  modifies the driving times obtained by look-up operation from table  630 , based on the control ratio for driving that was received in step S 5901 . The purpose of this step is to allow for modification of pre-stored values from look-up tables  630 , taking into consideration any difference between an actual print head mounted in printer  10 , and the print head combination stored in table  630 . In more detail, and as explained previously, although ROM  92  of printer  10  is pre-stored with plural tables for driving times, with each table tailored to a particular combination of print head/ink and resolution, it is not possible to anticipate each and every combination of print head/ink and resolution. Modification in step S 5912 , therefore, allows for use of previously unknown, or otherwise unstored, combinations of print head/ink and resolution. 
     Modification in step S 5912  is preferably through multiplication of the driving times obtained through look-up operation in step S 5911  by the control ratio received in step S 5901 . For this reason, the default control ratio is 100%. The control ratio that is commandable through the change pulse control ratio command [PCR] is constrained to lie between 1% to 200%, thereby allowing modification of pulse times from effectively negligible pulse times up to twice the values stored in tables  630 . 
     Flow then advances to step S 5914 , in which printer  10  looks up heat-up coefficients for head temperature calculations. As described previously in connection with tables  621  of FIG. 60, heat-up coefficients are obtained based on a particular combination of print head, ink and resolution, and are looked up from one of tables  622   a,  etc. based on the number of dots printed per cycle, each having a duration of approximately 50 msec. 
     Step S 5915  modifies the heat-up coefficients based on the control ratio received in step S 5903 . Again, the purpose of such modification is to permit usage of a particular combination of print head, ink and resolution not already stored in one of tables  621 . 
     Preferably, modification of the heat-up coefficients in step S 5915  is through multiplication of the coefficients obtained through look-up operation in step S 5914  by the control ratio received in step S 5903 . For this reason, the default control ratio is 100%. The control ratio that is commandable through the change pulse control ratio command [PCR] is constrained to lie between 1% to 200%, thereby allowing modification of heat-up coefficient from effectively negligible values up to twice the values stored in tables  221 . 
     In step S 5916 , printer  10  controls nozzle driving based on the modified driving times obtained in step S 5912 , all in response to a command from host processor  2  that sends print data to printer  10 , and a command for printer  10  to print such data (step S 5917 ). Flow repeats as before, with steps S 5906  through S 5915  being executed at 50 msec cyclic intervals, for example, and with control over nozzle driving based on modified driving times, as set out in step S 5916 , being executed as commanded by host processor  2 . In addition, it should be recognized that control ratios for driving, as well as control ratios for head temperature calculations, may be sent from host processor  2  at any time, and are responded to by printer  10  as set out in steps S 5901  and S 5903  described above. 
     FIG. 61 is a flowchart for describing use of a real-time environmental temperature for determination of driving times. In the preferred embodiment of the invention, environmental temperature T env  used for determination of driving times for nozzles of a print head is real-time environmental temperature T envR . 
     Accordingly, in step S 6101 , real-time temperature T envR  is measured using temperature sensor  103   a  shown in FIG.  9  and is retrieved through an A/D converter and I/O ports  96 . In step S 6102 , a hard-power-on timer is incremented. Then, in step S 6103 , real-time temperature T envR  is updated using the hard-power-on timer so as to account for effects of continued operation of printer  10  on environmental temperature. 
     In step S 6104 , it is determined if T envR  is less than zero degrees Celsius, in which T envR  is set equal to zero degrees Celsius in step S 6105 . Likewise, in step S 6106 , it is determined if T envR  is greater than seventy degrees Celsius, in which case T envR  is set equal to seventy degrees Celsius. 
     In step S 6108 , a head target temperature is retrieved as explained above with reference to FIGS. 39 and 40, using T envR  as T env . 
     FIG. 62 is a diagram for describing control of heat pulse width modulation after automatic prefire operations performed based on a fixed time interval. 
     As shown in FIG. 62, pulse width modulation varies across each scan line so as to maintain stable printing density. Prefire operations occur after scan lines during which a three second time interval from a previous prefire operation expires. 
     After each prefire operation, cartridge receptacle  405  must move from prefire area  439  to printing area  644  before printing can resume. A print head carried by cartridge receptacle  405  cools during this motion. As a result, after a prefire operation, maximum pulse width  647  is employed when printing resumes, as shown in FIG.  62 . It should be noted that the pulse widths actually comprise a pre-heat pulse, a quiescent interval, and then a main pulse as illustrated for maximum pulse width  647 . 
     The control of heat pulse modulation illustrated in FIG. 62 may be sufficient to maintain printing quality in a case that prefiring occurs based on a single short fixed time interval. However, the heat pulse modulation can be modified to accommodate better the prefire operations according to the invention, in which prefire operations can be separated by varying time intervals as discussed above in Section 8.1.1. 
