Patent Publication Number: US-7596339-B2

Title: Image forming apparatus including an interchangeable engine and an interchangeable paper feeding and outputting system

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus including an interchangeable engine as well as an interchangeable paper feeding and outputting system. 
   2. Description of the Related Art 
   Some image forming apparatuses including copiers and printers for performing only black and white printing or both black and white printing and color printing are known. 
   Also, by connecting a different apparatus to such an image forming apparatus, some image forming systems can be provided that have capabilities that otherwise could not be realized by the image forming apparatus alone. 
   Japanese Patent Laid-Open No. 11-292335 describes a copier formed by connecting an image reader unit to an image forming apparatus and capable of copying the image of an original document read out by the image reader unit. In addition, an image forming apparatus including a plurality of interchangeably stacked paper feeder units, which also serve as a mounting base of the image forming apparatus, has been proposed so that various types of feeder units can be used. Furthermore, some image forming apparatuses are known that are capable of being connected to a post-print processing apparatus (an accessory) that sorts or staples printed recording sheets. 
   Additionally, a variety of image forming apparatuses that are capable of having an optional unit attached thereto have been developed. For example, for some image forming apparatuses, although the standard functionality of the image forming apparatus is minimized, a duplex transport unit for turning over a recording sheet after one-side printing can be optionally attached to the image forming apparatuses. As noted above, by designing some units in the image forming apparatus to be removable, the image forming apparatus meets a user&#39;s specific requirements regarding usage of the image forming apparatus. 
   In general, a user selects an image forming apparatus that provides the functionality desired by the user, performance (such as black and white printing/color printing and the number of output pages per minute), and ease of use (such as the dimensions and the position of outputting sheets) from among various product lines of the image forming apparatus. Furthermore, when the user desires functionality and performance that are not provided by the image forming apparatus (such as duplex printing, sorting, or stapling), the user selects a configuration by, as described above, assembling an optional accessory, an optional apparatus, or an optional unit to the image forming apparatus so that the user can obtain the desired functionality and performance. By cooperating with the connected accessory, apparatus, or unit, the image forming apparatus can provide a variety of operations, which is convenient for the user. 
   However, the configuration and available accessories of an existing image forming apparatus are designed so that most typical users can comfortably use the image forming apparatus, and therefore, the image forming apparatus cannot flexibly provide the functionality that individual users desire. 
   That is, the existing image forming apparatus has a configuration so as to perform an operation in cooperation with the accessories, various apparatus, or units. Thus, the existing image forming apparatus can provide operations according to an operating mode, functionality, and performance available only in such a configuration. For example, when a feeder unit or an accessory is connected to the image forming apparatus, the total operating performance of the image forming apparatus may be limited depending on the combination of the image forming apparatus and a feeder unit (or an accessory). In addition, depending on the configuration of an image forming unit, a feeder unit, and a paper transport unit in the image forming apparatus, the total operating performance of the image forming apparatus is determined. As a result, the image forming apparatus does not flexibly provide the functionality that individual user desire. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the problems described above by providing an image forming apparatus that flexibly provides the functionality that individual users desire. 
   According to an aspect of the present invention, an image forming apparatus includes an interchangeable image forming subsystem having an image bearing member, an exposure unit, a charging unit, and a developing unit, an interchangeable sheet transport subsystem for transporting a sheet medium in the image forming apparatus, a mounting base for removably supporting the image forming subsystem and the sheet transport subsystem, and a control unit for controlling the operation of the image forming apparatus. The mounting base is capable of mounting one of a plurality of the image forming subsystems having different performances and one of a plurality of the sheet transport subsystem having different specifications thereon. The control unit controls the operation of the image forming apparatus in accordance with a combination of the mounted image forming subsystem and the sheet transport subsystem. 
   Further features and advantages of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate numerous exemplary embodiments of the invention, and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is an illustration of an exemplary hardware configuration of an image forming apparatus according to a first exemplary embodiment of the present invention. 
       FIG. 2  is a cross-sectional view of a first example of the interchangeable configuration of an image forming subsystem. 
       FIG. 3  is a cross-sectional view of a second example of the interchangeable configuration of an image forming subsystem. 
       FIG. 4  is a cross-sectional view of a second example of the interchangeable configuration of an image forming subsystem. 
       FIGS. 5A and 5B  are cross-sectional views of examples of the interchangeable configuration of a paper transport platform. 
       FIGS. 6A and 6B  are cross-sectional views illustrating an exemplary configuration of a feeder unit. 
       FIGS. 7A and 7B  are cross-sectional views illustrating an exemplary configuration of a transport unit. 
       FIG. 8  is a perspective view of a printer engine when the image forming subsystem is pulled out from the paper transport platform. 
       FIGS. 9A and 9B  are partial magnified views of a positioning mechanism of the image forming subsystem. 
       FIG. 10  is a cross-sectional view of an image forming subsystem for a 4D full-color printer. 
       FIG. 11  is a cross-sectional view of an image forming subsystem for a 1D full-color printer. 
       FIG. 12  is a cross-sectional view of an image forming subsystem for a 1D black and white printer. 
       FIG. 13  is a block diagram illustrating an exemplary configuration of electrical connection of an image forming apparatus according to the first embodiment. 
       FIG. 14  is a block diagram of a 4D full-color image forming subsystem. 
       FIG. 15  is a timing diagram illustrating the image forming timing of the 4D full-color image forming subsystem. 
       FIG. 16  is a block diagram of a 1D full-color image forming subsystem. 
       FIG. 17  is a timing diagram illustrating the image forming timing of the 1D full-color image forming subsystem. 
       FIG. 18  is a block diagram of a 1D black and white image forming subsystem. 
       FIG. 19  is a timing diagram illustrating the image forming timing of the 1D black and white image forming subsystem. 
       FIGS. 20A-20C  illustrate parameters of configuration communication when the power is turned on. 
       FIGS. 21A and 21B  illustrate parameters of configuration communication when the power is turned on. 
       FIGS. 22A and 22B  illustrate the command sequence of the configuration information in detail when the power is turned on. 
       FIGS. 23A-23F  illustrate communication parameters used when the image forming operation is performed. 
       FIGS. 24A and 24B  illustrate the command sequence during the image forming operation. 
       FIG. 25  is an illustration of an exemplary hardware architecture of an image forming apparatus according to a second embodiment of the present invention. 
       FIG. 26  is a block diagram of the electrical connection of the image forming apparatus according to the second embodiment. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   Exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The following description of exemplary embodiments is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. All of the features and the combinations thereof described in the embodiments are not necessarily essential to the invention. 
   Hardware Configuration according to First Exemplary Embodiment 
   System Architecture 
     FIG. 1  is an illustration of an exemplary hardware configuration of an image forming apparatus according to a first exemplary embodiment of the present invention. 
   According to the first exemplary embodiment, the image forming apparatus is a multifunction printer (MFP) including an electrophotographic printer engine  100 . The image forming apparatus receives data from a scanner, a facsimile, a copier, and a personal computer (PC) and serves as a printer that prints the received data. The printer engine  100  has color print capability using a photoconductor and intermediate transfer method. 
   The printer engine  100  is a main component of the image forming apparatus for printing. The printer engine  100  converts an original document image to image information and prints the image information. In the printer engine  100 , a paper transport subsystem (hereinafter referred to as a “paper transport platform”)  60  and an image forming subsystem  150  are mounted on an engine platform  101  serving as a mounting base. Additionally, on the paper transport platform  60 , a feeder unit  70  and a transport unit  80  are mounted. A power supply unit  90  is mounted on the engine platform  101 . 
   A document feeder unit  280  feeds a document set on the document feeder unit  280  to the readout position on an image reader unit  270 . An image of the document fed to the readout position on an image reader unit  270  is converted to image information by the image reader unit  270 . The image information is delivered to a controller  250 . The controller  250  performs a desired process on the image information and delivers the processed image information to the printer engine  100 . The information about the readout document image is printed by the printer engine  100  so that the copying function of the document image is realized. 
   An operation unit  260  is used when a user inputs a print mode, a print count, and print conditions or a service person performs a maintenance operation. When a print start key (not shown) of the operation unit  260  is depressed, the readout operation of a document image starts and a desired operation, such as a printing operation performed by the printer engine  100  or transmission of the document image, also starts. 
   Example of Replaceable Structure of Image Forming Subsystem 
   According to the present embodiment, by configuring an image forming subsystem primarily for forming an image to be interchangeable, the following advantages are provided to users and service persons. Hereinafter, as configurations of an interchangeable image forming subsystem, three types of printer engine  100  having different performances are described with reference to  FIGS. 2 to 4 . 
     FIG. 2  is a cross-sectional view of a first example of the interchangeable configuration of the image forming subsystem  150 . In this example, the color printer engine  100  including the color image forming subsystem  150 A as the image forming subsystem  150  is used. The color image forming subsystem  150 A is based on a four-drum tandem method (hereinafter referred to as a 4D method) and is assembled into the engine platform  101 . The color image forming subsystem  150 A includes four photoconductive drums serving as image bearing members, an exposure unit, a charging unit, and a developing unit. In particular, the image forming subsystem  150 A is suitable for high-productivity color image formation. The image forming subsystem  150 A may be used for high-volume printing, such as in an office, or may be used for low-volume printing. Additionally, the color image forming subsystem  150 A may be replaced with a variety of image forming subsystems  150 , one of which has, for example, A4 20-ppm (pages per minute) or 70-ppm color printing capability as needed. 
     FIG. 3  illustrates an example of the configuration of the color printer engine  100  in which a color image forming subsystem  150 B of a one-drum method is assembled in the engine platform  101  as the image forming subsystem  150 . The color image forming subsystem  150 B includes one photoconductive drum serving as an image bearing member, an exposure unit, a charging unit, and a developing unit. In particular, the image forming subsystem  150 B is suitable for high-quality color image formation, such as photo printing document or graphic design. The color image forming subsystem  150 A may be replaced with a variety of the image forming subsystems  150 , one of which has, for example, 400-dpi (dots per inch), 600-dpi, or 1200-dpi resolution printing capability or has the capability of using a variety of toner and transfer media as needed. 
     FIG. 4  illustrates an example of the configuration of the black and white printer engine  100  in which a black and white image forming subsystem  150 C of a one-drum method is assembled into the engine platform  101  as the image forming subsystem  150 . The black and white image forming subsystem  150 C includes one photoconductive drum serving as an image bearing member, an exposure unit, a charging unit, and a developing unit. In particular, the image forming subsystem  150 C may be used for high-volume printing, such as in an office, or may be used for low-volume printing. Additionally, the image forming subsystem  150 C may be replaced with a variety of image forming subsystems  150 , one of which has, for example, A4 20-ppm (pages per minute) or 100-ppm black and white printing capability as needed. 
   Example of Replaceable Structure of Paper Transport Platform  60   
     FIG. 5A  is a cross-sectional view of the paper transport platform  60  into which a feeder unit  70 A and a transport unit  80 A are assembled.  FIG. 5B  is a cross-sectional view of the paper transport platform  60  into which a feeder unit  70 B and a transport unit  80 B are assembled. The paper transport platform  60  is mounted on the engine platform  101 . In  FIGS. 5A and 5B , the paper transport platform  60  including the feeder units  70 A and  80 A having different specifications and the paper transport platform  60  including the feeder units  70 B and  80 B having different specifications are illustrated. However, the combinations of the units are not limited thereto. For example, the combination of the feeder unit  70  and the transport unit  80  appropriate for the requirement and specification for the product may be selected and may be assembled into the paper transport platform  60 . By identifying the assembled unit or communicating with the assembled unit, a platform control unit  65  collects control information associated with the assembled unit and exchanges that control information with a printer engine control unit  105 . The platform control unit  65  then performs control of the paper transport platform  60  on the basis of the control specification determined by the printer engine control unit  105 . 
   By configuring the paper transport platform  60 , which is mounted on the engine platform  101  and primarily provides a paper transport function, so that the transport unit  80  and the feeder unit  70  are interchangeable, many configurations of the product can be provided. 
   The examples of the configuration of the printer engine  100  are described next with reference to  FIGS. 5A  and  5 B as an interchangeable configuration of the paper transport platform  60 . That is, two types of printer engine  100  in which the same image forming subsystem  150  is used and the transport units  80  and the feeder units  70  on the paper transport platform  60  mounted on the engine platform  101  are interchangeable are illustrated. 
   For example, as shown in  FIG. 5A , the paper transport platform  60 A of a slow speed type including a feeder unit  70 A and a transport unit  80 A is combined with the image forming subsystem  150 . In contrast, as shown in  FIG. 5B , the paper transport platform  60 B of a high speed type including a feeder unit  70 B and a transport unit  80 B is combined with the image forming subsystem  150 . 
   Thus, the printer engine  100  can be selectively configured by combining the feeder unit and the transport unit in the paper transport platform  60  with the image forming subsystem  150  having the desired image quality and specification. 
   Hardware Configuration of Feeder Unit and Transport Unit 
   The feeder unit  70  and the transport unit  80  are described next. 
     FIGS. 6A and 6B  are cross-sectional views illustrating the configuration of the feeder unit  70 . Feeder units having different types of performance are interchangeably connected to the paper transport platform  60 . As the feeder units having different performances, the slow-speed feeder unit  70 A and the high-speed feeder unit  70 B are described next. As shown in  FIG. 6A , the slow-speed feeder unit  70 A includes a DC brushless motor  501 , a pickup roller  502  rotatably driven by the DC brushless motor  501 , a transport roller  503  rotatably driven by the DC brushless motor  501 , a paper feed path  511 , and a paper refeed path  512  in which a transfer medium P passes. 
