Patent Publication Number: US-8538309-B2

Title: Printing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2009-222188 filed on Sep. 28, 2009 and Japanese Patent Application No. 2010-057693 filed on Mar. 15, 2010, and the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a printing apparatus that performs duplex printing. More particularly, the present invention relates to a printing apparatus that is capable of performing duplex printing (hereinafter appropriately referred to as a high-speed duplex printing) where the printing apparatus prints one surface of a first sheet, prints one surface of a second sheet subsequent to the first sheet after the first sheet enters a reconveyer path, and prints the other surface of the first sheet in this order. 
     BACKGROUND 
     There is a related printing apparatus capable of high-speed duplex printing. The related printing apparatus adjust the number of the sheets to be conveyed based on the size of the sheets. Thereby, the related printing apparatus selects the most suitable feeding operation based on the size of the sheets. For example, such a related printing apparatus sets a size of sheets to be printed in advance, adjusts the number of sheets to a suitable number based on the sheet size set in advance, and conveys the suitable number of sheets. 
     However, the printing apparatus of the related art has the following problems. That is to say, in the related art technique, a sheet conveyance is controlled based on the assumption that the sheet size is set in advance and sheets of the set size are set on a feeding section. Since the size of the sheets is set in advance in the related art, it is not possible to convey the sheets in an appropriate manner in a case where a size of sheets actually conveyed is different from the size of sheets set in advance. Examples of such case are that users have made mistakes in sheet settings or that sheets were automatically fed from a different sheet tray in response to detection of absent of sheet. 
     SUMMARY 
     The exemplary embodiments of the present invention have been made to solve the problems of the printing apparatus according to the related art. That is, the exemplary embodiments of the present invention provide a printing apparatus that performs an appropriate duplex printing operation in accordance with the size of the sheet being actually conveyed. 
     The first aspect of the exemplary embodiments of the present invention is a printing apparatus comprising: a printing section comprising: a conveyer conveying a first sheet and a second sheet subsequent to the first sheet; a printer printing one surface and the other surface of the first sheet, and printing one surface and the other surface of the second sheet; and a reconveyer conveying the first sheet after printing the one surface of the first sheet and conveying the second sheet after printing the one surface of the second sheet, wherein the printing section performing a high-speed duplex printing where the printer prints the one surface of the first sheet, the one surface of the second sheet after the first sheet is in the reconveyer, and the other surface of the first sheet in this order; a sensor specifying a length of the first sheet while the conveyer conveys the first sheet and a length of the second sheet while the conveyer conveys the second sheet; a determination section determining whether a pair of the first sheet and the second sheet is eligible for the high-speed duplex printing based on the combination of the length of the first sheet and the length of the second sheet before the second sheet conveyed by the reconveyer; and a controller controlling the printing section to convey the first sheet and the second sheet to the reconveyer when the determination section determines the pair of the length of the first sheet and the length of the second sheet is eligible for the high-speed duplex printing, and controlling the printing section to eject at least one of the first sheet and the second sheet when the determination section determines the pair of the length of the first sheet and the length of the second sheet is not eligible for the high-speed duplex printing. 
     The printing apparatus of the exemplary embodiments of the present invention is a printing apparatus capable of high-speed duplex printing and specifies the sheet length of a sheet while conveying the target sheet. Especially, specifies the sheet length of the first sheet (preceding sheet) and the second sheet (target sheet) subsequent to the first sheet. Moreover, it is determined based on the combination of the identified sheet lengths of the preceding sheet and the target sheet whether the printing apparatus is able to continue the high-speed duplex printing or not. Then, based on the determination results, it is determined whether the respective sheets will be conveyed to the reconvey path or the respective sheets will be ejected without conveyance to the reconvey path. A combination for which high-speed duplex printing cannot be continued corresponds to a case, for example, where the sheet length of the target sheet is too large, and there is a possibility that conveying control based on the sheet length of the preceding sheet may result in a collision of the target sheet and the preceding sheet during the concurrent convey of both sheets. 
