Patent Publication Number: US-8985582-B2

Title: Sheet processing apparatus, method for controlling sheet processing apparatus, and storage medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus, a method for controlling the sheet processing apparatus, and a storage medium. 
     2. Description of the Related Art 
     There has conventionally been a sheet processing apparatus which has a plurality of sheet discharge trays, and discharges print products onto any one of the sheet discharge trays. 
     Such a sheet processing apparatus discharges print products onto a sheet discharge tray and, when the sheet discharge tray becomes tray-full, continues discharging remaining print products onto another sheet discharge tray. Even if the sheet discharge tray is not tray-full, this sheet processing apparatus can also divide print products to discharge thereof onto another sheet discharge tray for the purpose of sorting. 
     When print products are separately discharged onto a plurality of sheet discharge trays, it is difficult for a user to recognize the discharge order of the print products discharged onto the plurality of sheet discharge trays. Regularly, the user does not constantly keep monitoring print products being discharged onto the plurality of sheet discharge trays to confirm the discharge order. 
     Japanese Patent Application Laid-Open No. 2000-094808 discusses a technique for printing on a notification sheet the discharge order of print products separately discharged onto a plurality of sheet discharge trays, and attaching the notification sheet to print products discharged onto the respective sheet discharge trays. 
     However, when dividing print products to discharge thereof onto a plurality of sheet discharge trays with the conventional method, a user cannot recognize the discharge order of print products discharged onto the plurality of sheet discharge trays unless the user refers to the notification sheet. In this case, since notification sheets are required, sheets will be wasted. Further, after confirming the discharge order, the user must carefully make take-out instruction according to notification sheets. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a sheet processing apparatus for performing control to discharge sheets onto a plurality of discharge trays includes a storage unit configured to store discharge order of a plurality of sheets separately discharged onto equal to or more than two sheet discharge trays by executing a job, and a control unit configured to, upon reception of a take-out instruction for taking out in the discharge order the sheets discharged by executing the job, perform processing for allowing a user to take out the plurality of sheets discharged onto the equal to or more than two sheet discharge trays, in the discharge order stored in the storage unit. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a configuration of a system according to an exemplary embodiment of the present invention. 
         FIG. 2  illustrates a configuration of a personal computer (PC) according to an exemplary embodiment of the present invention. 
         FIG. 3  illustrates a configuration of a printing system according to an exemplary embodiment of the present invention. 
         FIG. 4  illustrates a configuration of the printing system according to an exemplary embodiment of the present invention. 
         FIG. 5  illustrates a configuration of an operation unit according to an exemplary embodiment of the present invention. 
         FIG. 6  illustrates an operation screen according to an exemplary embodiment of the present invention. 
         FIGS. 7A and 7B  illustrate a configuration of a mass stacker according to an exemplary embodiment of the present invention. 
         FIG. 8  illustrates a dolly according to an exemplary embodiment of the present invention. 
         FIG. 9  illustrates a configuration of a mass stacker according to an exemplary embodiment of the present invention. 
         FIG. 10  illustrates stacker trays according to an exemplary embodiment of the present invention. 
         FIG. 11  illustrates stacker trays according to an exemplary embodiment of the present invention. 
         FIG. 12  illustrates appearances of mass stackers according to an exemplary embodiment of the present invention. 
         FIG. 13  illustrates a screen of a printer driver according to an exemplary embodiment of the present invention. 
         FIG. 14 , which is composed of  FIG. 14A  and  FIG. 14B , is a flowchart illustrating processing according to an exemplary embodiment of the present invention. 
         FIG. 15 , which is composed of  FIG. 15A  and  FIG. 15B , is a flowchart illustrating processing according to an exemplary embodiment of the present invention. 
         FIG. 16  is a flowchart illustrating processing according to an exemplary embodiment of the present invention. 
         FIGS. 17A and 17B  illustrate discharge destination information blocks according to an exemplary embodiment of the present invention. 
         FIG. 18  is a flowchart illustrating processing according to an exemplary embodiment of the present invention. 
         FIG. 19  illustrates a job history screen according to an exemplary embodiment of the present invention. 
         FIG. 20  illustrates a take-out screen according to an exemplary embodiment of the present invention. 
         FIGS. 21A ,  21 B,  21 C, and  21 D illustrate a stacking status table according to an exemplary embodiment of the present invention. 
         FIG. 22  illustrates a notification screen according to an exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following describes exemplary embodiments of the present invention with reference to accompanying drawings. 
       FIG. 1  illustrates an example configuration of a system according to an exemplary embodiment of the present invention. A printing system according to the present exemplary embodiment includes a printing system  1000  and a PC  102  as an example of an external apparatus. The printing system  1000  and the PC  102  are connected via a network  101 . The network  101  may be any one of a wired local area network (LAN), a wireless LAN, and the Internet. 
     The PC  102  is provided with application software for generating image data. The PC  102  transmits the image data to the printing system  1000  via a printer driver in response to an instruction received from a user. 
     The PC  102  also checks the status of the printing system  1000  via the network  101 . 