   The feeder unit  70 A is controlled by the platform control unit  65  or a feeder unit controller (not shown) in the feeder unit  70 A. The DC brushless motor  501  rotates at a predetermined speed. In a paper feed operation, the pickup roller  502  is controlled by, for example, a solenoid (not shown) so as to be brought into contact with the transfer medium P or be separated from the transfer medium P at a predetermined timing. The transfer medium P is stored in a feeder cassette  505 . The pickup roller  502  driven by the DC brushless motor  501  is brought into contact with the transfer medium P to pick up the transfer medium P. The transfer medium P is then fed into the paper feed path  511  and is transported by the transport roller  503  in the paper feed path  511  towards the image forming subsystem  150  at a predetermined speed. The transfer medium P re-fed from a transport unit, which is described below, passes through the paper refeed path  512  and is transported by the transport roller  503  to the image forming subsystem  150 . 
   The high-speed feeder unit  70 B shown in  FIG. 6B  includes a stepping motor  504  for driving the pickup roller  502  and the transport roller  503 . The feeder unit  70 B is controlled by the platform control unit  65  or a feeder unit controller (not shown) in the feeder unit  70 B. The stepping motor  504  rotates at a predetermined variable speed. In a paper feed operation, the pickup roller  502  is controlled by, for example, a solenoid (not shown) so as to be brought into contact with the transfer medium P or be separated from the transfer medium P at a predetermined timing. 
   The transfer medium P is stored in a feeder cassette  505 . The pickup roller  502  driven by the stepping motor  504  is brought into contact with the transfer medium P to pick up the transfer medium P. The transfer medium P is then fed into the paper feed path  511  and is transported by the transport roller  503  in the paper feed path  511  towards the image forming subsystem  150  at a predetermined speed. The transfer medium P re-fed from a transport unit, which is described below, passes through the paper refeed path  512  and is transported by the transport roller  503  towards the image forming subsystem  150 . At that time, the transport speed of the transfer medium P is variably controlled in accordance with the variable rotational speed of the stepping motor  504  so that the transport speed of the transfer medium P and the spacing between the successively fed transfer media P can be controlled in a stepwise fashion in a wide range. 
   While the above-described description of the feeder unit  70  has been made with reference to a one-feeder station, the one-feeder station is not intended to be limited to such applications. The present embodiment is applicable to multiple stacked (joined or connected) feeder stations so that a plurality of types and sizes of the transfer medium P are available. 
     FIGS. 7A and 7B  are cross-sectional views illustrating the configuration of the transport unit  80 . 
   One of transport units having different performances is interchangeably connected to the paper transport platform  60 . As the transport units having different performances, the slow-speed transport unit  80 A and the high-speed transport unit  80 B are described next. 
   The slow-speed transport unit  80 A shown in  FIG. 7A  includes a stepping motor  520 , a DC brushless motor  521 , a paper output roller  522  rotatably driven by the stepping motor  520  in the clockwise and counterclockwise directions, transport rollers  523  and  524  driven by the DC brushless motor  521 , a paper output path  525 , and a transport path  526 . 
   The transport unit  80  is controlled by the platform control unit  65  or a transport unit controller (not shown) in the transport unit. The stepping motor  520  is controlled so as to rotate in the clockwise direction and the counterclockwise direction in accordance with an operating mode. The DC brushless motor  521  rotates at a predetermined speed. In a transport operation, the transfer medium P transported from a fixing unit  180  of the image forming subsystem  150  is fed into the paper output path  525 . To output the transfer medium P, the paper output roller  522  rotates in a direction so as to output the transfer medium P to an output tray  527 . Thus, the transfer medium P is output to the output tray  527 . When the transfer medium P is transported in a reverse direction for duplex printing, the paper output roller  522  rotates in a direction so as to output the transfer medium P. The stepping motor  520  stops and rotates in the reverse direction with the paper output roller  522  pinching the trailing edge of the transfer medium P. Thus, the paper output roller  522  stops and rotates in the reverse direction so that the transfer medium P is transported to the transport path  526 . The transfer medium P is transported in the transport path  526  by the transport rollers  523  and  524  driven by the DC brushless motor  521 , which is rotating at a predetermined speed. The transfer medium P is then fed to the paper refeed path  512  of the feeder unit  70 . 
   The high-speed transport unit  80 B shown in  FIG. 7B  includes stepping motors  531  and  532 . The stepping motor  531  rotates the transport roller  523  whereas the stepping motor  532  rotates the transport roller  524 . The transport unit  80 B is controlled by the platform control unit  65  or a transport unit controller (not shown) in the transport unit  80 B. The stepping motors  520 ,  531 , and  532  rotate in predetermined directions at predetermined variable speeds. 
   In a transport operation, the transfer medium P transported from the fixing unit  180  of the image forming subsystem  150  is fed into the paper output path  525 . To output the transfer medium P, the paper output roller  522  rotates in a direction so as to output the transfer medium P to an output tray  527 . Thus, the transfer medium P is output to the output tray  527 . When the transfer medium P is transported in a reverse direction for duplex printing, the paper output roller  522  rotates in a direction so as to output the transfer medium P. The stepping motor  520  stops and rotates in the reverse direction with the paper output roller  522  pinching the trailing edge of the transfer medium P. Thus, the paper output roller  522  stops and rotates in the reverse direction so that the transfer medium P is transported to the transport path  526 . 
   The transfer medium P is transported in the transport path  526  by the transport roller  523 , which is rotated by the stepping motor  531  at a variably controlled speed, and the transport roller  524 , which is rotated by the stepping motor  532  at a variably controlled speed. The transfer medium P is then fed to the paper refeed path  512  of the feeder unit  70 . At that time, the transport speed of the transfer medium P is variably controlled in accordance with the variable rotational speeds of the stepping motors  531  and  532  so that the transport speed of the transfer medium P and the spacing between the successively fed transfer media P can be controlled in a stepwise fashion in a wide range. 
   Method of Replacing Image Forming Subsystem and Unit 
     FIG. 8  is a perspective view of the printer engine  100  when the image forming subsystem  150  is pulled out from the engine platform  101 . 
   In the first embodiment, the image forming subsystem  150  is pulled out from the engine platform  101  with the front cover  810  of the printer engine  100  open. The image forming subsystem  150  is connected to the engine platform  101  and the paper transport platform  60  using left and right slide rails  811 . The image forming subsystem  150  can be pulled out and removed from the engine platform  101  by operating a removal knob  100   a.  When the image forming subsystem  150  is removed from the engine platform  101 , an image producing unit  170  and the fixing unit  180  mounted on the image forming subsystem  150  are also removed. 
   The feeder unit  70  and the transport unit  80  in the paper transport platform  60  are described next. 
   Like the image forming subsystem  150 , the feeder unit  70  is connected to the paper transport platform  60  via left and right slide rails  812  and can be pulled out and removed. Like the feeder unit  70 , the transport unit  80  is connected to the paper transport platform  60  via left and right slide rails  813  and is can be pulled out and removed. 
   When the weights of the image forming subsystem  150  and the other units are relatively small or strictly precise mounting positions are not required, the above-described slide rails may be inexpensive ones. In addition, when relatively high precision is required, a variety of linear sliding guides (guide rails) can be employed. Thus, the operability, precision, reliability, and durability can be increased. 
   To mount a relocatable (connectable and removable) component in an image forming apparatus, the maintainability should be taken into account as well as the position of the component. Additionally, when considering a market requirement for features and serviceability, some apparatuses allow a user to remove or relocate the component. In such a type of usage, in particular, the safety of the user when operating a heavy component should be taken into account. In addition, it is desirable that the apparatus has sufficient hardness and rigidity so that the apparatus is not damaged even when the user roughly operates the apparatus. 
   For example, the front cover  810  includes a locking mechanism. When a user can carry out a maintenance operation of the apparatus, the front cover  810  is unlocked so that the front cover  810  is openable. However, when the user cannot carry out the maintenance operation of the apparatus, the front cover  810  is locked so that the front cover  810  cannot be open. After a service person unlocks the locking mechanism, the image forming subsystem  150  can be pulled out from the apparatus. Accordingly, the user has little chance to unintentionally touch the internal component, and therefore, the safety can be maintained. In addition, since the service person pulls out the internal component after a predetermined procedure is carried out, the apparatus can be operated more safely. 
   Positioning Configuration of Image Forming Apparatus 
     FIGS. 9A and 9B  are partial magnified views of a positioning mechanism of the image forming subsystem  150 .  FIG. 9A  illustrates the image forming subsystem  150  and the engine platform  101  (or the paper transport platform  60 ) before installation.  FIG. 9B  illustrates the image forming subsystem  150  and the engine platform  101  (or the paper transport platform  60 ) after the installation. 
   It is important to design a configuration that can provide a removal operation having a high operational performance as well as the required precision and cost for removal of the image forming subsystem  150  by the user. To achieve such a configuration, the configuration of a removal mechanism and a method and configuration of the positioning mechanism  120  are key factors. 
   An example of a configuration that satisfies the required positioning precision by using a positioning pin  115 , a positioning hole  119 , and a release knob  100   a  while improving the user operability is described next. It should be understood that a variety of embodiments in addition to the present exemplary embodiment can be provided within the spirit and scope of the present invention. In the present embodiment, a method of using a positioning pin is discussed. 
   To obtain a smooth positioning operation, an optimal design of the shapes of the positioning pin  115  and the positioning hole  119  is discussed in addition to the positional relationship between the axis of the positioning pin  115  and a hole (i.e., a fitting method). That is, the positioning pin  115  is used when the positioning precision is required. The shape of the positioning pin  115  is determined depending on the required precision, the improvement level of the reliability, and ease of user operation. 
   The precision of the shape of a used component and the mounting precision of the component are determined depending on the required positioning precision and the level of the precision of components that form the positioning pin  115  and the positioning hole  119 . Additionally, the length of an interface between the positioning pin  115  formed on the image forming subsystem  150  (e.g., one of the image forming subsystem  150 A-C) and the positioning hole  119  formed on the engine platform  101  or the paper transport platform  60  is determined depending on the level of the operability and workability. 
   The inner diameter and the position of the positioning hole  119  are determined so that the required positioning tolerance with respect to the image forming subsystem  150  is satisfied. If needed, the precision of the right angle between the positioning hole  119  and the positioning pin  115  may be increased. The reference plane of the shape of the positioning pin  115  inserted into the positioning hole  119  is determined so that the position of the hole relative to the surface of the pin is precisely determined. Thus, by optimally designing the fitness between the positioning pin  115  and the positioning hole  119 , the precision of the relative position between the paper transport platform  60  and the image forming subsystem  150  can be set within the required precision range. 
   To increase the operability, it is desirable that the entrance of the fitness has a shape having a large chamfered edge so that the positioning pin  115  is smoothly inserted and removed. Accordingly, the diameter and the nose shape of the shaft of the positioning pin  115  are determined depending on the length of a tapered portion of the positioning pin  115  and the offset level of the center of the inserted positioning pin  115  from the center of the positioning hole  119 . 
   The guide length of positioning can be determined depending on the operability and the improvement level of the reliability of the apparatus. As shown in  FIGS. 9A and 9B , the nose of the positioning pin  115  is slightly tapered so as to be easily guided when inserted into the hole. In particular, since the image forming subsystem  150  includes a variety of components required for realizing an image forming function, the image forming subsystem  150  is anticipated to be heavy. For example, for the color image forming subsystems  150 A and  150 B that carry out color image formation, further careful consideration of the operability is desirable. 
   In contrast, for the image forming subsystem  150 C that carries out a black and white image formation, for example, the weight of the high-speed black and white image forming subsystem  150 C for providing high productivity is anticipated to be substantially the same as that of the color image forming subsystem  150 A or  150 B. Additionally, the weight of the middle-speed black and white image forming subsystem  150 C is anticipated to be substantially the same as that of the color image forming subsystem  150 A or  150 B or lighter than that of the color image forming subsystem  150 A or  150 B. 
   Thus, it is desirable that a positioning mechanism provides ease of user operation in addition to the desired safety, durability, reliability, and high precision even when any one of a variety of the image forming subsystems  150  is connected. 
   In contrast, if the model of the image forming subsystem  150  is relatively light-weighted or the required positioning precision is not strict, the removable mechanism and a positioning mechanism  120  can be changed to a relatively low-cost structure. Thus, cost reduction can be achieved. 
   According to the present exemplary embodiment, as shown in  FIG. 8 , the engine platform  101  of the image forming apparatus includes a removable mechanism using slide mechanisms  811  to  813  so that the image forming subsystem  150  can be pulled out. In such a structure in which the image forming subsystem  150  is removable, the positioning between a toner image that is to be transferred to the transfer medium P and the transfer medium P is critical. Therefore, a position detecting unit  112  is provided to the engine platform  101  or the paper transport platform  60  in order to detect a position between the image forming subsystem  150  and the engine platform  101  or between the image forming subsystem  150  and the paper transport platform  60  when the image forming subsystem  150  is mounted in the printer engine  100 . 