     That is, in the printing apparatus according to the exemplary embodiments of the present invention, the sheet length of a sheet (target sheet) being conveyed actually is specified, and conveyance of the respective sheets is controlled based on the determination results on the conveyability corresponding to the combination of the target sheet and the preceding sheet. Therefore, it is possible to perform the duplex printing operation more appropriately. For example, when the combination of sheets is inappropriate, by not conveying at least one of the target sheet and the preceding sheet to the reconvey path, it is possible to avoid troubles concerning the sheet convey. Moreover, there is a case where high-speed duplex printing is possible for a combination of sheets having different sheet sizes. For this reason, the high-speed duplex printing may be continued depending on the combination rather than uniformly applying error processing for stopping printing in all cases where the sheet sizes are different. 
     According to the present invention, it is possible to realize a printing apparatus that performs an appropriate duplex printing operation in accordance with the size of sheet being conveyed actually. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing a schematic configuration of a printer according to the embodiment. 
         FIG. 2  is a schematic diagram showing an internal configuration of the printer shown in  FIG. 1 . 
         FIGS. 3A to 3H  are schematic diagrams showing the flow of a high-speed duplex printing operation. 
         FIG. 4  is a block diagram showing an electrical configuration of the printer shown in  FIG. 1 . 
         FIG. 5  shows an example of the conveyability corresponding to a combination of sheets. 
         FIG. 6  is a schematic diagram showing a combination of sheets for which high-speed duplex printing is not possible. 
         FIG. 7  is a schematic diagram showing a combination of sheets for which high-speed duplex printing is possible. 
         FIG. 8  is a flowchart showing the flow of a duplex printing process. 
         FIG. 9  is a flowchart showing the flow of a convey adjustment process. 
         FIG. 10  shows an example of a convey operation selection screen. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of a printing apparatus according to the present invention will be described with reference to the accompanying drawings. This embodiment applies the present invention to an electrophotographic printer that is capable of performing single-sided printing on two subsequent sheets and then performing printing on the other side of the sheet at the time of performing duplex printing. 
     [Overall Configuration of Printer] 
     As shown in  FIG. 1 , a printer  100  according to this embodiment includes a main section  10  that forms images on a sheet and an operation panel  40  that is positioned on the surface of the main section  10 . The operation panel  40  includes a display section  41  formed of a liquid crystal display and a group of buttons  42  including a start key, a stop key, a number pad, and the like. With this operation panel  40 , an operation status can be displayed to users, and users can perform an input operation on the printer  100 . 
     [Internal Configuration of Printer] 
       FIG. 2  shows an internal configuration of the printer  100 . As shown in  FIG. 2 , the printer  100  includes a process section  50  (an example of a print section) that forms a toner image by a well-known electrophotographic method, feeder cassettes  91  and  92  in which non-printed sheets are placed, an eject tray  96  in which printed sheets are placed, and a sheet sensor  60  that detects passage of sheets. 
     Moreover, the printer  100  has an approximately S-shaped convey path  11  (depicted by a one-dot chain line in  FIG. 2 ). The convey path  11  allows a sheet accommodated in the feeder cassette  91  (or  92 ) positioned in the lower part to be passed through the feeder roller  71  (or  72 ) and the process section  50  and be guided to a eject tray  96  positioned in the upper part by a eject roller  76 . 
     The sheet sensor  60  is positioned upstream the process section  50  and downstream the feeder roller  71  in the convey direction of sheets and detects whether or not a sheet has passed through a predetermined position of the convey path  11 . That is to say, the printer  100  is able to detect passage of the leading edge of a sheet when an output signal from the sheet sensor  60  changes from a state indicative of non-presence of sheet to a state indicative of presence of sheet and to detect passage of the bottom edge of a sheet when the output signal from the sheet sensor  60  changes from a state indicative of presence of sheet to a state indicative of non-presence of sheet. Moreover, the printer  100  determines the time to convey a subsequent sheet in response to the passage of the bottom edge of the sheet, for example. 