     The printing system  1000  includes a printing apparatus  100  and a sheet processing apparatus  200 , receives print data from the PC  101 , and prints an image on a sheet according to the print data. The printing system  1000  further includes a scanner, and prints on a sheet an image of a document read by the scanner. 
     Although, in the present exemplary embodiment, the printing system  1000  and the PC  102  are connected via the network  101 , they may be connected via a local interface, such as a universal serial bus (USB) interface, or via a wireless network. 
     The following describes a configuration of the PC  102  with reference to  FIG. 2 . 
     The PC  102  includes a central processing unit (CPU)  104 , a read-only memory (ROM)  105 , a random access memory (RAM)  106 , a network communication unit  107 , a storage unit  108 , a display unit  109 , and an operation unit  110  which are mutually connected via a bus. 
     The CPU  104  reads a program (for example, application software, the printer driver, etc.) from the ROM  105 , and then executes it. The ROM  105  stores various programs to be read by the CPU  104 . 
     The RAM  106  stores data and programs and is used mainly as a work memory for the CPU  104 . 
     The network communication unit  107  performs interface control of data transmitted and received via the network  101 . 
     The storage unit  108  is a nonvolatile memory, such as a hard disk drive (HDD). The storage unit  108  stores programs and image data generated by the above-described application software. 
     The display unit  109 , such as a liquid crystal display (LCD) unit and a cathode ray tube (CRT) display unit, displays various operation screens and messages. 
     The operation unit  110 , such as a keyboard, a mouse, and a touch panel, receives an operation performed by the user on the PC  101 . 
     The PC  102  generates print data by the printer driver based on the image data generated by application software, and transmits the generated print data, together with print settings, to the printing system  1000  via the network  101 . The PC  102  also receives status information and a print result of the printing system  1000  from the printing system  1000 , and displays the received status information and print result on the display unit  109 . 
     The following describes a configuration of the printing system  1000  with reference to  FIG. 3 . 
     The printing system  1000  includes a scanner unit  201 , an external interface (I/F) unit  202 , a printer unit  203 , an operation unit  204 , a control unit  205 , a ROM  207 , a RAM  208 , and an HDD  209 . These components are connected via an internal bus of the printing system  1000 . 
     The control unit  205  reads a program stored in the ROM  207  and then executes the program to totally control the printing system  1000 . 
     The scanner unit  201  reads a document, generates image data of the read document, and transmits the generated image data to the control unit  205 . 
     The external I/F unit  202  controls data transmission and reception to/from the external network  101 . For example, the external I/F unit  202  receives image data transmitted from an external apparatus, such as the PC  102 , and transmits the image data to the control unit  205 . The external I/F unit  202  also transmits data received from the control unit  205  to an external apparatus, such as the PC  102 , via the network  101 . As described above, the network  101  may be a local interface or a wireless network. 
     The printer unit  203  prints an image on a sheet based on the image data received from the control unit  205  and the print settings (a print layout, number of print copies, and other print information) received from the control unit  205 . The printer unit  203  includes motors and rollers for conveying a sheet (not illustrated). 
     The operation unit  204  includes a display, a touch panel, hardware keys, etc. The operation unit  204  displays an operation screen on the display, and receives a user&#39;s instruction from the touch panel provided on the display unit. The operation unit  204  also receives a user&#39;s instruction via the hardware keys. The operation unit  204  transfers the received instruction to the control unit  205 . 
     The ROM  207  stores programs to be executed by the control unit  205 . 
     The RAM  208  serves as a work memory for the control unit  205 , and temporarily stores a program read from the ROM  207  and image data. 
     The HDD  209  is a nonvolatile storage medium. The HDD  209  stores data of a job to be performed together with the order to be executed. 
     For example, when executing a copy job, the control unit  205  stores in the HDD  209  as a copy job the image data read by the scanner unit  201  in association with the print settings received via the operation unit  204 , and executes the stored copy job. The control unit  205  executes the copy job, and instructs the printer unit  203  to print the image data stored in the HDD  209 , based on the print settings stored in association with the relevant image data. 
     Further, when executing a print job, the control unit  205  stores in the HDD  209  as a copy job the image data received via the external I/F unit  202  in association with the print settings, and executes the stored print job. The control unit  205  executes the print job stored in the HDD  209 , and instructs the printer unit  203  to print the image data stored in the HDD  209  based on the print settings stored in association with the relevant image data. 
     The HDD  209  stores a plurality of jobs. The control unit  205  performs the plurality of stored jobs in the reception order. 
     A compression/decompression unit  210  compresses or decompresses image data stored in the RAM  208  or the HDD  209 , by using various compression methods, such as Joint Bi-level Image Experts Group (JBIG) and Joint Photographic Experts Group (JPEG). 
     As illustrated in  FIG. 4  (described below), the sheet processing apparatus  200  is connected with the printing apparatus  100  to apply sheet processing, such as sheet stacking and saddle stitch bookbinding, to sheets printed by the printing apparatus  100 . The sheet processing apparatus  200  includes motors and rollers for conveying a sheet (not illustrated). The sheet processing apparatus  200  further includes motors and rollers for performing sheet processing (not illustrated). 