   For a position sensor used in the position detecting unit  112 , a small and low-cost optical displacement sensor has been developed and can be used in this embodiment. One of the examples of the sensor is a micro-displacement sensor available from OMRON Corporation. It should be appreciated that a position sensor other than an optical position sensor can be employed. The micro-displacement sensor available from OMRON Corporation (Z4DB02) has the following specification: the detectable distance is 9.5 mm±3 mm and the detecting resolution is ±50 μm. In the image forming subsystem  150  having a 400-dpi resolution, the size of one dot (pixel) is 25.4 mm/400 dots=63.5 μm. Therefore, the micro-displacement sensor can detect the displacement less than one-dot size (one-pixel size). In the image forming subsystem  150  having a 600-dpi resolution, the size of one dot (pixel) is 25.4 mm/600 dots=42.3 μm. Therefore, the detecting resolution corresponds to 1.18 dots. In the image forming subsystem  150  having a 1200-dpi resolution, the size of one dot (pixel) is 25.4 mm/1200 dots=21.2 μm. Therefore, the detecting resolution corresponds to 2.36 dots. 
   However, the detection of the relative position between the image forming subsystem  150  and the engine platform  101  or between the image forming subsystem  150  and the paper transport platform  60  is related to the detection of the relative position between an image to be printed and the transfer medium (transfer sheet) P. Accordingly, the resolution of about 50 μm is sufficient. For example, when a margin is 2.5 mm, a resolution of ±50 μm of the micro-displacement sensor in the position detecting unit  112  corresponds to 1/50 of the margin. Accordingly, this resolution is sufficient for a typical printing operation. If more precise position detecting resolution is required, the position detecting resolution can be increased from ±50 μm to ±10 μm by using, for example, the micro-displacement sensor Z4DB01 available from OMRON Corporation. Thus, the resolution of the position detecting unit  112  can be increased to five times higher than that of the micro-displacement sensor Z4DB02. 
   When a micro-displacement sensor is used in the position detecting unit  112 , the detection result from the micro-displacement sensor is output in the form of an analog signal so that the output voltage from the micro-displacement sensor linearly decreases as the distance between the detection object and the micro-displacement sensor increases. Such position information from the micro-displacement sensor in the position detecting unit  112  is used to control the proper image forming position at which an image is printed on the transfer medium P. 
   By operating the release knob  100   a,  the image forming subsystem  150  is horizontally translated so as to be inserted into the engine platform  101 . When the image forming subsystem  150  is contained in the engine platform  101 , a subsystem reference surface  113  provided to a stopper  117 , which serves as a reference position of the image forming subsystem  150 , is brought into contact with a stopper  116  provided to the engine platform  101  or the paper transport platform  60  disposed at a position facing the subsystem reference surface  113 . Thus, the positions of the image forming subsystem  150  in the axis direction of the positioning pin  115  are determined. 
   The position detecting unit  112  is disposed on the stopper  116 . The positioning pin  115  on the image forming subsystem  150  is inserted into the positioning hole  119  of the printer engine, and therefore, the image forming subsystem  150  is contained in the engine platform  101  while maintaining the desired positioning precision. At that time, to detect a physical position of the paper transport platform  60  relative to the image forming subsystem  150 , a position detecting sensor light beam is emitted from the position detecting unit  112  to the subsystem reference surface  113 . The position detecting unit  112  then receives a reflected light beam off the subsystem reference surface  113  so as to detect a position (distance) Ls of the image forming subsystem  150 . The position information (i.e., the distance Ls) detected by the position detecting unit  112  is delivered to the platform control unit  65  of the paper transport platform  60 . Subsequently, position control information is sent from the platform control unit  65  to an image formation control unit  160  so that the image forming position is set to an optimal position on the basis of the detected position information. 
   Alternatively, a reference surface may be provided to the engine platform  101  or the paper transport platform  60  and the position detecting unit  112  may be disposed on the image forming subsystem  150  so that the position information may be sent to the image formation control unit  160 . In addition, while the above-described description has been made with reference to the reference surface  113  as the reference surface of the stopper of the image forming subsystem  150 , a different method and a different location may be added to the subsystem reference surface  113  or may be replaced with the subsystem reference surface  113 . For example, the number of micro-displacement sensors in the position detecting unit  112  may be increased or the micro-displacement sensors may be relocated so that reference surfaces  1132  and  1133  are detected as the different reference surfaces of the stopper  117 . Additionally, for example, the positional offsets of the reference surfaces  113 ,  1132 , and  1133  in three directions may be detected, and therefore, three-dimensional positional offsets of the image forming subsystem  150  are more precisely detected and may be used for the correction control of the image position. 
   In addition, the positioning mechanism  120  may be advantageously located in the vicinity of a mechanism for transferring a toner image onto the transfer medium P. The positions of the transfer roller and the incoming transfer medium P can be more precisely controlled. 
   Details of Image Forming Subsystem  150   
   (A) Hardware Configuration of Image Forming Subsystem for 4D Full-Color Printer 
   The image forming subsystem  150  mounted on the engine platform  101  is described next. 
     FIG. 10  is a cross-sectional view of the detailed structure of the image forming subsystem  150 A for a 4D full-color printer. 
   As shown in  FIG. 10 , the color image forming subsystem  150 A includes an image producing unit  170 A and a fixing unit  180 A. These units can be replaced with different units having the same functionality. In addition, these units can be physically separated. 
   First, the image producing unit  170 A is described. 
   The image producing unit  170 A includes four image forming units, namely, an image forming unit  601 Y for forming an image of a yellow color, an image forming unit  601 M for forming an image of a magenta color, an image forming unit  601 C for forming an image of a cyan color, and an image forming unit  601 BK for forming an image of a black color. These four image forming units  601 Y,  601 M,  601 C, and  601 BK are arranged in a line with a predetermined spacing therebetween. 
   The image forming units  601 Y,  601 M,  601 C, and  601 BK include drum-shaped electrophotographic photoreceptors (hereinafter referred to as “photoconductive drums”)  602 A,  602 B,  602 C, and  602 D as image bearing members, respectively. Around the photoconductive drum  602 A, a primary charger  603 A, a developing unit  604 A, a transfer roller  605 A serving as a transfer unit, and a drum cleaner  606 A are disposed. Similarly, around the photoconductive drum  602 B, a primary charger  603 B, a developing unit  604 B, a transfer roller  605 B serving as a transfer unit, and a drum cleaner  606 B are disposed. Around the photoconductive drum  602 C, a primary charger  603 C, a developing unit  604 C, a transfer roller  605 C serving as a transfer unit, and a drum cleaner  606 C are disposed. Around the photoconductive drum  602 D, a primary charger  603 D, a developing unit  604 D, a transfer roller  605 D serving as a transfer unit, and a drum cleaner  606 D are disposed. Under positions between the primary charger  603 A and the developing unit  604 A, between the primary charger  603 B and the developing unit  604 B, between the primary charger  603 C and the developing unit  604 C, and between the primary charger  603 D and the developing unit  604 D, a laser exposure unit  607  is disposed. 
   The developing units  604 A,  604 B,  604 C, and  604 D contain yellow toner, cyan toner, magenta toner, and black toner, respectively. Each of the photoconductive drums  602 A,  602 B,  602 C, and  602 D includes a photoconductive layer on an aluminum drum base composed of a negatively-charged OPC (opto-photoconductor) and is driven by a drive unit (not shown) to rotate in a clockwise direction shown in  FIG. 11  at a predetermined process speed. 
   The primary chargers  603 A,  603 B,  603 C, and  603 D serving as a primary charging unit uniformly charge the surfaces of the photoconductive drums  602 A,  602 B,  602 C, and  602 D, respectively, at a predetermined negative potential by using a charge bias applied from a charge bias power supply (not shown). The developing units  604 A,  604 B,  604 C, and  604 D contain toner of the above-described colors and deposit the toner onto latent images formed on the photoconductive drums  602 A,  602 B,  602 C, and  602 D, respectively, so as to develop the latent images into visible toner images. 
   The transfer rollers  605 A,  605 B,  605 C, and  605 D serving as a primary transferring unit are disposed in primary transfer units  605 A to  605 D so as to be capable of being in contact with the photoconductive drums  602 A,  602 B,  602 C, and  602 D with an intermediate transfer belt  608  therebetween, respectively. The drum cleaners  606 A,  606 B,  606 C, and  606 D include cleaning blades for removing residual toner remaining on the photoconductive drums  602 A,  602 B,  602 C, and  602 D after primary transfer, respectively. 
   The intermediate transfer belt  608  is disposed above the photoconductive drums  602 A,  602 B,  602 C, and  602 D. The intermediate transfer belt  608  is stretched between a secondary transfer counter roller  609  and a tensioning roller  610 . The secondary transfer counter roller  609  is disposed so as to be in contact with a secondary transfer roller  611  via the intermediate transfer belt  608 . The intermediate transfer belt  608  is formed from a dielectric resin, such as a polycarbonate resin, a polyethylene terephthalate resin film, or polyvinylidene fluoride resin film. 
   Additionally, the intermediate transfer belt  608  has a primary transfer surface, which faces the photoconductive drums  602 A,  602 B,  602 C, and  602 D. The intermediate transfer belt  608  is disposed so that one end of the primary transfer surface adjacent to the secondary transfer roller  611  is tilted downward with respect to the other end. The laser exposure unit  607  includes a laser emitting unit (not shown) for emitting laser beams in response to given time-series electrical digital pixel signals, a polygon mirror  618 , a scanner motor  617 , and a reflecting mirror. The laser exposure unit  607  carries out exposure operations on the photoconductive drums  602 A,  602 B,  602 C, and  602 D so as to form electrostatic latent images of individual colors according to the image information on the surfaces of the photoconductive drums  602 A,  602 B,  602 C, and  602 D charged by the primary chargers  603 A,  603 B,  603 C, and  603 D, respectively. At the same time, a beam detection signal (BD) generating circuit (not shown) provided to the laser exposure unit  607  detects the laser beam deflected by the polygon mirror  618  in the main scanning direction. Furthermore, the laser exposure unit  607  includes an image-producing-unit controller (not shown) for controlling the operations of these components. Thus, the image-producing-unit controller controls the process speed of the image producing unit and the hue and density of an image. 
   The fixing unit  180 A is described next. 
   The fixing unit  180 A is disposed downstream of a secondary transfer unit  616  of the image producing unit  170 A in the transport direction of the transfer medium P. The fixing unit  180 A includes a fixing device  612  having a fuser roller  612 A and a pressure roller  612 B. The fuser roller  612 A incorporates a heat source, such as a halogen heater. The fixing device  612  is disposed so as to form a vertical paper path structure. Additionally, the fuser roller  612 A and the pressure roller  612 B are rotatably driven by a drive unit (not shown) and the electrical power of the heat source in the fuser roller  612 A is controlled so that the temperature of the surface of the fuser roller  612 A is controlled. Furthermore, a fixing unit controller (not shown) for controlling these components is provided to the fixing unit  180 A so that the rotational speed of the rollers, the temperature of the fuser roller  612 A, and the process for abnormal conditions are controlled. 
   In addition, the image forming subsystem  150 A for a 4D full-color printer includes the image formation control unit  160  that communicates with the image-producing-unit controller and the fixing unit controller. Thus, the image formation control unit  160  retrieves unit information from these control units and sends unit control information to these control units. Furthermore, the image formation control unit  160  exchanges various image signals with the controller  250  and exchanges control information with the printer engine control unit  105  and the platform control unit  65 . 
   While the description above has been made with reference to the image producing unit and the fixing unit both of which include the control units, the image producing unit and fixing unit can operate without the control units. In such a case, an image forming control unit (not shown) controls the components in the image producing unit and the fixing unit. 
   (B) Hardware configuration of Image Forming Subsystem for 1D Full-Color Printer 
     FIG. 11  is a cross-sectional view of the detailed structure of the image forming subsystem  150 B for a 1D full-color printer. 
   The color image forming subsystem  150 B includes an image producing unit  170 B and a fixing unit  180 B. Like the 4D color image forming subsystem  150 A having a vertical paper path structure, these units can be replaced with different units having the same functionality. In addition, these units can be physically separated. 
   First, the image producing unit  170 B is described in detail. 
   The image producing unit  170 B includes a scanner unit  631 , a photoconductive drum  632 , an intermediate transfer belt  633 , a developing rotary  637 , a primary transfer roller  644 , a secondary transfer roller  638 , and a cleaning blade  639 . The scanner unit  631  incorporates a laser unit  634 , a polyhedral mirror (polygon mirror)  635 , a scanner motor  636 , and a beam detection signal (BD signal) generating circuit  643 . The developing rotary  637  includes developer units  637 A- 637 D for individual colors. 
   The structure of each component of the image producing unit  170 B is described next. 
   The photoconductive drum  632  of the image producing unit  170 B includes a photoconductive layer on an aluminum drum base composed of an OPC (opto-photoconductor) and is driven by a drive unit (not shown) to rotate in a clockwise direction shown in  FIG. 11  at a predetermined process speed. A primary charger  642  serving as a primary charging unit uniformly charges the surface of the photoconductive drum  632  at a predetermined negative potential based on a charge bias applied by a charge bias power supply (not shown). 
   In the scanner unit  631 , the laser unit (hereinafter simply referred to as a “laser”)  634  emits laser beams modulated on the basis of time-series electrical digital pixel signals of given image information. The polyhedral mirror (polygon mirror)  635  is a rotating polyhedral mirror that deflects the laser beam emitted from the laser  634  so as to scan the surface of the photoconductive drum  632  and form an electrostatic latent image on the photoconductive drum  632 . The scanner motor  636  rotates the polygon mirror  635 . The beam detection signal (BD signal) generating circuit  643  detects the laser beam deflected by the polygon mirror  635  in the main scanning direction. 