     Moreover, the sheet sensor  60  is used for measurement of a sheet length. That is to say, the printer  100  acquires the time taken from the passage of the leading edge of a sheet to the passage of the bottom edge of the sheet based on the signal from the sheet sensor  60  and calculates the length of the sheet being actually conveyed along the convey path  11  based on the acquired time and the convey speed of the sheet. 
     The printer  100  of this embodiment picks up the sheets placed in the feeder cassette  91  (or  92 ) on a one-by-one basis, conveys the picked-up sheet to the process section  50 , and transfers a toner image formed in the process section  50  to the sheet. In addition, the sheet having the toner image transferred thereto is conveyed to a fixing section in the process section  50  so that the toner image is thermally fixed onto the sheet. Then, the sheet having the toner image fixed thereto is ejected to the eject tray  96 . 
     Moreover, the printer  100  has a duplex printing mechanism for performing printing on both sides of a sheet. A reconvey path  12  (depicted by a two-dot chain line in  FIG. 2 ) in  FIG. 2  is a convey path for reversing a sheet with one side (single side) printed and reconveying it to the process section  50  so as to perform printing on an obverse side (the other side) of the sheet. The reconvey path  12  is branched from the convey path  11  at a branch point  15  that is positioned upstream the eject roller  76  and downstream the process section  50  in the convey direction of sheets. Moreover, the reconvey path  12  forms a path that extends from the branch point  15  along the space between the process section  50  and the feeder cassettes  91  and  92  and converges with the convey path  11  at a side of the convey path  11  located upstream the process section  50 . 
     Specifically, by the duplex printing of the printer  100 , the sheets are reversed in the following order. First, a sheet for which single-sided printing is performed along the convey path  11  is conveyed to the eject roller  76 . When the bottom edge of the sheet is passed through the branch point  15 , the eject roller  76  is temporarily stopped with the sheet pinched. Subsequently, the rotation direction of the eject roller  76  is changed to reverse the convey direction of the sheet, thus conveying the sheet to the reconvey path  12 . Then, the sheet is conveyed back to the convey path  11  at a side of the convey path  11  located upstream the process section  50 . In this way, the sheet is reversed upside down with the other side printed. 
     Moreover, the printer  100  has a high-speed duplex printing function of performing single-sided printing on two subsequent sheets and then performing single-sided printing on the other sides of two subsequent sheets at the time of performing duplex printing. Specifically, the sheets are conveyed in the order as shown in  FIGS. 3A to 3H . 
     (A) A preceding sheet S 1  which is the first sheet is conveyed to the convey path  11 . 
     (B) Single-sided printing is performed on the preceding sheet S 1 . 
     (C) The preceding sheet S 1  is conveyed to the reconvey path  12 , and a subsequent sheet S 2  which is the second sheet is conveyed to the convey path  11 . 
     (D) The preceding sheet S 1  is conveyed back to the convey path  11 , and single-sided printing is performed on the subsequent sheet S 2 . 
     (E) Single-sided printing is performed on the other side of the preceding sheet S 1 , and the subsequent sheet S 2  is conveyed to the reconvey path  12 . 
     (F) The preceding sheet S 1  is ejected, and the subsequent sheet S 2  is conveyed back to the convey path  11 . 
     (G) Single-sided printing is performed on the other side of the subsequent sheet S 2 . 
     (H) The subsequent sheet S 2  is ejected. 
     That is to say, in the high-speed duplex printing of the printer  100 , two sheets are conveyed into the printer  100  and printing is performed in the order of single side (first sheet), single side (second sheet), the other side (first sheet), and the other side (second sheet). This convey order results in a shorter standby time for the process section  50  compared to a case (hereinafter referred to as “low-speed duplex printing”) where printing is performed in the order of single side of each sheet and the other side of each sheet, thus improving printing quality. 