     FIG. 63 is a diagram for describing heat pulse width modulation for a print head according to the invention in which a heat pulse width is maximized after a first time interval since a previous prefire operation. 
     In FIG. 63, prefire operation  651  occurs after six-second interval  652  since previous prefire operation  653 . Six second interval  652  is an example of a long interval during which a nozzle-number-change is not detected. Such long intervals are described above with reference to FIGS. 50 and 51. 
     During the first part of the long interval, the nozzles of a print head carried by cartridge receptacle  405  are operating in a “safe region”. This safe region is defined by a threshold before which prefire operations are not performed even if a change in a number of driven nozzles occurs, also as described above with reference to FIGS. 50 and 51. During the safe region of operation, the nozzles tend to remain free of drying or coagulating ink. Accordingly, pulse width modulation along the lines discussed above with respect to FIGS. 59 to  62  results in acceptable image quality. 
     After the safe region, the nozzles of a print head carried by cartridge receptacle  405  are operating in a “sensitive region”, again as described above with reference to FIGS. 50 and 51. During this sensitive region of operation, ink can begin to dry or coagulate in the nozzles of the print head. Therefore, according to the invention, maximum pulse width  654  is used to drive the nozzles when operating in the sensitive region. Likewise, if prefire is delayed until the “danger region” of operation discussed above with reference to FIGS. 50 and 51, a maximum pulse width continues to be used to drive the nozzles. 
     After a prefire operation, maximum pulse width  656  is used to drive the nozzles to account for cooling of the print head while cartridge receptacle  405  moves from prefire area  439  to the printing area. Then, pulse width modulation along the lines described above with respect to FIGS. 59 to  62  resumes, until the nozzles again are operating in a sensitive or danger region. 
     FIG. 64 is a flowchart for describing heat pulse width modulation according to the invention in which a heat pulse width is maximized after a first time interval since a previous prefire operation. This pulse width modulation control represents a modification of the pulse width modulation control described above with reference to FIGS. 59 to  62 . 
     The pulse width modulation control of FIG. 64 preferably is executed repeatedly by print control  110  at cyclic intervals of, for example, 50 msec so as to update pulse width modulation in real time. More specifically, the pulse width modulation control of FIG. 64 is executed every 50 msec, for example, from step S 1906  of FIG.  19 . 
     In step S 6401 , it is determined if a recording medium is loaded into printer  10 . If no recording medium is loaded, printing does not occur, and flow returns to FIG.  19 . Otherwise, flow proceeds to step S 6402 , where it is determined if a prefire operation is underway. If a prefire operation is underway, pulse width modulation is controlled according to print head configuration as described above with reference to FIG. 56, so flow returns to FIG.  19 . Otherwise, flow proceeds to steps S 6403  and S 6404 . 
     In steps S 6403  and S 6404 , pulse width modulation parameters are determined as described above with reference to FIGS. 59 to  62 . In this embodiment of the invention, the pulse width parameters are returned in the form of a pulse number. A higher pulse number represents a heat pulse that causes a nozzle to eject more ink, and a lower pulse number represents a heat pulse that causes a nozzle to eject less ink. 
     It is determined in step S 6405  if a prefire timer (PFT_A or PFT_B) is greater than a threshold (e.g., three seconds) defining a “sensitive region” for print head nozzle operation. If a prefire timer exceeds the threshold, flow proceeds to step S 6406 , and a maximum pulse width is used for pulse width modulation. By virtue of this step, a maximum pulse width is used during those times when nozzles are more likely to experience drying or coagulating of ink. The use of a maximum pulse width decreases the likelihood of nozzles becoming clogged, which would degrade image quality. Flow then proceeds to step S 6411 . 
     If a prefire timer does not exceed the threshold, step S 6407  determines if the pulse number from step S 6404  is less than the previously determined pulse width number. If the determined pulse number is less than the previous pulse width number, then in step S 6408  the current pulse number is set equal to the previous pulse number minus one. 
     Likewise, step S 6409  determines if the pulse number from step S 6404  is greater than the previously determined pulse width number. If the determined pulse width number is greater than the previous pulse width number, then in step S 6410  the current pulse number is set equal to the previous pulse number plus one. 
     By virtue of the operation of steps S 6407  through S 6410 , a rate of change in current pulse width numbers is limited to one per time that the pulse width modulation function is called. As a result, changes in pulse width modulation tend to be smoother than in conventional systems, evening out resulting print density across a scan line. 
     In step S 6411 , the current pulse width number is converted into heat pulse times, which are sent to control logic  94  in step S 6412 . Then, flow returns to FIG.  19 . 