   The developing rotary  637  develops the electrostatic latent image formed on the photoconductive drum  632  by the developer units  637 A,  637 B,  637 C, and  637 D corresponding to yellow (Y), magenta (M), cyan (C), and black (BK), respectively. Like the above-described  4 D color producing unit having the vertical paper path structure, the photoconductive drum  632  applies a primary transfer bias to the primary transfer roller  644  and primary-transfers a developer material developed on the photoconductive drum  632  by the developing rotary  637  to the intermediate transfer belt  633 . The secondary transfer roller  638  is in contact with the intermediate transfer belt  633  and secondary-transfers the developer material on the intermediate transfer belt  633  onto the transfer medium P. 
   The cleaning blade  639  is in contact with the photoconductive drum  632  at all times so as to strip off the residual toner on the surface of the photoconductive drum  632 . Thus, the photoconductive drum  632  is cleaned. Furthermore, like the above-described 4D color producing unit having the vertical paper path structure, an image-producing-unit controller (not shown) controls the operation of the components in the image producing unit. Thus, the image-producing-unit controller controls the process speed of the image producing unit and the hue and density of an image. 
   The fixing unit  180 B is described next. 
   The fixing unit  180 B is disposed downstream of the secondary transfer roller  638  of the image producing unit  170 B in the transport direction of the transfer medium P. Like the above-described 4D color producing unit having the vertical paper path structure, a fixing device  640  fixes a toner image transferred onto the transfer medium P by heating and pressing the toner image. Rollers of the fixing device  640  are rotatably driven by a drive unit (not shown) and the electrical power of a halogen heater in the fixing device  640  is controlled so that the temperature of the surface of a fuser roller is controlled. Furthermore, a fixing unit controller (not shown) for controlling these components is provided to the fixing unit  180 B so that the rotational speeds of the rollers, the temperature of the fuser roller, and the process for abnormal conditions are controlled. 
   Additionally, the image forming subsystem  150 B for a 1D full-color printer includes the image formation control unit  160  that communicates with the image-producing-unit controller and the fixing unit controller. Thus, the image formation control unit  160  retrieves unit information from these controllers and sends unit control information to these controllers. Furthermore, the image formation control unit  160  exchanges various image signals with the controller  250  and exchanges control information with the printer engine control unit  105  and the platform control unit  65 . 
   While the above-described description has been made with reference to the image producing unit and the fixing unit both of which include the control units, the image producing unit and fixing unit can operate without the control units. In such a case, an image forming control unit (not shown) controls the components in the image producing unit and the fixing unit. 
   (C) Hardware configuration of Image Forming Subsystem for 1D Black and White Printer 
     FIG. 12  is a cross-sectional view of the detailed structure of the image forming subsystem  150 C for a 1D black and white printer. 
   The black and white image forming subsystem  150 C includes an image producing unit  170 C and a fixing unit  180 C. Like the 4D color image forming subsystem  150 A having a vertical paper path structure, these units can be replaced with different units having the same functionality. In addition, these units can be physically separated. 
   First, the image producing unit  170 C is described in detail. 
   The image producing unit  170 C includes a scanner unit  661 , a photoconductive drum  662 , a developing unit  666 , and a transfer roller  667 . The scanner unit  661  incorporates a laser unit  663 , a polyhedral mirror (polygon mirror)  664 , a scanner motor  665 , and a beam detection signal (BD signal) generating circuit  672 . 
   Each component of the image producing unit  170 C and the operation thereof are described next. 
   The photoconductive drum  662  includes a photoconductive layer on an aluminum drum base composed of an OPC (opto-photoconductor) and is driven by a drive unit (not shown) to rotate in a counterclockwise direction shown in  FIG. 12  at a predetermined process speed. A primary charger  670  uniformly charges the surface of the photoconductive drum  662  at a predetermined potential based on a charge bias applied by a charge bias power supply (not shown). 
   In the scanner unit  661 , the laser unit  663  emits a laser beam modulated on the basis of time-series electrical digital pixel signals of given image information. The polyhedral mirror (polygon mirror)  664  is a rotating polyhedral mirror that deflects the laser beam emitted from the laser  663  so as to scan the surface of the photoconductive drum  662  and form an electrostatic latent image on the photoconductive drum  662 . The scanner motor  665  rotates the polygon mirror  664 . The beam detection signal (BD signal) generating circuit  672  detects the laser beam deflected by the polygon mirror  664  in the main scanning direction. 
   The developing unit  666  develops the electrostatic latent image formed on the photoconductive drum  662  using a black (BK) developer material. The transfer roller  667  is in contact with the photoconductive drum  662  and transfers the developer material on the photoconductive drum  662  to the transfer medium P. A cleaning blade  669  is in contact with the photoconductive drum  662  at all times so as to strip off the residual developer material on the surface of the photoconductive drum  662 . Thus, the photoconductive drum  662  is cleaned. Furthermore, like the above-described 1D color fixing system, an image-producing-unit controller (not shown) is provided to the image producing unit  170 C so as to control the operation of these components of the image producing unit. Thus, the process speed of the image producing unit and density of the image can be controlled. 
   The fixing unit  180 C is described next. 
   The fixing unit  180 C is disposed downstream of the transfer roller  667  of the image producing unit  170 C in the transport direction of the transfer medium P. Like the above-described 1D color fixing system, a fixing device  668  fixes a toner image transferred onto the transfer medium P by heating and pressing the toner image. A roller of the fixing device  668  is rotatably driven by a drive unit (not shown) and the electrical power of a halogen heater in the fixing device  668  is controlled so that the temperature of the surface of a fuser roller is controlled. Furthermore, a fixing unit controller (not shown) for controlling these components is provided to the fixing unit  180 C so that the rotational speeds of the roller, the temperature of the fuser roller, and the process for abnormal conditions are controlled. 
   Additionally, the image forming subsystem  150 C for a 1D black and white printer includes the image formation control unit  160  that communicates with the image-producing-unit controller and the fixing unit controller. Thus, the image formation control unit  160  retrieves unit information from these controllers and sends unit control information to these controllers. Furthermore, the image formation control unit  160  exchanges various image signals with the controller  250  and exchanges control information with the printer engine control unit  105  and the platform control unit  65 . 
   While the above-described description has been made with reference to the image producing unit and the fixing unit both of which include the control units, the image producing unit and fixing unit can operate without the control units. In such a case, an image forming control unit (not shown) controls the components in the image producing unit and the fixing unit. 
   Configuration of Electrical Connection According to First Exemplary Embodiment 
   Overall Configuration 
   The configuration of electrical connection of an image forming apparatus according to the first exemplary embodiment is described below. 
     FIG. 13  is a block diagram illustrating the configuration of electrical connection of an image forming apparatus according to the present embodiment. 
   As shown in  FIG. 13 , the image forming apparatus includes the printer engine control unit  105  for controlling the printer engine  100  and the platform control unit  65  for controlling the paper transport platform  60 . Here, the transport unit  80  includes a control unit incorporating a central processing unit (CPU) whereas the feeder unit  70  does not include a CPU. 
   The transport unit  80  communicates with the platform control unit  65  to exchange control information. Thus, the transport unit  80  controls the load of control components (such as the motors). Under the control of the platform control unit  65 , the feeder unit  70  controls the load of control components. The feeder unit  70  controls the load associated with a feeding operation of the transfer medium P. The transport unit  80  controls the load associated with an output operation, an inverting operation, and a duplex transporting operation of the transfer medium P. Using such controls, the paper transport platform  60  achieves a transport operation of the transfer medium P to form an image. 
   The image formation control unit  160  controls the image forming subsystem  150 . Here, the image producing unit  170  includes a control unit incorporating a CPU whereas the fixing unit  180  does not include a CPU. 
   The image producing unit  170  communicates with the image formation control unit  160  so as to exchange control information. Thus, the image producing unit  170  controls the load of control components. Under the control of the image formation control unit  160 , the fixing unit  180  controls the control load of components. The image producing unit  170  forms an image on the transfer medium P on the basis of image signals exchanged with the controller  250 . The fixing unit  180  heats and fixes the image on the transfer medium P. Examples of the exchanged image signals include video data (VIDEO), an image sync CLK (VCLK), a main scanning sync signal (BD), and a sub scanning sync signal (ITOP). 
   Here, the image forming subsystem  150  receives the transfer medium P transported by the paper transport platform  60 . Subsequently, in order to transfer an image formed by the image forming subsystem  150  to the transfer medium P at a proper position, the image forming subsystem  150  transmits a paper transport sync signal (REGI) generated on the basis of the sub scanning sync signal (ITOP) managed by the image formation control unit  160  to the platform control unit  65  via the printer engine control unit  105 . The platform control unit  65  controls the feeding and transporting operations on the basis of the paper transport sync signal (REGI) so that the transported transfer medium P is delivered to the image forming subsystem  150  at a predetermined timing. By performing such collaborative operations, the image forming subsystem  150  can achieve the image forming operation on the transported transfer medium P. 
   The printer engine  100  includes the power supply unit  90 , which receives an AC input and outputs DC outputs and rectified AC outputs. As the DC outputs, a plurality of controlled voltage outputs are supplied to the platforms, the subsystems, and the units in the image forming apparatus. The AC outputs are supplied to the platforms, the subsystems, and the units in the image forming apparatus as needed. In this embodiment, the AC output is supplied to the fixing unit  180 . 
   The printer engine control unit  105  manages control information on the paper transport platform  60  received via communication with the platform control unit  65 , control information on the image forming subsystem  150  received via communication with the image formation control unit  160 , and control information on the power supply unit  90  received from the power supply unit  90 . On the basis of all the received information, the printer engine control unit  105  transmits control information to the platform control unit  65 , the image formation control unit  160 , and the power supply unit  90  so as to cause the printer engine to carry out an image forming operation. 
   The platform control unit  65  communicates control information with the transport unit  80  on the basis of the control information determined by the printer engine control unit  105 . Also, the platform control unit  65  controls the load of the control components of the feeder unit  70  on the basis of the control information determined by the printer engine control unit  105 . The transport unit  80  controls the load of the control components on the basis of the received control information. 
   The image formation control unit  160  communicates control information with the image producing unit  170  on the basis of the control information determined by the printer engine control unit  105 . Also, the image formation control unit  160  controls the load of the control components of the fixing unit  180  on the basis of the control information determined by the printer engine control unit  105 . The image producing unit  170  controls the load of the control components on the basis of the received control information. The power supply unit  90  controls the output voltage on the basis of the control information determined by the printer engine control unit  105 . 
   The controller  250  exchanges image data and control information. That is, the controller  250  exchanges control information with the printer engine control unit  105  and exchanges image signals with the paper transport platform  60  of the printer engine  100 . The image reader unit  270  is connected to the controller  250  to receive image information. The document feeder unit  280  is connected to the image reader unit  270  to feed documents to be read out. The operation unit  260  for inputting user operations and displaying messages is connected to the controller  250  so as to exchange control information. The controller  250  is connected to a network  10  and can communicate image signals and control information with, for example, a computer (not shown) in the network  10 . 
   Electrical Configuration of Image Forming Subsystem 
   Components in the image forming apparatus, in particular, an image forming subsystem  150  and an image formation control unit  160  provided to the image forming subsystem  150  are described next. 
   (A) 4D Full-color Image Forming Subsystem  150 A 
     FIG. 14  is a block diagram of a 4D full-color image forming subsystem  150 A. 
   The 4D full-color image forming subsystem  150 A includes an image formation control unit  160 A including an image processing unit, an image producing unit  170 A, and a fixing unit  180 A. An image signal is input from the controller  250  to the image formation control unit  160 A in the form of an RGB color format. Thereafter, the image signal is processed as follows. 
   First, the image signal is subjected to a density conversion by a LOG conversion circuit  310  and is converted to YMCK data by an output masking circuit  311 . The output masking circuit  311  carries out the conversion so that the average color difference in a Lab space is minimal. The coefficient of the conversion depends on the hardware characteristics of the image producing unit  170 A. The YMCK data is input to a gradation correction circuit  312 , which corrects the gradation of the YMCK data using a lookup table (hereinafter referred to as an “LUT”). In the LUT, a table for correcting the hardware characteristics (such as an individual difference and a change over time), a density adjustment table that can be changed by a user, and an image mode table (such as a character mode and a print paper mode) are combined. 
   The LUT varies in accordance with a subsequent halftone process. Since a halftone processing circuit  313  carries out a plurality of halftone processes in parallel, the gradation correction circuit  312  has a number of LUTs equal to the number of parallel processings performed by the halftone processing circuit  313 . Thus, the gradation correction circuit  312  carries out all the halftone processes and outputs all the processing results at the same time. The gradation-corrected signal is input to the halftone processing circuit  313 , which generates print data. The halftone processing circuit  313  carries out error diffusion and a plurality of screen processes in parallel. One of the screens is selected and output in accordance with a Z signal, which is described below. The print data is subject to a delaying operation in accordance with the arrangement of the drums by an inter-drum delay memory  314  and is output to the image producing unit  170 A. 