     [Electrical Configuration of Printer] 
     Subsequently, an electrical configuration of the printer  100  will be described. As shown in  FIG. 4 , the printer  100  includes a control section  30  that includes a CPU  31 , a ROM  32 , a RAM  33 , a NVRAM (nonvolatile RAM)  34 , an ASIC  35 , and a network interface  36 . Moreover, the control section  30  is electrically connected to the process section  50 , the operation panel  40 , the sheet sensor  60 , and the like. 
     The ROM  32  stores various control programs for controlling the printer  100 , various setting values, initial values, and the like. The RAM  33  is used as a work area, to which various control programs are read, or a storage area, in which image data are temporarily stored. 
     The CPU  31  controls various constituent elements (for example, lighting timings of an exposure section, driving motors (not shown) of various rollers that constitute the convey path  11  or the reconvey path  12 ) of the printer  100  through intervention of the ASIC  35  while storing processing results in the RAM  33  or the NVRAM  34  in accordance with the control programs read from the ROM  32  or signals sent from various sensors. 
     The network interface  36  is connected to a network such as a LAN and enables connection to an external device in which a printer driver for the printer  100  is installed. The printer  100  is able to exchange print jobs via the network interface  36 . 
     [Duplex Printing Control of Printer] 
     Subsequently, duplex printing control of the printer  100  will be described. The printer  100  is capable of high-speed duplex printing and performs high-speed duplex printing upon receiving a duplex printing instruction. However, the high-speed duplex printing is not possible if the sheet length in the sheet convey direction is too large. In this case, the high-speed duplex printing is abandoned and low-speed duplex printing or single-sided printing is performed. 
     In this embodiment, it will be assumed that the three types “A4,” “Letter,” and “Legal” are supported as the sheet size for which duplex printing is possible. Moreover, it will be assumed that high-speed duplex printing is possible for “A4” and “Letter”, but high-speed duplex printing is not possible for “Legal”. The respective sheet sizes are as follows: 
     A4: (Main) 210 mm×(Sub) 297 mm 
     Letter: (Main) 216 mm×(Sub) 280 mm 
     Legal: (Main) 216 mm×(Sub) 356 mm 
     Here, “Main” means the main-scanning direction, and “Sub” means the sub-scanning direction (the sheet convey direction). 
     Moreover, in many cases, when high-speed duplex printing is performed, the preceding sheet S 1  and the subsequent sheet S 2  are feed from the same feeder cassette, and thus they have the same sheet size. However, when a feeder cassette that fed the preceding sheet S 1  runs short of sheets and sheets are automatically fed from a different feeder cassette, there is a possibility that a sheet of a different size is fed from the feeder cassette. Moreover, there is also a possibility that sheets of different sizes are accommodated in the same feeder cassette. 
     Furthermore, depending on a combination of the preceding sheet S 1  and the subsequent sheet S 2 , there may be a case where high-speed duplex printing can be continued and a case where high-speed duplex printing cannot be continued.  FIG. 5  shows the relationship between the continuability of high-speed duplex printing and combinations of the preceding sheet S 1  and the subsequent sheet S 2 . 
     In  FIG. 5 , “O” means that continuous high-speed duplex printing is possible. That is, for a combination of sheets for which high-speed duplex printing is basically possible, continuous high-speed duplex printing will be possible for any combination thereof. 
     In  FIG. 5 , “X” means that continuous high-speed duplex printing is not possible. That is, if the preceding sheet S 1  is “A4” and the subsequent sheet S 2  is “Legal,” there is a concern that as shown in  FIG. 6 , the preceding sheet S 1  and the subsequent sheet S 2  (in the figure, S 2  depicted by a dotted line) collide with each other in the course of convey at the convey order shown in  FIG. 3E . For this reason, for this combination, printing of the other side of the subsequent sheet S 2  is abandoned, and the subsequent sheet S 2  is ejected (in the figure, S 2  depicted by a solid line). 