     9.0 Color Printing Using Multiple Inks 
     As described above, printer driver  84  performs various functions to convert input multilevel RGB data to binary CMYK data for use in printing. FIG. 65 is a flow diagram of computer-executable process steps to convert RGB data of a single pixel into corresponding binary data for each of yellow ink, magenta ink, cyan ink, black high-penetration ink and black low-penetration ink. The process steps are preferably included in printer driver  84  and executed out of RAM  86  by CPU  70 . 
     Briefly, the FIG. 65 process steps include a first determining step to determine a first amount of low-penetration black ink corresponding to the multilevel value, a second determining step to determine a second amount of high-penetration black ink corresponding to the multilevel value and a printing step to print the pixel using the first amount of low-penetration black ink corresponding to the multilevel value and the second amount of high-penetration black ink corresponding to the multi-level value. 
     Specifically, flow begins at step S 6501 , in which RGB data for an input pixel is received. The input RGB data is preferably multi-value RGB data consisting of 8-bit red, green and blue values. The RGB data is converted to corresponding CMYK multi-bit values in step S 6502 . Next, in step S 6504 , a cyan data value resulting from step S 6502  is subjected to output correction. In this regard, a magenta data value from step S 6502  is subjected to output correction in step S 6505  and output correction is performed on a yellow data value and a black data value produced in step S 6502  in steps S 6506  and S 6507 , respectively. Output correction is also performed, in step S 6508 , on the black data value produced in step S 6502 . It should be noted that output correction performed in S 6508  is performed on the same black data upon which output correction is performed in step S 6507 , however, output correction in step S 6507  produces an output-corrected value corresponding to black high-penetration ink and output correction performed in step S 6508  results in a value corresponding to low-penetration black ink. 
     FIG. 66 shows a graph which may be used for performing steps S 6504  to S 6508 . In this regard, each graphed line in FIG. 66 represents corresponding input and output values used to perform output correction for a particular type of ink. For example, to perform output correction corresponding to dye black ink, an input black color value, produced in step S 6502 , is located on the horizontal axis, an imaginary vertical line is drawn to intercept the graphed line representing dye black ink, and an imaginary horizontal line is drawn from the interception point to the numbered vertical axis. The output-corrected value corresponding to the input color value is determined according to the point at which the imaginary horizontal line intersects the numbered vertical axis. 
     After output correction is performed as described above in steps S 6504  to S 6508 , each output-corrected value is subjected to halftoning. Specifically, output-corrected cyan data is subjected to halftoning in step S 6510 , output-corrected magenta data is subject to halftoning in step S 6511 , output-corrected yellow data is subjected to halftoning in step S 6512 , output-corrected dye black data is subjected to halftoning in step S 6513  and output-corrected pigment black data is subjected to halftoning in step S 6514 . It should be understood that the halftoning processes used in step S 6510  to step S 6514  will result in either a “0” or “1” value. As a result, when printing the pixel corresponding to the data input in step S 6501 , the pixel may be printed using no ink droplets, all ink droplets of each type of ink discussed above, or some combination thereof. Notably, and in contrast to conventional systems, both dye black ink and pigment black ink may be used to print the pixel. In step S 6516 , the halftoned data produced in each of steps S 6510  to S 6514  is placed in print buffer  109  for subsequent printing as described above. 
     In addition, the binarized data resulting from the process steps of FIG. 65 is preferably used to print yellow, magenta, cyan and high-penetration black ink droplets having a small droplet size and low-penetration black ink droplets having a larger droplet size. Such a configuration has been shown to produce high-quality text and black regions, while maintaining good quality within color regions. 
     10.0 Status-Based Control Over Printer 
     FIGS. 67 through 82 are used for explaining how the print driver obtains status of the printer and uses such status to control printer operations. In particular, these figures explain how the print driver uses status of the printer, and/or status of the printer coupled with the current environment of the computing equipment, so as to modify operational control parameters for the printer from their default values, and/or so as to modify the manner in which print data is derived by the print driver for printout by the printer, so that the print data is derived in a manner different from default methods taking into consideration the current status of the printer. 
     Many advantages result from status-based control over the printer. Specifically, operation of the printer is often fixed at design time with large operational margins, so as to accommodate all possible variations in status of the printer. While such large margins ensure operability across a large variety of possible status conditions, the large margins often result in inefficient usage at particular status conditions. Temperature, for example, is one example of printer status that causes large design margins in the printer, so as to accommodate good printer operation across a large variation in temperature. With large design margins, good printout can be obtained across a wide variety of temperatures. However, the cost of such printout is often decreased efficiency at one temperature (such as normal room temperature) so as to ensure good printout at another temperature (such a very cold or very warm temperatures). Representative embodiments of the invention, therefore, obtain printer status in the form of temperature, and modify how the printer is controlled based on the status, and/or modify how print data is derived based on such status. 
     10.1 Obtaining Status 
     FIGS. 67 through 69 illustrate generally how the print driver obtains status and uses the status to modify operational parameters of the printer and/or how the print driver derives print data, with such modifications departing from otherwise default processing. 