     The following describes the printing system  1000  with reference to the cross sectional view illustrated in  FIG. 4 . The following describes the configuration as well as the operation of the printing system  1000  when executing a copy job. 
     An automatic document feeder (ADF)  301  provided on the scanner unit  201  sequentially separates a plurality of sheets set on a document tray by the user, from a first sheet, and conveys the document sheet onto a document positioning glass plate. By using a charge-coupled device (CCD), a reading unit  302  reads an image of the document sheet conveyed onto the document positioning glass plate, and generates relevant image data. The generated image data is stored in a memory, such as the RAM  208  and the HDD  209 , by the control unit  205 . 
     The printing apparatus  100  is a tandem type color printer including a plurality of photosensitive members (drums). The printing apparatus  100  is provided with paper feed units (sheet accommodating units), such as sheet cassettes  317  and  318  and a manual feed tray  320 , and feeds a sheet from any one of these paper feed units. The printing apparatus  100  is also provided with a paper feed deck  319  connected thereto as a paper feed unit, capable of accommodating a large volume of sheets. Sheets can be fed from the paper feed deck  319 . 
     When a sheet fed from any one of the paper feed units is conveyed to a registration roller pair  306 , the printing apparatus  100  once stops the sheet to achieve synchronization with an intermediate transfer belt  305 . When there is a sheet waiting for transfer at the position of the registration roller pair  306 , a sheet for printing the next page can be fed from any one of the sheet cassettes  317  and  318 , the paper feed deck  319 , and the manual feed tray  320 . Feeding paper in this way enables shortening the conveyance interval of a plurality of sheets, thus improving the productivity of printing. 
     Meanwhile, image data temporarily stored in the RAM  206  and the HDD  208  is transmitted to the printer unit  207 , and then converted into recording laser beams of respective four colors (yellow (Y), magenta (M), cyan (C), and black (K)), by a laser recording unit (not illustrated). Then, the photosensitive members for respective colors are irradiated with the recording laser beams to form electrostatic latent images of the respective colors on the photosensitive members. Then, toner development is performed by using toner of 4 colors supplied from toner cartridges to form respective toner images. The visualized toner images are primarily transferred from the photosensitive members onto the intermediate transfer belt  305  to form a 4-color toner image. 
     The intermediate transfer belt  305  rotates in the clockwise direction at a constant speed. When the intermediate transfer belt  305  has rotated up to a predetermined position, the control unit  205  starts conveying the sheet which has been waiting at the position of the registration roller pair  306 . The predetermined position refers to a position where, when the leading edge of the toner image transferred onto the intermediate transfer belt  305  arrives at a secondary transfer position  316 , an approximate end of the sheet is conveyed to the secondary transfer position  316 . Thus, at the secondary transfer position  316 , the toner image on the intermediate transfer belt  305  is transferred onto the sheet. 
     The sheet having the toner image transferred thereon is further conveyed by a belt  307 , subjected to the application of pressure and heat by a fixing unit  308  to fix the toner, conveyed along the sheet conveyance path, and then discharged. 
     A sheet discharge flapper  309  is configured to be rotatable centering on a rotating shaft to restrict the sheet conveyance direction. When the sheet discharge flapper  309  rotates in the clockwise direction viewed in the figure and then is fixed at the relevant position, the sheet discharged from the fixing unit  308  is horizontally conveyed as it is, and then conveyed to a mass stacker  1  (one of the sheet processing apparatuses  200 ) by a sheet discharge roller pair  310 . One-sided printing is performed in this way. 
     On the other hand, in the case of two-sided printing, the sheet discharge flapper  309  rotates in the counterclockwise direction viewed in the figure and then is fixed at the relevant position, a sheet discharged from the fixing unit  308  is downwardly directed and then sent to a two-sided conveyance unit. The two-sided conveyance unit includes a reversing flapper  311 , a reversing roller  312 , a reversing guide  313 , and a two-sided tray  314 . The reversing flapper  311  rotates centering on a rotating shaft to restrict the sheet conveyance direction. In the case of two-side printing, the control unit  205  performs control to rotate the reversing flapper  311  in the counterclockwise direction viewed in the figure to send the sheet having an image printed on the first side to the reversing guide  313  via the reversing roller  312 . Then, the control unit  205  stops the reversing roller  312 , with the trailing edge of the sheet pinched by the reversing roller  312  provided at the entrance of the reversing guide  313 , and subsequently rotates the reversing flapper  311  in the clockwise direction viewed in the figure to rotate the reversing roller  312  in the reverse direction. Thus, the control unit  205  conveys the sheet on a switchback basis, i.e., the control unit  205  performs control to guide the sheet to the two-sided tray  314  with the leading and trailing edges interchanged. 