     In  FIG. 5 , “⊚” means that continuous high-speed duplex printing is possible. That is, if the preceding sheet S 1  is “Letter” and the subsequent sheet S 2  is “Legal,” the sheet length of the preceding sheet S 1  in the sheet convey direction is small. Thus, for example, as shown in  FIG. 7 , there is little possibility that the preceding sheet S 1  and the subsequent sheet S 2  collide with each other in the course of convey at the convey order shown in  FIG. 3E . For this reason, high-speed duplex printing is continued for this combination. 
     In  FIG. 5 , “-” means a combination that does not exist. That is, the printer  100  switches to low-speed duplex printing from the beginning if the preceding sheet S 1  has a size for which high-speed duplex printing is not possible. For this reason, if the preceding sheet S 1  is “Legal,” its combination with the subsequent sheet  52  does not exist. These sheet size types, and combinations and collision examples thereof are examples only and are not limited thereto. 
     [Duplex Convey Process] 
     Hereinafter, a duplex convey process (an example of a specifying section, a determination section, and a controller) for realizing the above-described duplex printing control will be described with reference to the flowchart of  FIG. 8 . This duplex convey process is executed in response to receiving of a duplex print job via the network interface  36 . 
     First, feeding of a preceding sheet S 1  is started (S 101 ). For example, when a print job in which “A4” is designated as a sheet size is received, sheets are fed from a feeder cassette that is designated to accommodate A4-size sheets, and conveying control for high-speed duplex printing is started. Thereafter, the sheet sensor  60  detects passage of the preceding sheet S 1 , thus acquiring the sheet length L 1  of the preceding sheet S 1  in the strain data (sub-scanning direction) (S 102 ). 
     Next, based on the sheet length L 1 , it is determined whether or not high-speed duplex printing is possible for the preceding sheet S 1  (S 103 ). That is, it is determined whether or not high-speed duplex printing is possible for the preceding sheet S 1  that has been conveyed actually. A threshold value T 1  of a sheet length for which high-speed duplex printing is possible is stored in the printer  100 . If the specified sheet length L 1  is smaller than the threshold value T 1 , it is determined that high-speed duplex printing is possible. In this embodiment, it is determined that high-speed duplex printing is possible for “A4” or “Letter”-size sheets, whereas it is determined that high-speed duplex printing is not possible for “Legal”-size sheets. 
     If it is determined that high-speed duplex printing is not possible (S 103 : NO), it is further determined whether or not duplex printing is possible for the preceding sheet S 1  (S 121 ). A threshold value T 2  (&gt;T 1 ) of a sheet length for which low-speed duplex printing is possible is stored in the printer  100 . If the specified sheet length L 1  is larger than the threshold value T 2 , it is determined that duplex printing is not possible. 
     If it is determined that duplex printing is possible (S 121 : YES), conveying control is switched so that low-speed duplex printing is performed for the preceding sheet S 1  (S 122 ). Especially, In a case where the preceding sheet S 1  is fed without precedent sheet either in the conveyer  11  or the reconveyer  12 , a subsequent sheet is fed after printing the other surface of the preceding sheet S 1 . On the other hand, if it is determined that duplex printing is not possible (S 121 : NO), duplex printing is abandoned and conveying control is switched so that single-sided printing is performed for the preceding sheet S 1  (S 123 ). 
     After S 122  or S  123 , it is determined whether or not next print data are present (S  109 ), and if print data is present (S 109 : YES), the flow returns to  5101  to start feeding of the preceding sheet S 1 , whereas if print data is not present (S 109 : NO), this process ends. 