     FIG. 67 is a functional block diagram similar to those shown in FIGS. 9 and 18, showing computing equipment  1  communicating with printer  10 . As shown in FIG. 67, computing equipment  1  includes operating system  81 , an application program  82   a  which effects a print request, print driver  84 , and print data store  107 . Computing equipment  1  communicates to printer  10  over a bi-directional interface  76 , such as a Centronix or a network interface. Printer  10  includes printer control software  110  which stores print data from print driver  84  in a print data buffer  109 , and causes such print data to be printed by print engine  101 . 
     FIG. 68 is a flow diagram illustrating how print driver  84  obtains status from printer  10  and modifies otherwise default processing of print data generation, and/or modifies otherwise default operational parameters for the printer, all based on the status so-obtained. In FIG. 68, processing shown on the left-hand side is processing performed by print driver  84  in computing equipment  1 , whereas processing shown on the right-hand side is processing performed by print controller  110  in printer  10 . All such operations are performed in response to a request from application program  82   a  to print a particular print job. 
     In step S 6801 , print driver  84  obtains the current environment of the computing equipment. Current environment includes, for example, time, date and location information, and other like environmental information available from the computer and its operating system  81 . As shown below, such environmental information may be used by driver  84  to make even further refinements to the modifications made based on printer status. For example, certain operations may be performed more or less frequently, or not at all, at certain times of day. 
     In step S 6802 , print driver  84  obtains printer status. Print driver  84  obtains printer status by sending a [STATUS] command over bi-directional interface  76  to printer  10 . Printer controller  110  responds in step S 6804  by providing its current status to the print driver over bi-directional interface  76 . Examples of status requested by print driver  84  and provided from printer  10  include printer temperature, firmware version for the printer as well as its capabilities and current configurations, current and on-going operations of the printer (such as cleaning, aligning, purging, sheet feeding), processor speed and power, and any of the variety of information available in the printer&#39;s EEPROM. 
     Flow in print driver  84  next advances to step S 6805  in which the print driver modifies operational parameters of the printer based on the printer status so obtained, and/or based on the environment of computing equipment  10 . Examples of operational parameters that may be changed in this step S 6805  include adjustment of times between printhead prefires, adjustment of smear time, adjustment of automatic-sheet-feeder (ASF), speed for sheet feeding, adjustment of printhead purge speed, and the like. Print driver  84  modifies these operational parameters from their default values by transmission of appropriate commands over the bi-directional interface to printer  10 , as described more fully below in connection with representative embodiments of the invention. Printer  10  responds to such commands in step S 6806  by storing the modified operational parameters in place of their default values. 
     Flow in print driver  84  next advances to step S 6807  in which print driver modifies its own operation, such a modification of its user interface, based on the printer status and/or based on the computer&#39;s environment. Examples of such operational modifications from otherwise default operations include the display of special messages to the user, such as a display of a message to delay insertion of a manually-fed sheet until after on-going printer operations have terminated. 
     Flow next advances to step S 6809  in which the print driver modifies the manner in which it derives print data from otherwise default data processing, all based on the status of the printer and/or based on the status of the printer and the current environment of computing equipment  10 . Examples of such print data processing modifications include modifications to printer correction tables so as to reduce effects of ink bleed and/or ink smear, modifications to data compression processing so as to change data compression algorithms to more efficient algorithms, or to turn off compression altogether in situations where the printer  10  can accommodate uncompressed data more quickly than compressed data, and the like. The print data so generated is sent over bi-directional interface  74  to printer  10  using the [DATA] command, in response to which printer  10  prints out such data in step S 6810 . 
     One important status variable obtained from printer  10  is current temperature of the printer. Here, temperature of the printer refers not to internal temperature of any of the printer components (such as the printer head or the printer circuit board), but rather to ambient temperature of the printer. Ambient temperature of the printer defines in large part the environment in which the printer is printing, and largely controls a variety of physical phenomena such as ink drying time, ink viscosity, recording media “slipperiness” (i.e., the ability of printer  10  to feed and to advance a recording medium from the sheet feed tray to the eject tray), and the like. 
     FIG. 69 illustrates a flow sequence executed by print controller  110  so as to obtain temperature. The flow steps illustrated in FIG. 69 are a more detailed explanation of step S 1916  of FIG. 19, and obtain the status temperature of the printer based on the real time environmental temperature TenvR derived according to the steps illustrated in FIG.  61 . 