   The Z signal for indicating the features of the image is input to the image formation control unit  160 A from the controller  250  at the same time as the image signal. The Z signal is a signal synchronized with the RGB signal. The Z signal is input to the LOG conversion circuit  310 , the output masking circuit  311 , the gradation correction circuit  312 , and the halftone processing circuit  313 . The Z signal includes data indicating the features on a page-by-page basis and data indicating the features on a pixel-by-pixel basis. More specifically, the data on a page-by-page basis is data identifying a copy image or a PDL image whereas the data on a pixel-by-pixel basis is data identifying a character/photograph and a BMP/object. 
   The image output timing of the controller  250  is controlled by the image sync signals ITOP and a PBD signal output from a timing generating unit  315 . The ITOP signal is a sync signal in the sub scanning direction. The PBD signal is a sync signal in the main scanning direction. In addition, an image clock PCLK is input to the controller  250 . The controller  250  outputs image signal in synchronization with the image clock PCLK. The PBD signal is generated on the basis of the BD signal output from the image producing unit  170 A. 
   The timing generating unit  315  further generates an REGI signal for controlling the driving timing of a registration roller. The REGI signal is output to the image producing unit  170 A, which includes the registration roller. The REGI signal is generated on the basis of the ITOP signal. The timing of the ITOP signal is determined depending on a relationship among the image producing position, the transfer position, and the registration roller. Thus, the timing of the ITOP signal is uniquely determined for the image forming subsystem  150 A. The REGI signal is also delivered to the platform control unit  65  at the same time in order to synchronize with the registration roller. 
   (B) Image Forming Timing of 4D Full-color Image Forming Subsystem 
     FIG. 15  is a timing diagram illustrating the image forming timing of the 4D full-color image forming subsystem  150 A. 
   In  FIG. 15 , images for two pages are continuously produced. RGB images are output from the controller  250  in accordance with the ITOP timings. After an image processing delay t 1  has elapsed, YMCK data are sequentially output to the image producing unit  170 A. The YMCK data have a phase difference of t 2 , which is the time delay of an inter-drum. The delaying operation is carried out by the inter-drum delay memory  314 . 
   The timing generating unit  315  generates the REGI signal after a registration delay t 3  has elapsed from the time the ITOP signal was generated. At that time, the registration roller is driven so that the transfer medium P is transported to the secondary transfer unit. The secondary transfer starts after a transfer delay t 4  has elapsed from the time the REGI signal occurs. The process of the second page starts during the transfer operation of the first page. If more pages are present, the above-described process is repeated in the same manner. 
   (C) Electric Configuration of 1D Full-color Image Forming Subsystem 
     FIG. 16  is a block diagram of a 1D full-color image forming subsystem  150 B. 
   The 1D full-color image forming subsystem  150 B includes an image formation control unit  160 B including an image processing unit, the image producing unit  170 B, and a fixing unit  180 B. An image signal is input from the controller  250  to the image formation control unit  160 B in the form of an RGB color format. Thereafter, the image signal is processed as follows. 
   The difference between the image processing performed by the 1D full-color image forming subsystem  150 B shown in  FIG. 16  and that performed by the 4D full-color image forming subsystem  150 A shown in  FIG. 14  is that the inter-drum delay memory  314  is changed to a page memory  320 . Other blocks are similar to those of the 4D full-color image forming subsystem  150 A, and therefore, descriptions thereof are not repeated. 
   (D) Image Forming Timing of 1D Full-Color Image Forming Subsystem 
     FIG. 17  is a timing diagram illustrating the image forming timing of the 1D full-color image forming subsystem  150 B. 
   In  FIG. 17 , images for two pages are continuously produced. RGB images are output from the controller  250  in accordance with the ITOP timing. After an image processing delay t 1  has elapsed, YMCK print data is stored in the page memory  320 . The YMCK data are sequentially delivered to the image producing unit  170 B. According to this configuration, an image is formed for each color. Therefore, after image formation for all colors are completed, the next print data is supplied. 
   The timing generating unit  315  generates the REGI signal after a registration delay t 3  has elapsed from the time the ITOP signal was generated. At that time, the registration roller is driven so that the transfer medium P is transported to the secondary transfer unit. The secondary transfer starts after a transfer delay t 4  has elapsed from the time the REGI signal occurs. The process of the second page starts at a certain time so that the image formation of the fourth color for the first page does not overlap the image formation of the first color for the second page. If more pages are present, the above-described process is repeated in the same manner. 
   (E) Electrical Configuration of 1D Black and White Image Forming Subsystem 
     FIG. 18  is a block diagram of a 1D black and white image forming subsystem  150 C. 
   The 1D full-color image forming subsystem  150 C includes an image formation control unit  160 C including an image processing unit, the image producing unit  170 C, and a fixing unit  180 C. Like the full-color image forming subsystems, an image signal is input from the controller  250  to the image formation control unit  160 C in the form of an RGB color format. The image formation control unit  160 C generates a BK signal. 
   First, a BK generating circuit  330  converts the RGB signal to the BK signal. Thereafter, the BK signal is subject to a density conversion by a LOG conversion circuit  331  and is subjected to gradation correction by a gradation correction circuit  332 . Finally, a halftone processing circuit  333  generates print data from the BK signal. 
   The functions of the LOG conversion circuit  331 , the gradation correction circuit  332 , and the halftone processing circuit  333  are similar to those of the full-color image forming subsystems except that the number of channels is one (for BK single color). 
   (F) Image Forming Timing of 1D Black and White Image Forming Subsystem 
     FIG. 19  is a timing diagram illustrating the image forming timing of the 1D black and white image forming subsystem  150 C. 
   In  FIG. 19 , images for two pages are continuously produced. RGB images are output from the controller  250  in accordance with the ITOP timings. After an image processing delay t 20  has elapsed, BK data is output to the image producing unit  170 C. The timing generating unit  315  generates the REGI signal after a registration delay t 23  has elapsed from the time that the ITOP signal was generated. At that time, the registration roller is driven so that the transfer medium P is transported to the transfer unit. The transfer starts after a transfer delay t 24  has elapsed from the time the REGI signal occurs. 
   The process of the second page starts during the transfer operation of the first page. If more pages are present, the above-described process is repeated in the same manner. 
   Operation according to First Exemplary Embodiment 
   Simplex Image Forming Operation Corresponding to High-Speed Color Throughput 
   The simplex image forming operation performed by the printer engine  100  is described next for a case in which the above-described image forming subsystem  150 A corresponding to a high-speed color throughput is mounted on the paper transport platform  60 . 
   Upon receiving a user instruction for starting an image forming procedure via the operation unit  260  of the image forming apparatus, the printer engine control unit  105  transmits a paper feed request command to the platform control unit  65 . Thereafter, the transport unit  80  and the feeder unit  70  start the operations. Similarly, when the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 , the image producing unit  170 A and the fixing unit  180 A start an image forming operation. The photoconductive drums  602 A,  602 B,  602 C, and  602 D of the image forming units,  601 Y,  601 M,  601 C, and  601 BK, which are rotatably driven at a predetermined process speed by a driving mechanism of the image producing unit  170 A, are uniformly and negatively charged by the primary chargers  603 A,  603 B,  603 C, and  603 D, respectively. Thereafter, the laser exposure unit  607  emits externally input color-separated image signals from a laser emitting element to the polygon mirror  618  rotatably driven by the scanner motor  617 . Thus, the image signals reflected by the reflection mirror form electrostatic latent images for four colors on the photoconductive drums  602 A,  602 B,  602 C, and  602 D, respectively. 
   Subsequently, yellow toner is deposited on the electrostatic latent image formed on the photoconductive drum  602 A by the developing unit  604 A to which a developing bias having the same polarity as the charged polarity of the photoconductive drum  602 A (i.e., negative polarity) is applied. Thus, the electrostatic latent image is visualized as a toner image. This yellow toner image is primary-transferred onto the moving intermediate transfer belt  608  by the transfer roller  605 A to which a primary transfer bias having a polarity opposite to that of the primary transfer biased toner (i.e., positive polarity) is applied in the primary transfer unit  615 A disposed between the photoconductive drum  602 A and the transfer roller  605 A. 
   The intermediate transfer belt  608  having the yellow toner image formed thereon is moved towards the image forming unit  601 M. Similarly, in the image forming unit  601 M, a magenta toner image formed on the photoconductive drum  602 B is transferred to the intermediate transfer belt  608  while overlapping the yellow toner image by the primary transfer unit  615 B. 
   At that time, residual toner on the photoconductive drums  602 A,  602 B,  602 C, and  602 D is removed and collected, for example, by cleaner blades provided to the drum cleaners  606 A,  606 B,  606 C, and  606 D, respectively. 
   Similarly, cyan and black toner images, which are formed on the photoconductive drums  602 C and  602 D of the image forming units  601 C and  601 BK, respectively, are sequentially overlapped on the overlap-transferred yellow and magenta toner images on the intermediate transfer belt  608 . Thus, a full-color toner image is formed on the intermediate transfer belt  608 . 
   In synchronization with the time when the leading edge of the full-color toner image on the intermediate transfer belt  608  is moved to the secondary transfer unit  616  disposed between the secondary transfer counter roller  609  and the secondary transfer roller  611 , the feeder cassette  505  of a feeder unit  60 A is selected. Then, the top sheet of the transfer media P stacked in the feeder cassette  505  is picked up by the pickup roller  502  and is transported to the paper feed path  511 . Additionally, the transport roller  503  delivers the transported transfer medium P to a registration roller  613  of the image producing unit  170 A. Subsequently, the registration roller  613  of the image producing unit  170 A delivers the transfer medium P to the secondary transfer unit  616 . The full-color toner image is secondary-transferred to the transfer medium P transported to the secondary transfer unit  616  by the secondary transfer roller  611  to which a secondary transfer bias having a polarity opposite to that of the toner (i.e., positive polarity) is applied. 
   The transfer medium P having the full-color toner image formed thereon is transported to the fixing unit  180 A. In a fixing nip unit  614  disposed between the fuser roller  612 A and the pressure roller  612 B, the full-color toner image is affixed to the surface of the transfer medium P by heating and pressing the full-color toner image. Thereafter, the transfer medium P is transported to the transport unit  80 A. The transfer medium P then passes through the paper output path  525  of the transport unit  80 A and is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 . Thus, the series of image forming operations is completed. 
   So far, the simplex image forming operation has been described. 
   Duplex Image Forming Operation Performed by Image Forming Apparatus corresponding to High-Speed Color Throughput 
   The duplex image forming operation performed by image forming apparatus corresponding to high-speed color throughput is described next. 
   The processes before the transfer medium P is delivered to the fixing unit  180 A are similar to those for the simplex image forming operation. In the fixing nip unit  614  disposed between the fuser roller  612 A and the pressure roller  612 B, the full-color toner image is heated and pressed and is heat-fixed to the surface of the transfer medium P. Thereafter, the transfer medium P passes through the paper output path  525  of the transport unit  80 A. When most of the transfer medium P is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 , the rotation of the paper output roller  522  is stopped. At that time, the trailing edge of the transfer medium P is located at a reversible position of the transfer medium P, that is, at a position downstream of the branching position of the paper output path  525  and the transport path  526 . 
   Subsequently, in order to deliver the transfer medium P, which is stopped due to the stop of the rotation of the paper output path  525 , to the transport path  526  having the transport rollers  523  and  524 , the paper output roller  522  rotates in a direction opposite to the direction of the simplex image forming operation. By rotating the paper output roller  522  in the reverse direction, the trailing edge of the transfer medium P, which is located at the reversible position, becomes the leading edge and reaches the transport roller  523 . 
   Thereafter, the transport roller  523  transports the transfer medium P to the transport roller  524 . The transfer medium P is then transported to the paper feed path  511  of the feeder unit  60 A. The transport roller  503  transports the delivered transfer medium P to the registration roller  613  of the image producing unit  170 A. During the transportation, the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 . Like the above-described simplex image forming operation, in synchronization with the time when the leading edge of the full-color toner image on the intermediate transfer belt  608  moves to the secondary transfer unit  616  disposed between the secondary transfer counter roller  609  and the secondary transfer roller  611 , the registration roller  613  moves the transfer medium P to the secondary transfer unit  616 . 
   After the leading edge of the toner image is aligned with the leading edge of the transfer medium P in the secondary transfer unit  616  and the toner image is transferred to the transfer medium P, the fixing unit  180 A fixes the image onto the transfer medium P, as in the simplex image formation. The transfer medium P is then transported by the paper output roller  522  of the transport unit  80 A again. Finally, the transfer medium P is output onto the output tray  527 . Thereafter, the series of image forming operations is completed. 
   Simplex Image Forming Operation corresponding to Low-Speed Color Throughput 
   The simplex image forming operation performed by the printer engine  100  is described next for a case in which the above-described image forming subsystem  150 B corresponding to a low-speed color throughput is mounted in the engine platform  101  to form the printer engine  100  along with the paper transport platform  60 . 
   Upon receiving a user instruction of starting an image forming job via the operation unit  260  of the image forming apparatus, the printer engine control unit  105  transmits a paper feed request command to the platform control unit  65 . Thereafter, the transport unit  80  and the feeder unit  70  start the operations. Similarly, when the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 , the photoconductive drum  632  is rotatably driven by a driving mechanism (not shown) of the image producing unit  170 B at a predetermined process speed. In addition, the photoconductive drum  632  is uniformly charged to a negative polarity by the primary charger  642 . 