     The sheet is once supported by the two-sided tray  314  and then conveyed again to the registration roller pair  306  by a paper re-feed roller  315 . In this case, the sheet is conveyed with the second side facing the intermediate transfer belt  305  (the second side is the side opposite to the first side on which toner has been transferred in the above-described first-side transfer process). Then, the control unit  205  forms an image on the second side of the sheet in a similar way to the above-described first-side transfer process. Then, after the fixing unit  308  has fixed the image formed on the second side of the sheet, the sheet discharge roller pair  310  conveys the sheet having the image formed thereon to the mass stacker  1  (one of the sheet processing apparatuses  200 ). Two-side printing is performed in this way. 
     The sheet having an image printed on one side or on both sides by the printing apparatus  100  is selectively conveyed to any one of the mass stacker  1 , a mass stacker  2 , and a saddle stitch bookbinding machine (the sheet processing apparatuses  200 ) based on the print settings set from the operation unit  203 . 
     When sheets are to be discharged onto the mass stacker  1 , a sheet having an image printed thereon by the printing apparatus  100  is conveyed to the mass stacker  1 . The mass stacker  1  discharges the received sheet onto a stacker tray  331  or  332 . The mass stacker  1  can also discharge the received sheet onto an escape tray  330 . When sheets are to be discharged onto the mass stacker  2 , a sheet having an image printed thereon by the printing apparatus  100  is conveyed to the mass stacker  2  via the sheet conveyance path of the mass stacker  1 . The mass stacker  2  discharges the received sheet onto a stacker tray  333  or  334 . The mass stacker  2  can also discharge the received sheet onto an escape tray  335 . 
     When sheets are not to be discharged onto the mass stackers  1  and  2 , the sheet having an image printed thereon by the printing apparatus  100  is conveyed to the saddle stitch bookbinding machine via the sheet conveyance path of the mass stackers  1  and  2 . When no post-processing is set to be performed, the saddle stitch bookbinding machine discharges the received sheet onto the sheet discharge unit  337  via the sheet conveyance path  336 . 
     When stapling is set to be performed, the saddle stitch bookbinding machine stores the received sheet in an intermediate tray via the sheet conveyance path  336 . Then, when sheets to be stapled as one bundle have been prepared in the intermediate tray, the saddle stitch bookbinding machine applies stapling to the sheets for one bundle, and discharges the bundled sheets onto a sheet discharge unit  338 . 
     When saddle stitch bookbinding is set to be performed, the saddle stitch bookbinding machine applies saddle stitch bookbinding to received sheets, and discharges the bound sheets onto a sheet discharge unit  339 . 
     The printing system  1000  performs paper feeding, printing, post-processing, and sheet discharge processing in this way. 
     In the printing system  1000 , each of the sheet conveyance paths of the printing apparatus  100 , the mass stacker  1 , the mass stacker  2 , and the saddle stitch bookbinding machine is provided with a sheet detection sensor. Specifically, a sheet detection sensor is provided at the entrance and exit of each apparatus, and branching points and junction points of the sheet conveyance paths. Referring to  FIG. 4 , example positions of sheet detection sensors A to O are indicated by triangular marks. The control unit  205  receives signals from the sheet detection sensors A to O to detect the presence or absence of a sheet being conveyed through the respective sheet conveyance paths, and the position of the sheet. 
     For example, if a signal from a certain sheet detection sensor continues for a predetermined time period, the control unit  205  determines that a sheet retention jam has occurred at the position corresponding to the relevant sheet detection sensor. Alternatively, if a sheet that has passed a certain sheet detection sensor does not pass the following sheet detection sensor within a predetermined time period, the control unit  205  determines that a sheet delay jam has occurred between the relevant two sheet detection sensors. 
     If a sheet jam occurs in a sheet conveyance path of the printing system  1000 , the control unit  205  interrupts printing and then displays as a guidance the position of a sheet which should be removed and procedures for removing the sheet based on a signal from the relevant sheet detection sensor. Thus, the user can recognize where in the printing system  1000  the sheet which should be removed exists, open a door of an apparatus according to the guidance, and remove the sheet from the sheet conveyance path. 
     Although, in the present exemplary embodiment, the printing apparatus  100  is a four-drum (4D) type color multifunctional peripheral (MFP), the printing apparatus  100  is not limited thereto, and may be a monochrome MFP or a one-drum (1D) type color MFP. The operation and configuration of the printing system  1000  has specifically been described above based on a copy job. However, in the case of a print job, the control unit  205  performs a similar print operation with this configuration by using print data from the external I/F unit  202  instead of image data from the scanner unit  201 . 
       FIG. 5  illustrates a configuration of the operation unit  204 . 
     The operation unit  204  includes a touch panel unit  401  provided with software keys, and a key input unit  402  configured with hardware keys. 
     The touch panel unit  401  includes a liquid crystal display (LCD) unit and a touch panel attached thereon. The touch panel unit  401  receives an instruction from the user, and displays various messages to notify the user of information. 
     When the user presses the COPY tab of the touch panel unit  401 , an operation screen for the copy function is displayed. When the user presses the TRANSMIT tab, an operation screen for data transmission functions, such as fax and E-mail transmission, is displayed. When the user presses the BOX tab, an operation screen for the box function is displayed on a display  401 . The box function refers to a function of storing in the HDD  209  image data read by the scanner unit  201 , selecting print data stored in the HDD  209  at a desired timing, and instructing the printer unit  203  to print the selected print data. 