     The overall effect of the process steps shown in FIG. 69 is to set the printer status temperature to the real time environmental temperature TenvR after the printer has remained inoperative in the capped state for at least two hours. The process steps shown in FIG. 69 are executed at the one minute interrupt level (see FIG.  19 ), and cause an increment in a running minute counter (step S 6901 ). In step S 6902 , the capping state of the printer is investigated. If the printer is not currently in the capped state, flow branches to step S 6904  in which a capping counter is reset to zero, whereafter flow terminates until the next one minute interrupt. On the other hand, if step S 6902  determines that the printer is currently capped, then step S 6905  increments a capping counter. Steps S 6906  and S 6907  determine whether the capping counter has reached 120, corresponding to 120 minutes in the capped state. If the capping counter has not reached a count of 120, then flow terminates until the next one minute increment. On the other hand, if the capping counter has reached 120, then the printer status temperature TenvL is set to the current value of the real time temperature TenvR. Flow thereafter terminates until the next one minute increment. 
     10.2 Bleed Reduction 
     FIGS. 70 through 72 illustrate how driver  84  modifies its processing of print data from otherwise default processing, so as to reduce bleed, based on printer status. In the embodiments illustrated in FIGS. 70 through 72, modifications are based on printer status temperature TenvL, and the modifications act so as to reduce the overall amount of ink ejected by the printheads in high temperature situations where there is more possibility for ink bleeding. 
     FIG. 70 illustrates process steps for bleed reduction in which print driver  84  makes a selection of color tables based on the printer status. Thus, in step S 7001 , print driver  85  obtains printer status temperature TenvL. Step S 7002  tests the printer status temperature against a fixed predetermined amount, preferably 32° C. If the printer status temperature TenvL is not less than or equal to the fixed predetermined temperature, then flow branches to step S 7003  in which a color correction table is selected based on the higher possibility for ink bleed. Specifically, step  7003  selects color Table  2  which limits the amount of ink ejected by printer  10  for high temperatures. In this regard, it is inferred that high temperatures also involve high humidities, which increase overall ink drying time. 
     On the other hand, if step S 7002  determines that the printer status temperature TenvL is less than or equal to the predetermined threshold, then flow advances to step S 7004  to select a color correction table that does not limit the amount of ink ejected by printer  10  as much as color Table  2 . Specifically, since printer temperature TenvL is relatively cooler, there is less possibility for ink bleed, and color table  1  is selected that allows for default processing. 
     FIG. 71 illustrates values stored in color table  1  as opposed to values stored in Color Table  2 . FIG. 71 is a graph of such values, for each of cyan, magenta, yellow and black inks. The graphs give an output multilevel value obtained from the color table as a function of an input multilevel value. Values for table  1  are shown with solid lines. As seen in FIG. 71, output values for table  1 , for each of cyan, magenta, yellow and black inks, increase gradually for increasing input values. 
     Values for table  2  are shown in dotted line, and for input values of zero through  240  are identical to values in table  1 . However, beyond input value  240 , values for table  2  are maintained at a constant level, thereby limiting the amount of ink ejected at higher temperatures and reducing the possibility of ink bleed. 
     In the embodiment shown in FIG. 70, different color tables were selected by print driver  84  based on the printer status temperature TenvL. It is also possible for print driver  84  to modify values in a look-up table, rather than to select between different look-up tables. FIG. 72 illustrates this alternative embodiment. 
     Thus, in step S 7200 , print driver  84  obtains printer status temperature TenvL. Next, in step  7201 , a standard printer color correction table is loaded into memory. Step S 7202  tests the printer status temperature against a fixed predetermined threshold such as 32° C. If the printer status temperature is less than or equal to the fixed threshold, then no modifications are made to the loaded printer color correction table. On the other hand, if the printer status temperature exceeds the fixed predetermined threshold, then flow branches to step S 7203  where print driver  84  modifies the values in the color correction look-up table so as to reduce the possibility of ink bleed. Suitable modifications are modifications to values so as to obtain the values shown in FIG.  71 . 
     By virtue of the foregoing, where the print driver modifies data processing from otherwise default data processing based on printer status, it is possible to reduce ink bleed. 
     10.3 Smear Reduction 
     “Smear” is a phenomenon by which ink on a recording medium currently in the ejection tray has not sufficiently dried, which allows the leading edge of a second recording medium currently being ejected from (or printed on by) the printer to smear the undried ink. 
     To control smear, print controller  110  implements smear control processing shown in FIGS. 73A and 73B. The processing in FIG. 73B is simple, and merely decrements a non-zero smear timer at the one second interrupt level (see step S 1911  in FIG.  19 ). FIG. 73A shows how the smear timer is used in connection with currently printed dot density so as to reduce the possibility of undried ink being smeared by the leading edge of a subsequent recording medium. 