   Thereafter, the scanner unit  631  emits externally input color-separated image signals from a laser emitting element to the polygon mirror  635  rotatably driven by the scanner motor  636 . Thus, the image signals reflected by the reflection mirror form a yellow (Y) electrostatic latent image on the photoconductive drum  632 . At a position at which the photoconductive drum  632  is in contact with the yellow (Y) developer unit  637 A in the developing rotary  637 , the latent image is visualized using the yellow (Y) developer material. The photoconductive drum  632  is further rotated by the driving mechanism and reaches a position at which the photoconductive drum  632  is in contact with the intermediate transfer belt  633 . At that point, the yellow (Y) developer material is primary-transferred to the moving intermediate transfer belt  633  by a transfer roller  630  to which a primary transfer bias having a polarity opposite to that of the toner (i.e., positive polarity) is applied. At that time, residual toner on the photoconductive drum  632  is removed, for example, by the cleaner blade  639  provided to a drum cleaner unit and is collected into a recycling container. Thereafter, a driving unit (not shown) rotates the developing rotary  637  about 90 degrees to prepare for the next print operation for magenta (M). 
   To produce an image from magenta (M) data, a latent image for the magenta (M) data is written onto the photoconductive drum  632 , as in the formation of the yellow (Y) data. Subsequently, the driving mechanism rotates the photoconductive drum  632 . Additionally, the primary charger  642  uniformly and negatively charges the photoconductive drum  632 . The scanner unit  631  then emits externally input color-separated image signals from the laser emitting element to the polygon mirror  635  rotatably driven by the scanner motor  636 . Thus, the image signals reflected by the reflection mirror form a magenta (M) electrostatic latent image on the photoconductive drum  632 . At the rotational position of the intermediate transfer belt  633  that is the same as that in the yellow (Y) image formation, the latent image on the photoconductive drum  632  is visualized using the magenta (M) developer material. The photoconductive drum  632  is further rotated by the driving mechanism and reaches a certain position at which the photoconductive drum  632  is in contact with the intermediate transfer belt  633 . At that point, the magenta (M) developer material is primary-transferred to the moving intermediate transfer belt  633  by a transfer roller  644  to which a primary transfer bias having a polarity opposite to that of the toner (i.e., positive polarity) is applied. 
   Subsequently, a similar image forming steps are carried out for cyan (C) and black (BK). When the yellow (y), magenta (M), cyan (C), and black (BK) developer materials overlap at a predetermined position, the feeder cassette  505  of a feeder unit  70 B is selected. Then, the top sheet of the transfer media P stacked in the feeder cassette  505  is picked up by the pickup roller  502  and is transported to the paper feed path  511 . Additionally, the transport roller  503  delivers the transported transfer medium P to a registration roller  641  of the image producing unit  170 B. Subsequently, the registration roller  641  of the image producing unit  170 B delivers the transfer medium P to a secondary transfer unit formed by the secondary transfer roller  638  and the intermediate transfer belt  633 . The full-color toner image is secondary-transferred to the transfer medium P transported to the secondary transfer unit by the secondary transfer roller  638  to which a secondary transfer bias having a polarity opposite to that of the toner (i.e., positive polarity) is applied. 
   The transfer medium P having the full-color toner image formed thereon is transported to the fixing unit  180 B. In the fixing unit  180 B, the full-color toner image is heated and pressed and is heat-fixed to the surface of the transfer medium P by the fixing device  640 . Thereafter, the transfer medium P is transported to the transport unit  80 B. The transfer medium P then passes through the paper output path  525  of the transport unit  80 B and is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 . Thus, the series of image forming operations is completed. 
   So far, the simplex image forming operation has been described. 
   Duplex Image Forming Operation Corresponding to Low-Speed Color Throughput 
   The duplex image forming operation corresponding to low-speed color throughput is described next. 
   The processes before the transfer medium P is delivered to the fixing unit  180 B are similar to those of the simplex image forming operation. In the fixing device  640 , the full-color toner image is heated and pressed and is heat-fixed to the surface of the transfer medium P. Thereafter, the transfer medium P passes through the paper output path  525  of the transport unit  80 B. When most of the transfer medium P is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 , the rotation of the paper output roller  522  is stopped. At that time, the trailing edge of the transfer medium P is located at a reversible position of the transfer medium P, that is, at a position downstream of the branching position of the paper output path  525  and the transport path  526 . 
   Subsequently, in order to deliver the transfer medium P, which is stopped due to the stop of the rotation of the paper output path  525 , to the transport path  526  having the transport rollers  523  and  524 , the paper output roller  522  rotates in a direction opposite to the direction of the simplex image forming operation. By rotating the paper output roller  522  in the reverse direction, the trailing edge of the transfer medium P, which is located at the reversible position, becomes the leading edge and reaches the transport roller  523 . 
   Thereafter, the transport roller  523  transports the transfer medium P to the transport roller  524 . The transfer medium P is then transported to the paper feed path  511  of the feeder unit  60 B. The transport roller  503  transports the delivered transfer medium P to the registration roller  613  of the image producing unit  170 B. During the transportation, the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 . Like the above-described simplex image forming operation, in synchronization with the time when the leading edge of the full-color toner image on the intermediate transfer belt  608  moves to the secondary transfer unit  616  disposed between the secondary transfer counter roller  609  and the secondary transfer roller  611 , the registration roller  613  moves the transfer medium P to the secondary transfer unit  616 . 
   After the leading edge of the toner image is aligned with the leading edge of the transfer medium P in the secondary transfer unit  616  and the toner image is transferred to the transfer medium P, the fixing unit  180 B fixes the image onto the transfer medium P, as in the simplex image formation. The transfer medium P is then transported by the paper output roller  522  of the transport unit  80 B again. Finally, the transfer medium P is output onto the output tray  527 . Thereafter, the series of image forming operations is completed. 
   Simplex Image Forming Operation corresponding to High-Speed Black and White Throughput 
   The image forming operation performed by the printer engine  100  is described next for a case in which the above-described image forming subsystem  150 C corresponding to a high-speed black and white throughput is mounted in the engine platform  101  to form the printer engine  100  along with the paper transport platform  60 . 
   Upon receiving a user instruction for starting an image forming procedure via the operation unit  260  of the image forming apparatus, the printer engine control unit  105  transmits a paper feed request command to the platform control unit  65 . Thereafter, the transport unit  80  and the feeder unit  70  start the operations. Similarly, when the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 , the photoconductive drum  662  is rotatably driven by a driving mechanism (not shown) of the image producing unit  170 C at a predetermined process speed. In addition, the photoconductive drum  662  is uniformly charged to a negative polarity by the primary charger  670 . 
   Thereafter, the scanner unit  661  externally emits input image signals from a laser emitting element to the polygon mirror  664  rotatably driven by the scanner motor  665 . Thus, the image signals reflected by the reflection mirror form an electrostatic latent image on the photoconductive drum  662 . At a position at which the photoconductive drum  662  is in contact with the developing unit  666 , the latent image on the photoconductive drum  662  is visualized using the developer material. Additionally, the feeder cassette  505  of a feeder unit  70 A is selected. Then, the top sheet of the transfer media P stacked in the feeder cassette  505  is picked up by the pickup roller  502  and is transported to the paper feed path  511 . Additionally, the transport roller  503  delivers the transported transfer medium P to a registration roller  671  of the image producing unit  170 A. Subsequently, the toner image is transferred to the transfer medium P transported to a transfer unit  34  by the transfer roller  667  to which a secondary transfer bias having a polarity opposite to that of the toner (i.e., positive polarity) is applied. The transfer medium P having the toner image formed thereon is transported to the fixing unit  180 C. In the fixing unit  180 C, the toner image is heated and pressed and is heat-fixed to the surface of the transfer medium P by the fixing device  668 . Thereafter, the transfer medium P is transported to the transport unit  80 A. The transfer medium P then passes through the paper output path  525  of the transport unit  80 A and is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 . Thus, the series of image forming operations is completed. Furthermore, at that time, residual toner on the photoconductive drum  662  is removed, for example, by a cleaner blade  669  provided to a drum cleaner unit and is collected into a recycling container. 
   So far, the simplex image forming operation has been described. 
   Duplex Image Forming Operation Corresponding to High-Speed Black and White Throughput 
   The duplex image forming operation corresponding to the above-described low-speed color throughput is described next. 
   The processes before the transfer medium P is delivered to the fixing unit  180 C are similar to those for the simplex image forming operation. In the fixing device  668 , the toner image is heated and pressed and is heat-fixed to the surface of the transfer medium P. Thereafter, the transfer medium P passes through the paper output path  525  of the transport unit  80 A. When most of the transfer medium P is output onto the output tray  527  disposed on the top of the image forming apparatus by the paper output roller  522 , the rotation of the paper output roller  522  is stopped. At that time, the trailing edge of the transfer medium P is located at a reversible position of the transfer medium P, that is, at a position downstream of the branching position of the paper output path  525  and the transport path  526 . 
   Subsequently, in order to deliver the transfer medium P, which is stopped due to the stop of the rotation of the paper output path  525 , to the transport path  526  having the transport rollers  523  and  524 , the paper output roller  522  rotates in a direction opposite to the direction of the simplex image forming operation. By rotating the paper output roller  522  in the reverse direction, the trailing edge of the transfer medium P, which is located at the reversible position, becomes the leading edge and reaches the transport roller  523 . 
   Thereafter, the transport roller  523  transports the transfer medium P to the transport roller  524 . The transfer medium P is then transported to the paper feed path  511  of the feeder unit  60 A. The transport roller  503  transports the delivered transfer medium P to the registration roller  671  of the image producing unit  170 C. During the transportation, the printer engine control unit  105  transmits an image forming request command to the image formation control unit  160 . Thus, like the above-described simplex image forming operation, the transfer medium P is moved to a transfer unit by the registration roller  613 . 
   After the leading edge of the toner image is aligned with the leading edge of the transfer medium P in the transfer unit and the toner image is transferred to the transfer medium P, the fixing unit  180 C fixes the image onto the transfer medium P, as in the simplex image formation. The transfer medium P is then transported by the paper output roller  522  of the transport unit  80 A again. Finally, the transfer medium P is output onto the output tray  527 . Thereafter, the series of image forming operations is completed. 
   Communication Data used for Image Forming Operation and Timing of Communication Data 
   (A) Parameter of Configuration Communication when Power is turned ON 
   The communication data and the timing of the communication data used for communication between the printer engine control unit  105  and the image formation control unit  160  in the image forming subsystem  150 , between the printer engine control unit  105  and the platform control unit  65  in the paper transport platform  60 , and between the printer engine control unit  105  and the power supply unit  90  in order to achieve the image forming operation performed by the printer engine  100  are described next with reference to  FIGS. 20A to 24B . 
     FIGS. 20A-C ,  21 A, and  21 B illustrate parameters of the configuration communication when the power is turned ON. 
   Data structure  701  shown in  FIG. 20A  illustrate data that is common to the configuration information data for all the units. The configuration information data is transmitted to the printer engine control unit  105  when the power is turned ON. When the power supply unit  90  starts supplying the power and the printer engine control unit  105  and the platform control unit  65  start the operations thereof, the configuration information data is transmitted from the platform control unit  65  to the printer engine control unit  105 , and similarly, from the image formation control unit  160  to the printer engine control unit  105 . The transmitted configuration information data is used for notifying the printer engine control unit  105  of the capabilities of the platform control unit  65  and the image formation control unit  160 . 
   For example, the transmitted configuration information data includes the following data items: a unit ID for identifying a unit associated with this transmitted configuration information data and a process speed at which the unit can operate. At that time, for example, when the image forming subsystem  150  is capable of performing color printing, the process speed for the fixing operation may vary depending on the selection of a full-color mode or a black color mode even when the same type of the transfer medium P is used. Accordingly, in order to properly notify the printer engine control unit  105  of the capabilities of the image forming subsystem  150 , a data set including information about the process speed and the color mode (full-color or black-color mode) needs to be transmitted. In contrast, in most cases for the paper transport platform, the capability of transporting the transfer medium P does not vary regardless of mode (full-color mode or black color). Therefore, at that time, the process speed is sent together with information indicating that the process speed is applied to both full-color mode and black-color mode. 
   Additionally, when the type of transfer medium P is different, for example, when a thick paper sheet and a plain paper sheet are compared, the fixing conditions and the transport conditions for the paper sheets tend to be different. Therefore, the process speed needs to be sent for each type of transfer medium P. That is, a set of the material condition and the process speed needs to be sent. 
   Furthermore, since a required fixing heater temperature depends on the color mode and material conditions, data about the color mode and material conditions needs to be sent together with data about electric power consumed by the unit under these conditions. 
   Accordingly, the sent configuration data in the data structure  701  contains a set of the process speed, the color mode which determines the process speed, the power consumption, and the material conditions. 
   The data structure  701  shown in  FIG. 20A  illustrates an example in which three types of process speed are sent. However, for a unit that requires only one type of process speed, one speed should be sent. Furthermore, since the distance between the transfer media P (i.e., an inter-print gap) may vary depending on the transport conditions associated with the type of unit (e.g., a sensor response time and a fixing performance), the data structure  701  contains that data as the data to be sent. 
   A data structure  702  shown in  FIG. 20B  illustrates available electric power supply data that is sent from the power supply unit  90  to the printer engine control unit  105 . According to the present exemplary embodiment, since the image forming apparatus includes the interchangeable image forming subsystem  150  and the paper transport platform  60  having different capabilities, the data about the available electric power supply from the power supply unit  90  and the configuration data about the power supply system are important for determining whether the power supply unit  90  can supply sufficient electric power to the units. Accordingly, like the data in the data structure  701 , these data should be sent to the printer engine control unit  105  when the power is turned on. 