     A power switch  403  is a button for switching between two different operating modes of the printing system  1000 : the standby mode (a normal operating state) and the sleep mode (state of reducing power consumption while programs are deactivated in an interrupt wait state for being prepared for network printing, facsimile, etc.). 
     The START key  404  is used to instruct the printing system  1000  to start the copy operation and transmit operation. 
     The numeric keypads  405  are used to set the number of copies and input a password. 
     The USER MODE key  406  is used to make various settings for the printing system  1000 . 
     The SET SHEET PROCESSING key  407  is used to set sheet processing to be performed by the sheet processing apparatus  200 . When the user presses the SET SHEET PROCESSING key  407 , the control unit  205  displays on the touch panel unit  401  the screen illustrated in  FIG. 6 . 
     The screen illustrated in  FIG. 6  displays buttons for receiving settings of sheet processing executable by the printing system  1000 . Types of executable sheet processing are changed depending on the configuration of the printing system  1000 . 
       FIG. 6  illustrates keys for performing the following processing. 
     (1) Stapling (key  701 ) 
     (2) Punching (key  702 ) 
     (3) Cutting (key  703 ) 
     (4) Shift sheet discharge (key  704 ) 
     (5) Saddle stitch bookbinding (key  705 ) 
     (6) Folding (key  706 ) 
     (7) Mass stacking (keys  707  and  708 ) 
     The control unit  205  performs control to apply selected sheet processing from (1) to (7) to the sheets printed by the printing apparatus  100 . 
     For example, with the key  705  selected in the copy function, when the user presses the OK key  711  and then the START key  404 , the control unit  205  reads a document via the scanner unit  201 . Then, the control unit  205  prints image data of the read document based on the print settings received via the operation unit  204 . Then, the control unit  205  conveys the printed sheet to the saddle stitch bookbinding machine illustrated in  FIG. 4 , and instructs the saddle stitch bookbinding machine to perform saddle stitch bookbinding. 
     Further, with the key  707  selected in the copy function, when the user presses the OK key  711  and then the START key  404 , the control unit  205  reads a document via the scanner unit  201 . Then, the control unit  205  prints image data of the read document based on the print settings received via the operation unit  204 . Then, the control unit  205  conveys the printed sheet to the mass stacker  1  illustrated in  FIG. 4 , and instructs the mass stacker  1  to perform mass stacking. However, when sheets cannot be stacked onto the mass stacker  1 , the printed sheet is conveyed to the mass stacker  2  and then stacked onto the mass stacker  2 . 
     Further, with the key  708  selected in the copy function, when the user presses the OK key  711  and the START key  404 , the control unit  205  reads a document via the scanner unit  201 . Then, the control unit  205  prints image data of the read document based on the print settings received via the operation unit  204 . Then, the control unit  205  conveys the printed sheet to the mass stacker  2  illustrated in  FIG. 4 , and instructs the mass stacker  2  to perform mass stacking. However, when sheets cannot be stacked onto the mass stacker  2 , the printed sheet is conveyed to the mass stacker  1  and then stacked onto the mass stacker  1 . 
     Mass Stacker 
       FIG. 7A  is a cross sectional view illustrating the mass stacker  1 . The mass stacker  1  includes two stacker trays (corresponding to the stacker trays  331  and  332  illustrated in  FIG. 4 ). In the following descriptions, the stacker tray closest to the sheet discharge slot of the stack path is referred to as stacker tray A, and the other stacker tray is referred to as stacker tray B. 
       FIG. 7B  is a cross sectional view illustrating the mass stacker  2 . The mass stacker  2  also includes two stacker trays (corresponding to the stacker trays  333  and  334  illustrated in  FIG. 4 ). In the following descriptions, the stacker tray closest to the sheet discharge slot of the stack path is referred to as stacker tray C, and the other stacker tray is referred to as stacker tray D. 
       FIGS. 7A and 7B  illustrate example configurations of the mass stackers  1  and  2 , respectively. The form of the sheet conveyance path is not limited to the one illustrated in  FIGS. 7A and 7B , and may be the form of the sheet conveyance path of the mass stacker illustrated in  FIG. 4 . 
     Each mass stacker includes a straight path, an escape path, and a stack path. 
     The straight path is a sheet conveyance path for conveying a sheet conveyed from the apparatus (the printing apparatus  100  in the present exemplary embodiment) in a previous-stage to the apparatus (the saddle stitch bookbinding machine in the present exemplary embodiment) in a latter-stage. For example, a sheet printed by executing a job set to execute saddle stitch bookbinding is conveyed to the saddle stitch bookbinding machine in a latter-stage via the straight path. 
     The escape path is a sheet conveyance path for conveying a sheet to an escape tray. 
     The stack path is a sheet conveyance path for conveying a sheet printed by executing a job set to stack onto each mass stacker, to stack it onto a stacker tray. 