     On the other hands, returning to the description of S 103 , if it is determined that high-speed duplex printing is possible (S 103 : YES), conveying control of the preceding sheet S 1  is continued so that high-speed duplex printing is performed (S 104 ). Moreover, during the convey of the preceding sheet S 1 , feeding of a subsequent sheet S 2  is started (S 105 ). Then, similar to S 102 , the sheet sensor  60  detects a passage of the subsequent sheet S 2 , thus acquiring the sheet length L 2  of the subsequent sheet S 2  in the strain data (sub-scanning direction) (S 106 ). 
     Next, based on the sheet length L 2 , it is determined whether or not high-speed duplex printing is possible for the subsequent sheet S 2  (S 107 ). In S 107 , the determination is made using the same threshold value T 1  as S 103 . If it is determined that high-speed duplex printing is possible (S 107 : YES), convey of the subsequent sheet S 2  is continued so that high-speed duplex printing is performed (S 108 ). Thereafter, it is determined whether or not next print data are present (S 109 ), and if print data is present (S 109 : YES), the flow returns to S 101  to start feeding of the preceding sheet S 1 , whereas if print data is not present (S 109 : NO), this process ends. 
     On the other hand, if it is determined that high-speed duplex printing is not possible (S 107 : NO), it is further determined whether high-speed duplex printing is possible for a combination of the preceding sheet S 1  and the subsequent sheet S 2  (S 141 ). A database storing the conveyability corresponding to a combination of sheets as shown in  FIG. 5  is stored in the printer  100 . Based on a sheet type predicted from the sheet lengths L 1  and L 2 , it is determined whether or not high-speed duplex printing is possible for that combination. 
     If it is determined that high-speed duplex printing is possible (the “©” case in  FIG. 5 ) (S 141 : YES), the flow proceeds to S 108  to continue the conveyance of the subsequent sheet S 2  so that high-speed duplex printing is performed. If it is determined that high-speed duplex printing is not possible (the “X” case in  FIG. 5 ) (S 141 : NO), a convey adjustment process is performed (S 142 ). 
     That is to say, even when the subsequent sheet S 2  has a sheet length for which high-speed duplex printing is not possible, there may be a case where the possibility of collision is low depending on its combination with the preceding sheet S 1 . For this reason, rather than uniformly applying error processing in all cases where the subsequent sheet S 2  has a sheet length for which high-speed duplex printing is not possible, high-speed duplex printing is continued for a combination with a low possibility of causing troubles. 
     Subsequently, details of the convey adjustment process (an example of a command section) in S 142  will be described with reference to the flowchart of  FIG. 9 . First, sheet convey is interrupted (S 151 ), and an error notification concerning the sheet size is sent to a user (S 152 ). 
     In this notification in S 152 , a screen as shown in  FIG. 10 , allowing the user to select whether the user will stop or retry printing is displayed on the display section  41  of the operation panel  40  and the user&#39;s selection is received. 
     Next, it is determined whether or not the user has selected to stop printing (S 153 ). When the user has selected to stop printing (S 153 : YES), the subsequent sheet S 2  being conveyed is ejected (S 154 ), the preceding sheet S 1  is ejected subsequently (S 155 ), and sheet convey is halted (S 156 ). After S 156 , the duplex convey process ends. By stopping printing, convey errors such as a paper jam can be avoided. Moreover, the user is able to check sheets in the feeder cassette, replace sheets appropriately, and then start printing all over again. 
     On the other hand, when the user has selected retry printing (S 153 : NO), the subsequent sheet S 2  is conveyed for single-sided printing and the subsequent sheet S 2  is ejected without being conveyed to the reconvey path  12  (S 161 ). In S 161 , single-sided printing of the subsequent sheet S 2  may be performed or not performed actually. 