   A data structure  703  shown in  FIG. 20C  illustrates data about the capability of the image forming subsystem  150  that the image formation control unit  160  should send in addition to the data in the configuration data structure  701 . More specifically, this data represents configuration information indicating the selection of a 4D color image forming subsystem (e.g., the 4D color image forming subsystem  150 A) or a 1D color image forming subsystem (e.g., the 1D color image forming subsystem  150 B). Additionally, for the color image forming subsystems (such as the image forming subsystem  150 A or  150 B), in order to develop and transfer four color images, ITOP signals for the four colors need to be generated at an appropriate time period. An “ITOP period” field represents such data. Furthermore, for the color image forming subsystems, in order to align the position of the image data with the position of the transfer medium P, the following data may be required: a time period from the time when an ITOP signal for controlling color image data of a given page that is developed first is generated to the time when the image for the fourth color is developed and transferred and the head of the image data for subscanning reaches a secondary transfer unit ( 150 A) formed by the secondary transfer roller  611  and the intermediate transfer belt  608  or a secondary transfer unit ( 150 B) formed by the secondary transfer roller  638  and the intermediate transfer belt  633 . This data can be contained in the data structure  703  as needed. 
   A data structure  704  shown in  FIG. 21A  illustrates data on printer engine operating conditions determined by the printer engine control unit  105  in order to allow the printer engine  100  to function as an image forming apparatus. For example, the following operating conditions can be derived from the data structure  704 : operating conditions for allowing all the units to normally operate and allowing the printer engine  100  to stably operate as an image forming apparatus on the basis of the process speed and power consumption data determined by the color mode/material conditions sent from the paper transport platform  60  and the image forming subsystem  150  using the data structures  701  and  703  and the available power supply data using the data structure  702 . Additionally, the printer engine control unit  105  may prestore some operating conditions as default values and select the operating conditions that are consistent with the data collected from the units. In the example shown by the data structure  704 , process speeds and PPMs (print per minute) for three color mode/material conditions are determined. In addition, a combination of the color mode and a material that is not supported can be sent as needed. 
   A data structure  705  shown in  FIG. 21B  illustrates data sent from the image formation control unit  160  and the platform control unit  65  to the printer engine control unit  105  again after the image formation control unit  160  and the platform control unit  65  receive the operating conditions from the printer engine control unit  105  and redetermine the power consumption under the received conditions. The printer engine control unit  105  uses this data for comparing the available electric power received from the power supply unit  90  using the data structure  702  with the sum of electric power consumed by the units under the determined conditions and then determining the operability or correcting the conditions. 
   So far, the parameters of the configuration communications when the power is on have been described. 
   In the foregoing description, it has been assumed that each unit of the paper transport platform  60  and the image forming subsystem  150 , for example, the image producing unit  170  and the fixing unit  180  of the image forming subsystem  150  have no control unit (controller) (such as a CPU). That is, the subsystem itself stores and controls the capability information about its accompanying units. However, if the accompanying units include control units (controllers) thereof, the platform control unit  65  and the image forming subsystem  150  may receive the configuration information having the data structure  701  from the accompanying units and put this information together. Subsequently, the platform control unit  65  and the image forming subsystem  150  may communicate this information with the printer engine control unit  105 . 
   (B) Command Sequence of Configuration Information when Power is Turned ON 
     FIGS. 22A and 22B  illustrate the command sequence of the configuration information in detail when the power is turned on. 
   In an example shown in  FIG. 22A , the paper transport platform  60  and the image forming subsystem  150  function as a system that stores and controls the capability information about its accompanying units. 
   When a power switch SW (not shown) is turned on and the power supply unit  90  supplies power to the units, the platform control unit  65  and the image formation control unit  160  transmit the capability information based on the data structure  701  to the printer engine control unit  105  as configuration data. At that time, the image formation control unit  160  appends the data indicated by the data structure  703  to the data indicated by the data structure  701 . At almost the same time as this data communication, the power supply unit  90  transmits the available electric power data based on the data structure  702  to the printer engine control unit  105 . 
   On the basis of the received configuration data, the printer engine control unit  105  determines the operating conditions for the image forming apparatus (such as process speeds and the PPM for each of materials and the color mode). Thereafter, the printer engine control unit  105  transmits the determined operating conditions to the platform control unit  65  and the image formation control unit  160  using the data structure  704 . 
   The platform control unit  65  and the image formation control unit  160  operate under the operating conditions based on the data in the data structure  704  and prepare the image forming operation (e.g., generation of the operation parameters). At the same time, the platform control unit  65  and the image formation control unit  160  recalculate the power consumption under the provided operating conditions. The platform control unit  65  and the image formation control unit  160  then transmit the calculation result to the printer engine control unit  105  using the data structure  705 . 
   After the above-described command sequence is carried out, a series of the configuration communication when the power is on is completed. 
     FIG. 22B  illustrates an example of a sequence when the units accompanying the paper transport platform  60  and the image forming subsystem  150  include control units (controllers) thereof. 
   When a power SW (not shown) is turned on and the power supply unit  90  supplies power to the units, the feeder unit  70  and the transport unit  80  accompanying the platform control unit  65  transmit the capability information based on the data structure  701  to the platform control unit  65  as configuration data. Similarly, the fixing unit  180  accompanying the image formation control unit  160  transmits the capability information based on the data structure  701  to the image formation control unit  160 . The image producing unit  170  transmits the data indicated by the data structure  703  to the image formation control unit  160  in addition to the data indicated by the data structure  701 . 
   On the basis of the capability information transmitted from the feeder unit  70  and the transport unit  80 , the platform control unit  65  determines the capability information thereof. The image formation control unit  160  carries out the similar operation. Thereafter, to the printer engine control unit  105 , the platform control unit  65  transmits the capability information based on the data structure  701  and the image formation control unit  160  transmits the capability information based on the data structure  703  in addition to the capability information based on the data structure  701  as the configuration data. At almost the same time as the data communication, the power supply unit  90  transmits the available electric power data based on the data structure  702  to the printer engine control unit  105 . 
   On the basis of the received configuration data, the printer engine control unit  105  determines the operating conditions for an image forming apparatus (such as a process speed and a PPM for each of materials and the color modes). 
   Thereafter, the printer engine control unit  105  transmits the determined operating conditions to the platform control unit  65  and the image formation control unit  160  using the data structure  704 . The platform control unit  65  and the image formation control unit  160  operate under the operating conditions based on the data in the data structure  704  and transmit that information to the accompanying feeder unit  70 , the transport unit  80 , the image producing unit  170 , and the fixing unit  180 . 
   The feeder unit  70 , the transport unit  80 , the image producing unit  170 , and the fixing unit  180  recognize requests to operate under the provided operating conditions and prepare the image forming operation (e.g., generation of the operation parameters). At the same time, the feeder unit  70 , the transport unit  80 , the image producing unit  170 , and the fixing unit  180  recalculate the power requirements under the provided operating conditions. The feeder unit  70 , the transport unit  80 , the image producing unit  170 , and the fixing unit  180  then transmit the calculation result to the platform control unit  65  and the image formation control unit  160  using the data structure  705 . 
   Each of the platform control unit  65  and the image formation control unit  160  computes the sum of electric power on the basis of the consumed power data transmitted from the accompanying units and then transmits the computation result to the printer engine control unit  105  using the data structure  705 . 
   After the above-described command sequence is carried out, a series of the configuration communication when the power is turned on is completed. 
   (C) Communication Parameter and Command Sequence During Image Forming Operation 
   The communication parameters and command sequence between the units during an image forming operation performed by the printer engine  100  are described next with reference to  FIGS. 23A-F  and  FIGS. 24A and 24B .  FIGS. 23A-F  illustrate communication parameters exchanged between the units during the image forming operation.  FIGS. 24A and 24B  illustrate a communication command sequence during the image forming operation. 
   A data structure  711  shown in  FIG. 23A  is common part of the paper-feed request commands and the parameters transmitted from the printer engine control unit  105  to the platform control unit  65  and the image formation control unit  160  in order to start transporting the transfer medium P during the image forming operation. 
   Since command data shown in the data structure  711  relates to a paper feed request, this data can be transmitted only to the platform control unit  65 . Alternatively, this data can be also transmitted to the image formation control unit  160  in order to make an appointment to form an image. In this exemplary embodiment, the data is also transmitted to the image formation control unit  160  in order to make an appointment to form an image. 
   Examples of data required for the paper-feed start request in the data structure  711  include a command ID that indicates a paper-feed start request command, a page ID corresponding to requesting image data, a color mode, a paper size, material information, a printed surface (one side, a first side of two sides, a second side of two sides). 
   Command data shown by a data structure  712  shown in  FIG. 23B  is data to be transmitted that is not necessary for the image formation control unit  160  as appointment information on the image forming operation, but is necessary for the platform control unit  65  to control the transport of the transfer medium P and is not included in the command data in the data structure  711 . More specifically, this command data includes feeder station information and an output direction required for transporting the transfer medium P in the transport unit. 
   A data structure  713  shown in  FIG. 23C  represents paper-feed request ACK command data used for the platform control unit  65  to inform the printer engine control unit  105  of the determination result of start of the paper feed operation. The parameters of the command include a page ID, feeder station information, feed status information indicating whether the paper feed normally starts or not, and “NOT OK” factor information indicating the cause of failure when the paper feed does not normally start. Examples of the cause include the paper presence status, the error status, and a paper jam status. In addition, in this exemplary embodiment, the time when the platform control unit  65  transmits the paper-feed request ACK command indicates the time when the start of image formation is allowed. 
   A data structure  714  shown in  FIG. 23D  represents image-formation request command data that is transmitted from the printer engine control unit  105  to the image formation control unit  160  when the platform control unit  65  informs the printer engine control unit  105  of the start of paper feed using the data structure  713 . When the printer engine control unit  105  is ready for image formation, the printer engine control unit  105  issues this command. Examples of the parameter include a page ID and a color mode. 
   A data structure  715  shown in  FIG. 23E  represents an image forming operation start notification command sent from the image formation control unit  160  to inform the printer engine control unit  105  of the start of the image forming operation after the image formation control unit  160  receives the image forming request using the data structure  714 . In accordance with the configuration of the image formation control unit  160 , the image formation control unit  160  generates the ITOP signal serving as a trigger that starts the image forming operation. At the same time, the image formation control unit  160  issues this image forming operation start notification command. Upon receiving this command based on the data structure  715 , the printer engine control unit  105  transmits this data structure  715  to the platform control unit  65  in order to control the transport of the transfer medium P. Examples of the parameter include a page ID. 
   A data structure  716  shown in  FIG. 23F  represents data of an image formation and transport termination acknowledgment command sent from the platform control unit  65  when the platform control unit  65  detects the completion of the image forming operation and transport operation. At that time, the transfer medium P may be output to outside the apparatus or the transfer medium P may remain in the apparatus due to a paper jam. On the basis of this command, the printer engine control unit  105  determines whether the image formation of the target image (page) is normally completed or not. Examples of the parameter include a completion status indicating a normal completion or abnormal completion and a “not OK” cause indicating the cause of the abnormal completion. Examples of the “not OK” cause include an error status and a jam status. 
   So far, the parameters of the command data communicated between the printer engine control unit  105  and the platform control unit  65  and between the printer engine control unit  105  and the image formation control unit  160  during an image forming operation have been described in detail. 
   In the foregoing description, it has been assumed that each unit of the paper transport platform  60  and the image forming subsystem  150 , for example, the image producing unit  170  or the fixing unit  180  of the image forming subsystem  150  has no control unit (controller) (such as a CPU). That is, the subsystem itself controls its accompanying units. However, if the accompanying units include control units (controllers) thereof, the platform control unit  65  and the image formation control unit  160  may transmit command data to the accompanying units thereof on the basis of the received command data at appropriate timings so that the accompanying units partially control the image forming operation. When needed, the platform control unit  65  and the image formation control unit  160  may receive the result of the image forming operation from the accompanying units and, subsequently, communicate with the printer engine control unit  105 . 
   The command sequence during an image forming operation is described in detail next with reference to  FIGS. 24A and 24B . 
   In the present exemplary embodiment, the description is provided when a typical 1-page image forming operation normally starts and ends. 
     FIG. 24A  illustrates an example of the sequence when the paper transport platform  60  and the image forming subsystem  150  control the accompanying units thereof. To start the image forming operation, the printer engine control unit  105  transmits a paper feed request command to the platform control unit  65  and the image formation control unit  160 . At that time, the printer engine control unit  105  transmits the data represented by the data structure  712  and the data represented by the data structure  711  to the platform control unit  65 . The printer engine control unit  105  transmits the data represented by the data structure  711  to the image formation control unit  160 . 
   Upon receiving the paper feed request command, the platform control unit  65  determines whether the platform control unit  65  can start feeding the paper. The platform control unit  65  then transmits the result of the determination as a paper feed request ACK command represented by the data structure  713  to the printer engine control unit  105 . Examples of conditions that allow the start of the paper feed include the presence of the transfer medium P and the non-occurrence of a jam of the transfer medium P previously fed. 
   Upon receiving the paper feed request ACK command  713  and determining that the platform control unit  65  can start feeding the transfer medium P, the printer engine control unit  105  transmits an image formation start request represented by the data structure  714  to the image formation control unit  160 . 