     According to an instruction from the control unit  205 , the mass stacker switches between the stacker trays A and B on which a sheet discharged from the stack path is to be stacked, by using the flapper illustrated in  FIG. 7A . When stacking a sheet on the stacker tray A, the sheet is guided to the stacker tray A under the conveyance belt by the flapper and then discharged onto the stacker tray A. When stacking a sheet on the stacker tray B, the sheet is guided above the conveyance belt by the flapper to discharge the sheet onto the stacker B and then discharged onto the stacker tray B by the conveyance belt. 
     When sheets are to be stacked onto the stacker tray A, the control unit  205  performs control to move the abutting plate to the position of the stacker tray A, and stack sheets onto the stacker tray A in an aligned way. Otherwise, when sheets are to be stacked onto the stacker tray B, the control unit  205  performs control to move the abutting plate to the position of the stacker tray B, and stack sheets onto the stacker tray B in an aligned way. 
     Each mass stacker includes two stacker trays. Each of the two stacker trays is placed on an extendable stay so as to be elevatable. Each stacker tray is raised to a first position at which a sheet can be received, and lowered to a second position on the dolly by a extendable stay, and then set onto the dolly. The first position changes within a range indicated by the arrow illustrated in  FIG. 7A , and is determined by the amount of the sheets stacked on the stacker tray. Referring to  FIG. 7A , the stacker tray A exists at a position where the upper surface of the sheets stacked on the stacker tray A can stably support a sheet discharged from the stack path. This position is referred to as the first position. The second position refers to a position surrounded by the dotted lines illustrated in  FIG. 7A , at which the stacker tray is set onto the dolly. The dolly can be provided with a handle, as illustrated in  FIG. 8 . The dolly is used by the user to carry sheets to another sheet processing apparatus. 
     Each mass stacker is provided with two front doors (covers) which can be opened and closed to enable the user to take out the sheets stacked on each stacker tray. When an instruction for opening the front door of the dolly is made, each stacker tray is lowered and then set onto the dolly. Then, with each stacker tray set on the dolly, each stacker tray together with the dolly is attached to each mass stacker. When the dolly has been attached to the mass stacker, each stacker tray is raised to a position where a sheet discharged from the stack path can be easily stacked. When no sheet exists on a stacker tray, the stacker tray is raised to a height at which the upper surface of the stacker tray can receive a sheet discharged from the stack path. Otherwise, when sheets have already been stacked on a stacker tray, the stacker tray is raised to a height at which the uppermost surface of the sheets stacked on the stacker tray can receive a sheet discharged from the stack path. After sheets have been stacked, upon reception of a sheet take-out instruction from the user, the stacker tray is lowered to a position (second position) at which the user can take out the sheets discharged onto the stacker tray. 
     The mass stacker  1  can discharge sheets over the two stacker trays, as illustrated in  FIG. 9 . This also applies to the mass stacker  2 . In this case, a sheet is guided toward the downward conveyance path by the flapper. In this case, the control unit  205  performs control to move the abutting plate according to the sheet size, and stack sheets onto the stacker trays A and B in an aligned way. For example, when the width of a sheet to be discharged in the sheet conveyance direction is larger than the width of one stacker tray, the control unit  205  performs control to discharge sheets over the two stacker trays. When discharging sheets over the two stacker trays, a leading edge of the first sheet may be guided onto the stacker tray B by a pinching unit provided on the abutting plate to prevent the sheet from entering a gap between the stacker trays A and B. 
     In the following descriptions, a sheet having a width in the sheet conveyance direction larger than the width of one stacker tray is referred to as a large-size sheet. Further, a sheet having a width in the sheet conveyance direction equal to or smaller than the width of one stacker tray is referred to as a small-size sheet. 
       FIGS. 10 and 11  illustrate the stacker trays A and B of the mass stacker  1  when viewed from the top. 
     Referring to  FIG. 10 , a small-size sheet  801  is stacked on the stacker tray A. The width of the sheet  801  in the sheet conveyance direction is smaller than the width of the stacker tray A. 
     Referring to  FIG. 11 , a large-size sheet  805  is stacked on the stacker trays A and B. When stacking the large-size sheet  805 , the control unit  205  performs control to discharge the sheet  805  to extend over the two stacker trays A and B, as illustrated in  FIG. 9 . 
     This stacking method is performed also for the stacker trays C and D of the mass stacker  2 . 
       FIG. 12  illustrates the appearances of the mass stackers  1  and  2 . 
     The mass stacker  1  is provided with a take-out button for each stacker tray. A take-out button  2001  is used to take out the print products discharged onto the stacker tray A (existing on the right-hand side of the mass stacker  1  illustrated in  FIG. 12 ). When the user presses the take-out button  2001 , the stacker tray A is lowered onto the top of the dolly provided at the bottom inside the front doors of the mass stacker  1  and then placed onto the dolly, and a front door  2005  opens. 
     A take-out button  2002  is used to take out the print products discharged onto the stacker tray B (existing on the left-hand side of the mass stacker  1  illustrated in  FIG. 12 ). When the user presses the take-out button  2002 , the stacker tray B is lowered onto the top of the dolly and then placed onto the dolly, and the front doors  2005  and  2006  open. 