     Thus, in step S 7301 , printer  10  loads a recording medium from a print tray, and in step S 7302  the print controller  110  sets the smear timer to zero. Step S 7304  represents normal printout by the printer, during each scan of which the print controller  110  determines whether dot density for any one scan exceeds a driver-settable threshold for dot density (step S 7305 ). Unless the dot density for any one scan exceeds the threshold, no special processing is needed because such low amounts of ink are being ejected onto the recording medium that the possibility for smear is greatly reduced. On the other hand, if step S 7305  determines that the print dot density for any one scan exceeds the driver-settable threshold, then flow branches to step S 7306  in which the smear timer is set to a driver controlled value. Since the smear timer is now non-zero, the smear timer will be decremented in accordance with the processing of FIG. 73B, explained above. 
     As will be appreciated in consideration of the following explanations of FIGS. 74 and 75, both the driver-settable print density threshold and the driver-controlled smear timer value are determined in accordance with printer status, thereby achieving a control in smear parameters based on printer status. 
     Returning to FIG. 73A, step S 7307  determines whether an end of page has been reached, until which flow loops back up through step S 7304 . If end of page has been reached, then if the printed page is the last page (step S 7309 ), the currently-printed recording medium is simply ejected (step S 7310 ). On the other hand, if the currently-printed recording medium is not the last page, then flow branches to step S 7311  which checks to determine whether the smear timer has yet been decremented to zero. Until the smear timer has been decremented to zero, the currently-printed recording medium is not permitted to be ejected. However, as soon as the smear timer has been decremented to zero, then flow advances to step S 7312 , where the currently-printed recording medium is ejected to the eject tray, a new recording medium is loaded from the supply tray, and flow loops back up to step S 7304 . 
     FIGS. 74 and 75 are flow diagrams illustrating how print driver  84  sets the value for the smear timer, and sets the density threshold for smear control, based on current status of printer  10 . Thus, in FIG. 74, driver  84  calculates the value of the smear timer based on printer status and sends the value of the smear timer to printer  10 . Specifically, in step S 7401 , driver  84  obtains printer status in the form of printer status temperature TenvL. Step S 7402  tests the value of the temperature to determine whether it is in a nominal range between T 1  and T 2 . Typical values for the range are between 15° C. and 35° C. If the printer status temperature TenvL is within the range T 1  and T 2 , then the smear timer is set to a first value which contemplates short ink dry times coupled with lowered probability of ink smear (step S 7404 ). On the other hand, if the printer status temperature TenvL is outside the range of T 1  and T 2 , then driver  84  selects a second smear timer value which is larger than the first smear timer value, and which contemplates both longer ink drying times coupled with higher probability of smear. In step S 7407 , driver  84  sends the selected smear timer value to printer  10 . 
     FIG. 75 illustrates process steps by which driver  84  modifies the dot density threshold based on printer status, and sends the modified value to printer  10 . Thus, in step S 7501 , driver  84  obtains printer status in the form of printer status temperature TenvL. In step S 7502 , driver  84  tests the printer status temperature to determine whether it falls within a range of T 1  to T 2 , such as between 15° C. and 35° C. If the printer status temperature falls within the range of T 1  to T 2 , then a first density threshold value is selected which contemplates relatively fast ink drying times coupled with a correspondingly high density threshold. On the other hand, if the printer status temperature falls outside the range T 1  to T 2 , then flow advances to steps S 7505 , or S 7506 , as appropriate, in which the smear threshold is set to a second value lower than the first value, which contemplates relatively long ink drying times coupled with a correspondingly lower density threshold. In step S 7507 , driver  84  sends the selected density threshold to printer  10 . 
     10.4 Automatic Sheet Feed (ASF) Speed 
     FIGS. 76 and 77 are flow diagrams for explaining how print driver  84  modifies speed at which printer  10  feeds sheets from the feed tray, based on printer status or based on printer status and current environment of computing equipment  1 . 
     In the embodiments of FIGS. 76 and 77, printer status that is used to modify feed speed is the printer status temperature TenvL. Specifically, at lower temperatures, sheets in the feed tray tend to be more slippery, because of a combination of reduced friction at lower temperatures coupled with a hardening of the rubberized sheet feed rollers in printer  10 . Accordingly, at lower temperatures, a slower but more certain feed speed is selected; on the other hand, at higher temperatures, a quicker feed speed is selected because of the relative ease at which recording media are fed. 
     Thus, as shown in FIG. 76, in step S 7601  print driver  84  obtains printer status temperature TenvL from printer  10 , and in step S 7602  determines whether the temperature is below a predetermined threshold such as 18° C. If the printer status temperature is less than or equal to the determined threshold, then the speed at which sheets are fed by the automatic sheet feeder is reduced to a slower speed (step S 7604 ). On the other hand, if the temperature is high enough, meaning that recording media may be fed with greater certainty even at a high speed, then print driver  84  selects a high speed for automatic sheet feeding. 
     In step S 7606 , print driver  84  sends the selected feeding speed to printer  10 , using a parameter in the [LOAD] command. 