   Upon receiving the image formation start request represented by the data structure  714 , the image formation control unit  160  determines the period of image formation obtained from the PPM setting value and the elapsed time since the previous image formation was completed. If the image formation control unit  160  determines that the image formation can be carried out, the image formation control unit  160  generates the ITOP signal so as to start the image forming operation. At the same time, the image formation control unit  160  transmits an image forming operation start notification represented by the data structure  715  to the printer engine control unit  105 . 
   Upon receiving the image forming operation start notification represented by the data structure  715  and recognizing that the image formation normally starts, the printer engine control unit  105  transmits the data represented by the data structure  715  to the platform control unit  65  in order to control the transport of the transfer medium P. Upon receiving the data represented by the data structure  715 , the platform control unit  65  recognizes that the transport of the target transfer medium P is controlled by the registration roller and the transfer to the transfer medium P is controlled by the secondary transfer units  16  and  34 . At the same time, the image formation control unit  160  controls the registration roller so that the position of the developed image is aligned with the position of the transfer medium P after a predetermined time elapses from the time the ITOP signal is generated. The image formation control unit  160  also transmits a registration signal to the platform control unit  65  to inform the platform control unit  65  of the start of the transport operation of the transfer medium P. Upon receiving the registration signal, the platform control unit  65  starts driving the load (such as the driving motor) upstream of the registration roller. 
   After the platform control unit  65  and the image formation control unit  160  control the image forming operation and the transport operation, the target transfer medium P is delivered from the image forming subsystem  150  to the paper transport platform  60 . Subsequently, upon recognizing that the transfer medium P is output from the paper transport platform  60  to outside the apparatus, the platform control unit  65  issues an image forming and transport termination command represented by the data structure  716  to the printer engine control unit  105 . 
   Upon receiving the image forming and transport termination command represented by the data structure  716 , the printer engine control unit  105  recognizes that the series of image forming operations for the transfer medium P corresponding to the target image has been completed. 
   So far, the details of a command sequence from the start to the end of a 1-page image forming operation in the system in which the paper transport platform  60  and the image forming subsystem  150  control the accompanying units thereof have been described. 
     FIG. 24B  illustrates an example of the sequence when the units accompanying the paper transport platform  60  and the image forming subsystem  150  include dedicated control units (dedicated controllers). To start the image forming operation, the printer engine control unit  105  transmits a paper feed request command to the platform control unit  65  and the image formation control unit  160 . At that time, the printer engine control unit  105  transmits the data represented by the data structure  712  as well as the data represented by the data structure  711  to the platform control unit  65 . The printer engine control unit  105  transmits the data represented by the data structure  711  to the image formation control unit  160 . 
   Upon receiving the paper feed request command, the platform control unit  65  directly transmits the received paper feed request command  711  and the data represented by the data structure  712  to the feeder unit  70 . 
   In addition, the image formation control unit  160  directly transmits the received paper feed request command  711  to the image producing unit  170  and the fixing unit  180 . 
   Upon receiving the paper feed request command, the feeder unit  70  determines whether the feeder unit  70  can start feeding the paper. The feeder unit  70  then transmits the result of the determination as a paper feed request ACK command represented by the data structure  713  to the platform control unit  65 . Examples of condition that allows the start of the paper feed include the presence of the transfer medium P and the non-occurrence of a jam of the transfer medium P previously fed. 
   Similarly, the platform control unit  65  transmits a paper feed request ACK command having the data structure  713  that is the same as the paper feed request ACK command received from the feeder unit  70  to the printer engine control unit  105 . Upon receiving the paper feed request ACK command  713  and recognizing that the platform control unit  65  can start feeding paper, the printer engine control unit  105  transmits an image formation start request having the data structure  714  to the image formation control unit  160 . 
   The image formation control unit  160  transmits the received image formation start request command  714  to the image producing unit  170  and the fixing unit  180  without changing any information. Upon receiving the image formation start request having the data structure  714 , the image producing unit  170  determines a period of image formation obtained from the PPM setting value and an elapsed time since the previous image formation has been completed. If the image producing unit  170  determines that the image formation can be carried out, the image producing unit  170  generates the ITOP signal so as to start the image forming operation. At the same time, the image producing unit  170  transmits an image forming operation start message having the data structure  715  to the image formation control unit  160 . 
   The image formation control unit  160  transmits a message that is the same as the image forming operation start message having the data structure  715  transmitted from the image producing unit  170  to the printer engine control unit  105 . Similarly, the image formation control unit  160  transmits the image forming operation start message having the data structure  715  to the fixing unit  180  in order to inform the fixing unit  180  of the arrival of the transfer medium P since the image producing unit  170  starts the image forming operation. 
   The printer engine control unit  105  receives the image forming operation start message having the data structure  715  and recognizes that the image forming operation starts normally. The printer engine control unit  105  then transmits the image forming operation start message having the data structure  715  to the platform control unit  65  in order to control the transport of the transfer medium P. Upon receiving the data having the data structure  715 , the platform control unit  65  transmits data that is the same as the image forming operation start message having the data structure  715  to the feeder unit  70 . 
   Upon receiving the data represented by the data structure  715 , the platform control unit  65  and the feeder unit  70  recognize that the transport of the target transfer medium P is controlled by the registration roller and the transfer to the transfer medium P is controlled by the secondary transfer units  16  and  34 . At the same time, the image producing unit  170  controls the registration roller so that the position of the developed image is aligned with the position of the transfer medium P after a predetermined time passes since the ITOP signal is generated. The image producing unit  170  also transmits a registration signal to the platform control unit  65  via the image formation control unit  160  so as to inform the platform control unit  65  of the start of the transport operation of the transfer medium P. Upon receiving the registration signal, the platform control unit  65  sends the registration signal to the feeder unit  70  without delay so that the feeder unit  70  starts driving the load (such as the driving motor) upstream of the registration roller. 
   When the transfer medium P is delivered from the image forming subsystem  150  to the paper transport platform  60  after a predetermined time has elapsed from the time the platform control unit  65  received the image forming operation start message having the data structure  715 , the platform control unit  65  sends a paper feed start request command generated from the information already received in the data structures  711  and  712  to the transport unit  80 . Thus, the transport unit  80  prepares for receiving the transfer medium P. 
   Subsequently, the transport unit  80  receives the transfer medium P and transports the transfer medium P. Finally, upon recognizing that the transfer medium P is output to outside the apparatus, the transport unit  80  issues an image forming and transport termination command represented by the data structure  716  to the platform control unit  65 . 
   Upon receiving the image forming and transport termination command represented by the data structure  716 , the platform control unit  65  transmits a message having the same information as the received image forming and transport termination command to the printer engine control unit  105 . Upon receiving the received image forming and transport termination command having the data structure  716 , the printer engine control unit  105  recognizes that the series of image forming operations for the transfer medium P corresponding to the target image has been completed. 
   So far, the details of a command sequence from the start to the end of a 1-page image forming operation have been described when the units accompanying the paper transport platform  60  and the image forming subsystem  150  include dedicated control units thereof. 
   Advantages of Present Embodiment 
   When users purchase an image forming apparatus (such as a copier), the users are forced to select a desired one from among the lineup of the image forming apparatuses that the product provider (manufacturer) provides. Therefore, if the user needs a color copier due to changes in use environment after the user purchased a black and white copier, the user must replace the black and white copier with a color copier or additionally purchase the color copier. This places the economical burden on the user. That is, existing image forming apparatuses cannot flexibly support the user needs. 
   Therefore, according to the present embodiment, a structure is provided in which a plurality of subsystems having a variety of capabilities (e.g., the paper transport platform  60  and the image forming subsystem  150 ) can be connected to a basic platform (the engine platform  101 ). Each of the subsystems includes, for example, a plurality of types of units having different performance (e.g., the feeder unit  70  and the transport unit  80  in the paper transport platform  60 , and the image producing unit  170  and the fixing unit  180  in the image forming subsystem  150 ). The printer engine control unit  105  controls the operations of the subsystems so that a series of image output operations are carried out in parallel or independently. 
   In such a structure, the subsystem is replaced in accordance with the user needs, serviceability, and expandability so that various subsystems are interchangeably assembled into the platform. Thus, an apparatus that performs a desired image forming operation is achieved. This structure facilitates the system configuration change and the system functionality change in accordance with individual user needs when the user uses the image forming apparatus. Accordingly, a customizable image forming apparatus can be provided to individual users. Furthermore, the latest technology, service, and solution can be provided to the user at an optimal time. 
   While the foregoing description has been made with reference to an image forming apparatus using an electrophotographic recording method or an electrostatic recording method, the embodiment of the present invention is also applicable to an image forming apparatus using a recording method other than the electrophotographic recording method. In particular, the exemplary embodiment of the present invention relates to an image forming apparatus having the image forming functionality, paper transport functionality, and control functionality and is suitably applied to a copier, a printer, a multi-function printer, and various image forming apparatuses. Additionally, by changing a platform and combining appropriate subsystems, the number of models of the image forming apparatus can be increased. 
   Second Exemplary Embodiment 
   In a second exemplary embodiment, a system in which the printer engine control unit  105  operates on the basis of the same CPU resources as those of the platform control unit  65  is described. 
     FIG. 25  is an illustration of an exemplary hardware architecture of an image forming apparatus according to the second embodiment of the present invention.  FIG. 26  is a block diagram of the electrical connection of an image forming apparatus according to the second embodiment of the present invention. 
   As shown in  FIG. 26 , the printer engine control unit  105  manages the control information on a platform control unit  65  included in the printer engine control unit  105 , the control information on an image forming subsystem acquired via communication with the image formation control unit  160 , and the control information on a power supply unit acquired via communication with a power supply unit  90 . For the other components, the connections and controls similar to those described referring to  FIGS. 1 and 13  can be applied. 
   While the foregoing description has been made with reference to a system in which each unit of the paper transport platform  60  and each unit of the image forming subsystem  150  include control units having CPUs and a system in which each unit has no CPUs, the combination of the units having CPUs and units having no CPUs is not limited thereto. This combination can be appropriately determined depending on the control of the units. 
   In addition, the foregoing description has been made with reference to a system in which the printer engine  100  includes the paper transport platform  60  and the image forming subsystem  150 , the paper transport platform  60  includes the feeder unit  70  and the transport unit  80 , and the image forming subsystem  150  includes the image producing unit  170  and the fixing unit  180 . However, the structure of subsystems in a printer engine, the platform, and the structure of the units in the subsystem are not limited thereto. The structure of the system can be appropriately determined depending on the control of the subsystem and the units. 
   The second exemplary embodiment can provide the same advantage as that of the first exemplary embodiment. By reexamining the hardware, mechanism, software, and automatic cassette change (ACC) of the subsystems that have different functions and that can be assembled in a base platform, the subsystems can be designed to be interchangeable. The subsystem may include a plurality of units. By replacing the subsystem, a system configuration change, a system functionality change, a service of replacement and examination, and the operation performed by a user and a service person can be efficiently carried out in terms of the user needs, serviceability, and expandability. Additionally, the number of models of the image forming apparatus can be increased so that a plurality of platforms have the compatibility. Thus, the latest technology, service, and solution can be provided to the user at an optimal time. Furthermore, a customizable print system can be provided to the user. 
   The present invention can also be achieved by supplying a recoding medium storing software program code that achieves the functions of the above-described embodiments to a system or an apparatus and by causing a computer (central processing unit (CPU) or micro-processing unit (MPU)) of the system or apparatus to read and execute the software program code. 
   In such a case, the program code itself read out of the recording medium realizes the functions of the above-described embodiments. Therefore, the storage medium storing the program code can also realize the present invention. 
   Examples of the recording medium for supplying the program code include a flexible disk, a hard disk, a magneto optical disk, a CD-ROM (compact disk-read only memory), a CD-R (CD recordable), a CD-RW (CD-rewritable), a DVD-ROM (digital versatile disk-read only memory), a DVD-RAM (DVD-random access memory), a DVD-RW (DVD-rewritable), a DVD+RW (DVD-rewritable), a magnetic tape, a nonvolatile memory card, a ROM (read only memory) or the like. Alternatively, the program code can be downloaded via a network. 
   Additionally, the functions of the above-described embodiments can be realized by another method in addition to executing the program code read out by the computer. For example, the functions of the above-described embodiments can be realized by a process in which an operating system (OS) running on the computer executes some of or all of the functions in the above-described embodiments under the control of the program code. 
   The present invention can also be achieved by writing the program code read out of the storage medium to a memory of an add-on expansion board of a computer or a memory of an add-on expansion unit connected to a computer. The functions of the above-described embodiments can be realized by a process in which, after the program code is written, a CPU in the add-on expansion board or in the add-on expansion unit executes some of or all of the functions in the above-described embodiments under the control of the program code. 
   In such a case, the program code can be supplied directly from the storage medium that stores the program or by downloading from another computer and a database (not shown) connected to the Internet, a commercial network, or a local area network. 
   The present invention can be applied to a system including a plurality of devices, or to a single-device apparatus. Furthermore, the invention is applicable also to a case where the object of the invention is attained by supplying a program to a system or apparatus. 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
   This application claims the benefit of Japanese Application Nos. 2005-258385 filed Sep. 6, 2005 and 2006-205677 filed Jul. 28, 2006, which are hereby incorporated by reference herein in their entirety.