     Subsequently, the user can take out the dolly from the mass stacker  1  and then carry the print products. 
     In a case where large-size sheets have been stacked on the mass stacker  1  and the user presses either of the take-out buttons  2001  and  2002 , both the stacker trays A and B are lowered. 
     The mass stacker  2  is provided with a take-out button for each stacker tray. A take-out button  2003  is used to take out the print products discharged onto the stacker tray C (existing on the right-hand side of the mass stacker  2  illustrated in  FIG. 12 ). When the user presses the take-out button  2003 , the stacker tray C is lowered onto the top of the dolly and then placed onto the dolly, and the front doors  2007  and  2008  open. 
     The take-out button  2004  is used to take out the print products discharged onto the stacker tray D (existing on the left-hand side of the mass stacker  2  illustrated in  FIG. 12 ). When the user presses the take-out button  2004 , the stacker tray D is lowered onto the top of the dolly and then placed onto the dolly, and the front doors  2007  and  2008  open. 
     In a case where large-size sheets have been stacked on the mass stacker  2  and the user presses either of the take-out buttons  2003  and  2004 , both the stacker trays C and D are lowered. 
     According to an operation received via the operation unit  204 , the control unit  205  of the printing system  1000  executes a copy job for reading an image of a document by the scanner unit  201  and printing the read image by the printer unit  203 . Then, the control unit  205  applies the sheet processing set on the screen illustrated in  FIG. 6  to the sheet having an image printed thereon. 
     The printing system  1000  executes a print job for receiving print data from the PC  102  and printing an image by the printer unit  203  based on the print data. The print data includes printing settings and image data. The control unit  205  prints the image data according to the print settings. 
     The print settings are set by the user via the printer driver illustrated in  FIG. 13 . This printer driver is displayed on the display unit  109  by the CPU  104  of the PC  102 . 
     When “STACKER  1  (STACKER TRAY)” is selected for the item “SHEET DISCHARGE DESTINATION” and a printing instruction is made, the printer driver generates a print job for discharging print products onto the mass stacker  1 , and transmits the print job to the printing system  1000 . The printing system  1000  receives a print job specifying the mass stacker  1  and then executes it, whereby sheets are discharged onto the stacker tray A. If sheets of another size have already been stacked on the stacker tray A, or if the stacker tray  331  is tray-full, sheets are discharged onto the stacker tray B. 
     Otherwise, when “STACKER  2  (STACKER TRAY)” is selected for the item “SHEET DISCHARGE DESTINATION” and a printing instruction is made, the printer driver generates a print job for discharging print products onto the mass stacker  2 , and transmits the print job to the printing system  1000 . 
     The printing system  1000  receives a print job specifying the mass stacker  2  and then executes it, whereby sheets are discharged onto the stacker tray C. If sheets of another size have already been stacked on the stacker tray C, or if the stacker tray C is tray-full, sheets are discharged onto the stacker tray D. 
     As described above, the printing system  1000  has four different stacker trays, and can discharge print products onto each stacker tray. Then, the user can collectively handle print products stacked on each stacker tray. For example, if print products are separately discharged onto a plurality of stacker trays because of tray-full by executing one job, the user needs to get together in the discharge order these print products into one print product afterwards. Further, if these print products are discharged onto different stacker trays by executing different jobs, the user may want to put together in the discharge order these print products for the jobs into one print product afterwards. In this case, however, the user cannot easily know the order in which print products are stacked onto the respective stacking trays. 
     According to the present invention, therefore, the control unit  205  stores the discharge order of print products discharged onto a plurality of stacker trays, and displays the discharge order. If an automatic sheet take-out instruction is received from the user, the control unit  205  performs sheet take-out processing in the discharge order. The sheet take-out processing refers to processing for lowering a stacker tray, setting it onto the dolly, and opening the front doors for the stacker trays. Thus, the user can easily take out the sheets separately discharged onto the plurality of stacker trays in the discharge order. 
       FIGS. 14 and 15  indicate processing for determining the discharge destination performed by the control unit  205 . Processing of each step illustrated in the flowchart in  FIGS. 14 and 15  is implemented by the control unit  205  reading a program stored in the ROM  207  and then executing it. 
     In step S 2001 , the control unit  205  receives a job. For example, the control unit  205  receives settings for a copy job via the operation unit  204 . 
     In this case, the control unit  205  receives a setting for reading a document, a setting for a sheet cassette to be used, a setting for sheet processing, etc., via the operation unit  204 , and stores the received setting information in the RAM  208 . The sheet cassette to be used may be determined by the user by directly specifying a specific sheet cassette, or determined by the control unit  205  by automatically selecting a sheet cassette based on the document size and the image magnification rate. Then, by the START key  404  being pressed, the control unit  205  starts the copy job based on the received settings. 
     When a copy job is started, the control unit  205  starts processing for determining a discharge destination based on the sheet stacking status of the stacker trays and the received setting information. 