     Subsequently, the preceding sheet S 1  is conveyed for printing the other side thereof (S 162 ), and then the preceding sheet S 1  is ejected (S 163 ). In this way, on the eject tray  96 , the preceding sheet S 1  with both sides printed is placed on the subsequent sheet S 2  on which duplex printing is not yet completed. Thereafter, the flow returns to S 101  of  FIG. 8  to start feeding of the preceding sheet. That is to say, printing subsequent to the printing of the preceding sheet S 1  with both sides printed is retried. 
     In the retry, the subsequent sheet S 2  that resulted in an inappropriate combination is ejected to avoid a sheet collision, and then duplex printing of the preceding sheet S 1  is completed. When the preceding sheet S 1  is ejected, printing subsequent to the printing of the preceding sheet S 1  is retried. In this way, by performing the retry autonomously, it is possible to suppress decrease in productivity. In addition, there is no change in printing order of output sheets after the retry. 
     In the convey adjustment process, although the timing of asking whether printing will be stopped or retried occurs later than the combination determination and earlier than the eject of the sheets S 1  and S 2 , the asking timing may occur earlier than the start of printing on the other side of the preceding sheet S 1 . That is to say, the subsequent sheet S 2  is ejected with the other side not printed regardless of whether printing is stopped or retried. Therefore, the asking as to whether printing will be stopped or retried may be made after the subsequent sheet S 2  is ejected. 
     As described in detail above, the printer  100  of this embodiment specifies the lengths of sheets being conveyed actually and switches conveying control of the sheets based on the results of determination on the combination of the preceding sheet S 1  and the subsequent sheet S 2  (target sheet) in addition to the sheet lengths of the sheets S 1  and S 2 . In this way, depending on the size of the sheets being conveyed actually, it is possible to perform an appropriate duplex printing operation. For example, when the combination of sheets is inappropriate, by discharging the subsequent sheet S 2  without conveying the sheet to the reconvey path  12 , it is possible to avoid a collision with the preceding sheet S 1 . Moreover, for a combination where the preceding sheet S 1  has the “Letter” size and the subsequent sheet S 2  has the “Legal” size, even if the sheet sizes are different, high-speed duplex printing is possible. For this reason, rather than uniformly applying error processing for stopping printing in all cases where the sheet sizes are different, by continuing high-speed duplex printing of that combination, it is possible to suppress decrease in productivity resulting from stopped convey. 
     Moreover, the printer  100  does not halt the sheet convey even if the preceding sheet S 1  has a sheet length for which high-speed duplex printing is not possible. That is to say, the sheet convey is continued by switching the sheet conveying control to low-speed duplex printing or single-sided printing. In this way, it is possible to avoid convey errors and suppress decrease in productivity resulting from stopped convey. 
     This embodiment is an example only and does not limit the present invention. Accordingly, various improvements and modifications may be made without departing from the scope of the present invention. The present invention is not limited to a printer but can be applied to any apparatus having a printing function such as a multi-functional peripheral or a FAX machine. Moreover, the image forming method of the process section is not limited to an electrophotographic method but may be an ink jet method. Furthermore, the printer may be capable of forming color images and may be configured to form only monochrome images. 
     Moreover, although the high-speed duplex printing according to the embodiment involves single-sided printing on both sides of two subsequent sheets, the number N of subsequent printing sheets is not limited to 2. The maximum number N of subsequent printing sheets differs depending on the number of sheets that can be conveyed along the sheet convey path. Moreover, the number of sheets that can be conveyed along the sheet convey path is determined by the length of the sheet convey path, the sheet length in the convey direction, and the like. That is to say, the number N of subsequent printing sheets is not limited to 2 but may be 3 or more. 
     Moreover, in the example of the high-speed duplex convey according to the embodiment, single-sided printing is performed on single sides of two subsequent sheets and then on the other sides of the same two subsequent sheets. However, single-sided printing may be performed on single sides of plural subsequent sheets and then alternately on the other side and the single side of sheets. For example, the number N of subsequent printing sheets may be set to 2 at the time of starting the sheet convey operation, and then single-sided printing may be performed alternately on the single side and the other side of sheets. In this case, when duplex printing of 4 sheets is performed, for example, printing is performed in the order of single side (first sheet), single side (second sheet), the other side (first sheet), single side (third sheet), the other side (second sheet), single side (fourth sheet), the other side (third sheet), and the other side (fourth sheet). The present invention can be applied to this convey order. 