     FIG. 77 illustrates an embodiment in which both the printer status and the current environment of computing equipment  10  are used in coordination by driver  84  so as to select the speed of the sheet feeding. Specifically, in the embodiment of FIG. 77, a slower (and consequently less noisy) speed of feed is always selected at nighttime, as determined by print driver  84  from the current configuration of computing equipment  1 . On the other hand, in daytime, a high feed of sheet feed is selected so long as printer status temperature is large enough; otherwise, a low speed of sheet feed is selected. 
     Thus, in step S 7701 , print driver  84  gets current printer status temperature TenvL, and in step  7702  print driver  84  obtains current configuration and time of day from computing equipment  1 . In step S 7703 , print driver  84  determines, based on time of day, whether it is nighttime, for example, by comparing time of day to determine whether it lies in the range of 5:00 a.m. to 10:00 p.m. If time of day is outside the normal daytime range, then flow advances to step S 7705 , in which a slow speed for sheet feed is always selected. 
     On the other hand, if in step S 7703  the print driver  84  determines that it is not nighttime, then flow advances to step S 7706  in which print driver  84  determines whether printer status temperature TenvL is high enough so as to select a high speed of sheet feed. If printer status temperature is large enough, then a high speed is selected (step S 7708 ), whereas if temperature is not high enough, then a low speed is selected (step S 7707 ). 
     Flow then advances to step S 7710  in which print driver  84  sends the selected speed of sheet feed to printer  10  using a parameter in the [LOAD] command. 
     10.5 Prefire Timing 
     FIG. 78 is a flow diagram for explaining how print driver  84  modifies the operational parameter of printer  10  that controls the timing for prefire operations, based on status of the printer. 
     In the embodiment of FIG. 78, the printer status that affects prefire timing is printer status temperature TenvL. Specifically, at lower operating temperatures, ink tends to be more viscous, meaning that more frequent prefirings are needed; consequently, a lower prefire timing interval is selected. On the other hand, at higher operating temperatures, ink is less viscous, meaning that less frequent prefirings are needed with a correspondingly higher prefire timing interval. 
     Thus, in step S 7801 , print driver  84  obtains printer temperature status TenvL, and in step S 7802  compares the printer status temperature to a fixed threshold such as 18° C. If the temperature is less than the threshold, then a default relatively short prefire interval is selected, such as prefiring every three seconds. On the other hand, if the temperature is larger than the threshold, then a relatively long prefire interval is selected, such as six seconds. In any event, flow thereafter advances to step S 7806  in which print driver  84  sends the selected prefire interval to printer  10  using the [PREFIRE_CYC] command. 
     10.6 Delay of Manual Feed 
     FIGS. 79 and 80 are views for explaining how print driver  84  modifies its own operation based on status of printer  10 . 
     FIG. 79 shows a portion of user interface  690  displayed by print driver  84  on display  2 . FIG. 79 shows a “setting” tabbed dialog for user interface  690 , and as shown in FIG. 79, the tabbed dialog includes a region  691  which permits the user to set media type, size and orientation, as well as a check box  692  which permits the user to specify that he will feed paper manually and that automatic sheet feed operations should be bypassed. Upon selection of check box  692 , the print driver will command printer  10  so as to cause media inserted at manual feed slot  17  (see FIG. 3) to be drawn into printer  10 , rather than automatic sheet feeding from supply tray  14 . 
     However, as explained in connection with FIGS. 5A and 5B, a single motor  34  is utilized for many different functions including line feed operations for a currently-printing sheet, sheet feed operations for a sheet from feed tray  14 , and purging operations in purge unit  46 . It is therefore possible for a user to encounter difficulties if he attempts to feed a sheet manually before the printer is ready, for example, before the printer has completed a purge operation. 
     According to this embodiment of the operation, therefore, print driver  84  modifies its operation based on status of printer  10 , so as to display a message requesting the user to delay manual insertion of a sheet until the printer has completed a purge operation, in situations where print driver  84  has been set to a manual feed configuration (through check box  692 ) and current status of the printer indicates that a purge operation is on-going. 
     Thus, referring to FIG. 80, in step S 8001 , print driver  84  determines whether check box  692  has been selected by the user, thereby setting the print driver into the manual feed mode. If the check box has not been selected, then automatic sheet feeding proceeds in accordance with operations described above. 
     On the other hand, if manual feed mode has been selected, then in steps S 8002  and S 8004 , print driver  84  obtains status from the printer so as to determine whether a purge operation is on-going. If in step S 8005  the print driver  84  determines that a purge operation is not on-going, then flow proceeds directly to step S 8010  in which the print driver displays a message to the user on display  2 , signifying to the user that a sheet should be inserted manually into the manual feed slot. On the other hand, if a purge operation is on-going, flow branches to step S 8006  in which print driver  84  displays a message on display  2 , signifying that the user should delay insertion of a sheet into the manual feed slot. Specifically, and as explained above, because a single motor is used both for purge operations and sheet feed operations, manual insertion of a sheet into the manual feed slot during purge operations might possibly result in a failed sheet feed operation. 