     Moreover, in the embodiment, although as a method of specifying the sheet length, the time passed from passage of the leading edge of a sheet to passage of the bottom edge of the sheet is measured using the sheet sensor  60 , the specifying method is not limited to this. For example, a method of reading a sheet length from information (for example, information stored in an IC tag) embedded in a sheet may be used. 
     Moreover, in the embodiment, although the subsequent sheet S 2  is ejected if it is determined that continued high-speed duplex printing is not possible for the combination of the preceding sheet S 1  and the subsequent sheet S 2 , the present invention is not limited to this. That is to say, the preceding sheet S 1  may be ejected instead of the subsequent sheet S 2 . In this case, it should be made sure that the determination on the combination with the subsequent sheet S 2  is completed before the preceding sheet S 1  is conveyed to the reconvey path  12 . At that time, only the preceding sheet S 1  may be ejected, and both the preceding sheet S 1  and the subsequent sheet S 2  may be ejected. 
     OTHER ASPECTS OF THE EXEMPLARY EMBODIMENTS 
     The second aspect of the exemplary embodiments of the present invention is that the conveyer ejects the second sheet without conveying the second sheet into the reconveyer in a case where the determination section determines the pair of the first sheet and the second sheet is eligible for the high-speed duplex printing. According to this configuration, it is possible to make the inappropriate pair of the sheets less effective to the printing process. For example, if the printing apparatus ejects the first sheet, print the one surface and the other surface of the second sheet in advance, then retries to print the contents to be printed on the first sheet, the printing apparatus prints the sheets in a different order from initially intended. To the contrary, when the second sheet is ejected, the printing order does not change. 
     The third aspect of the exemplary embodiments of the present invention is that the controller controls the printing section to continue printing on the other surface of the first sheet and the controller controls the printing section to print contents to be printed on the one surface and the other surface of the second sheet on a third sheet, when the determination section determines the pair of the first sheet and the second sheet is not eligible for the high-speed duplex printing. According to this configuration, even though the pair of the first sheet and the second sheet is inappropriate for the high-speed duplex printing, it is expected to suppress the reduction in productivity by automatically reprinting. 
     The fourth aspect of the exemplary embodiments of the present invention is that the printing apparatus further comprises a command section ordering the controller to cancel or continue printing before the printing section continue printing the other surface of the first sheet, wherein the controller controls the printing section to eject the first sheet after ejecting the second sheet when the command section orders the controller to cancel printing; and the controller controls the printing section to print the contents to be printed on the one surface and the other surface of the second sheet on the third sheet when the command section orders the controller to continue printing. According to this configuration, the user&#39;s intention is clearly reflected by making it possible to select a command to cancel the printing or a command to continue the printing. 
     The fifth aspect of the exemplary embodiments of the present invention is that the controller controls the printing section to eject the first sheet after ejecting the second sheet and to cancel printing, when the determination section determines the pair of the length of the first sheet and the length of the second sheet is not eligible for the high-speed duplex printing. According to this configuration, it is possible to restart the printing while preventing sheet jam in a case where the pair of the first sheet and the second sheet is not eligible for the high-speed duplex printing. 
     The sixth aspect of the exemplary embodiments of the present invention is that the second sheet is fed after printing the other surface of the first sheet in a case where the first sheet is fed without precedent sheet in either the conveyer or the reconveyer and the first sheet is determined not to be eligible for the high-speed duplex printing. According to this configuration, the printing can be continued even when the first sheet initially fed into the printing section is not eligible for the high-speed duplex printing.