     The message of step S 8006  remains displayed until printer status returned from the printer to print driver  84  signifies that the purge operation has been completed (steps S 8007  and S 8008 ). When printer status indicates that the purge operation has been completed, flow advances to step S 8010  where, as before, print driver  84  displays a message to the user signifying that it is safe to insert a sheet into the manual feed slot. 
     In step S 8011 , print driver  84  waits for the user to signify that he has inserted a sheet into the manual feed slot, whereafter flow advances to step S 8012  in which print driver  84  commands printer  10  to load paper from the manual feed slot using the [LOAD] command. 
     10.7 Purge Speed 
     FIG. 81 illustrates modification of purge speed in printer  10  by print driver  84  based on status of printer  10  or based on status of printer  10  coupled with current configuration of computing equipment  1 . 
     Operations in FIG. 81 that are performed by print driver  84  are delineated with dotted line  695 . As shown within those dotted lines, functions performed by print driver  84  include a step to obtain current status of printer  10 , to obtain current configuration of computing equipment  1 , to modify purge speed so as to achieve either a slow purge speed or a fast purge speed, and to command a purge operation. 
     In more detail, in steps S 8101  and S 8102 , print driver  84  obtains printer status temperature TenvL and configuration information for computing equipment  1 . In step S 8103 , print driver  84  sets the purge speed. The purge speed is set based on the printer status temperature, or based on the printer status temperature coupled with the current configuration of computing equipment  1 . Specifically, and as shown in connection with similar operations for selection of sheet feed speed in FIGS. 76 and 77, purge speed can be selected based only on printer status (for example, a high purge speed for a quick purge at the low ink viscosities that occur at high printer status temperatures), or based on printer status temperature coupled with time of day (for example, a low and quiet purge speed for nighttime operations, and a purge speed selected based on printer status temperature for daytime operations). 
     In step S 8104 , and at times when printer purging is needed, print driver  84  causes the printer to execute purge operation, for example, by transmission of a [RECOVER] command. 
     In response to receipt of a command for purging, printer  10  is controlled by print controller  110  to execute purge operations as shown in FIG. 81, in accordance with either the slow or the high speed set by the print driver. 
     By virtue of the foregoing arrangement, good purge results are obtained, even at a high purge speed that results in a quick purge operation, since the purge speed is selected based on printer status and is consequently tailored for specific aspects of the printer status. 
     10.8 Compression Mode 
     FIG. 82 illustrates modification of print driver operations based on status of printer  10 . Here, modifications of the print driver operations concern modifications over whether or not compression of print data is performed prior to transmission of such print data to the printer. The decision as to whether or not compressed data is sent is made based on printer status, which in this case is printer status that indicates whether or not DMA (direct memory access) is enabled in the printer firmware. 
     By way of explanation, print data compression is performed as a default operation in print driver  84  so as to compress and thereby minimize the amount of print data that is transmitted to the printer. Although transmission time is minimized by transmitting compressed data, time is also expended in compressing the data on the print driver side, and in decompressing the data on the printer side. 
     If DMA mode is enabled in the printer firmware, then print driver  84  is able to send print data directly to print data buffer  109 , ordinarily without the involvement of print controller  110 . In DMA mode, the time needed to deposit uncompressed print data directly into print data buffer  109  is less than the amount of time to compress print data, to transmit compressed print data, and to decompress the print data into the print data buffer. Accordingly, if print driver  84  determines that DMA mode is enabled in the firmware for printer  10 , then print driver  84  modifies its operation by transmitting uncompressed data directly into print data buffer  109 , rather than by compressing the print data and transmitting the compressed print data to controller  110 . FIG. 82 illustrates this operation. 
     Thus, in step S 8201 , print driver  84  obtains printer status in the form of status information that indicates whether firmware in the printer has a DMA capability and whether such a capability is enabled. If driver  84  determines that DMA mode is enabled (step S 8202 ), then print driver  84  turns off print data compression (step S 8204 ), and DMA&#39;s uncompressed print data directly to print data buffer  109  (step S 8205 ). On the other hand, if print driver  84  determines from the printer status that DMA is not enabled, then print driver  84  maintains its default mode of operation, whereby it compresses print data prior to transmission (step S 8206 ) and transmits compressed print data to print controller  110  (step S 8207 ). 
     The invention has been described with respect to particular illustrative embodiments. It is to be understood that the invention is not limited to the above-described embodiments and that various changes and modifications may be made by those of ordinary skill in the art without departing from the spirit and scope of the invention.