     In step S 2002 , the control unit  205  prepares a new discharge destination information block in a discharge order management area illustrated in  FIG. 17A  for the received job. The discharge destination information block includes a job ID for identifying a job, an area for storing the discharge destination of print products for the job in the discharge order, and a pointer for referring to the following discharge destination information block. The control unit  205  further updates the reference destination of a root pointer to a prepared discharge destination information block. If a preceding discharge destination information block exists, the control unit  205  updates the reference destination of a pointer to the preceding discharge destination information block to the prepared discharge destination information block. The discharge order management area illustrated in  FIG. 17A  is provided in the HDD  209 . 
     In step S 2003 , the control unit  205  stores the job ID of the job received in step S 2001  in the prepared discharge destination information block  7001 . 
     In step S 2004 , the control unit  205  determines whether the discharge destination of the sheets discharged by executing the job is the mass stacker  1  based on the setting information stored in the RAM  208 . If the control unit  205  determines via the screen illustrated in  FIG. 6  (YES in step S 2004 ) that the received job is set to perform mass stacking (mass stacker  1 ), the processing proceeds to step S 2005 . Otherwise, If the control unit  205  determines that the received job is not set to perform mass stacking (mass stacker  1 ) (NO in step S 2004 ), the processing proceeds to A (to step S 2027  illustrated in  FIG. 15 ). 
     In step S 2005 , the control unit  205  determines whether the size of the sheets to be discharged by executing the job is the large size or small size. If the sheet size of the received job is the A4 or B5 size, the control unit  205  determines the sheet size as the small size. If the sheet size of the received job is the A3 or B4 size, the control unit  205  determines the sheet size as the large size. If the control unit  205  determines the sheet size of the received job as the large size (LARGE SIZE in step S 2005 ), the processing proceeds to step S 2006 . Otherwise, If the control unit  205  determines the sheet size of the received job as the small size (SMALL SIZE in step S 2005 ), the processing proceeds to step S 2015 . 
     In step S 2006 , the control unit  205  determines whether sheets have already been stacked on the mass stacker  1 . In determining whether sheets have already been stacked on the mass stacker  1 , the control unit  205  uses, for example, a stacking status management table as illustrated in  FIGS. 21A ,  21 B,  21 C, and  21 D. The stacking status management table is stored in the HDD  209 . 
       FIG. 21A  illustrate the stacking status management table If no sheet is stacked onto the stacker trays A to D. Suppose that the control unit  205  subsequently executes a job  1  to discharge A4-size sheets onto the stacker tray C. In this case, the control unit  205  changes PRESENCE OF SHEET for the stacker tray C from “NONE” to “PRESENT”, and changes SHEET SIZE from “-” to “A4”. The stacking status management table used in this case is illustrated in  FIG. 21B . 
     Suppose that the control unit  205  subsequently executes a job  2  to discharge B5-size sheets onto the stacker tray D. In this case, the control unit  205  changes PRESENCE OF SHEET for the stacker tray D from “NONE” to “PRESENT”, and changes SHEET SIZE from “-” to “B5”. The stacking status management table in this case is illustrated in  FIG. 21C . 
     Suppose that the control unit  205  further executes a job  3  to discharge A3-size sheets onto both stacker trays A and B. In this case, the control unit  205  changes PRESENCE OF SHEET for the stacker trays A and B from “NONE” to “PRESENT”, and changes SHEET SIZE from “-” to “A3”. The sheet discharge status management table in this case is illustrated in  FIG. 21D . 
     Each of the stacker tray A to D is provided with a sensor for detecting the presence or absence of stacked sheets. If the relevant sensor detects that no sheet is present, the control unit  205  restores a discharge status table to the state illustrated in  FIG. 21A . For example, if the user takes out sheets stacked on the stacker tray A to D, the mass stackers  1  and  2  have no sheet. In this case, the control unit  205  resets the discharge status table to the state illustrated in  FIG. 21A  based on the sensor information. 
     In step S 2006 , the control unit  205  determines whether sheets have already been stacked on the mass stacker by using the above-described stacking status management table. 
     If the control unit  205  determines that sheets have already been stacked on at least one of the two stacker trays of the mass stacker  1  (YES in step S 2006 ), the processing proceeds to step S 2007 . Otherwise, if the control unit  205  determines that no sheets have been stacked on the two stacker trays of the mass stacker  1  (NO in step S 2006 ), the processing proceeds to step S 2014 . 
     In step S 2014 , the control unit  205  determines that large-size sheets will be discharged over the two stacker trays A and B of the mass stacker  1  by executing the job. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. In this case, the same sheets are stacked over the two stacker trays, as illustrated in  FIG. 11 . 
     In step S 2007 , the control unit  205  determines whether the size of the stacked sheets is the same as the size of the sheets to be discharged by executing the job. If the control unit  205  determines that the size of the stacked sheets is the same as the size of the sheets to be discharged by executing the job (YES in step S 2007 ), the processing proceeds to step S 2008 . Otherwise, if the control unit  205  determines that the size of the stacked sheets is different from the size of the sheets to be discharged by executing the job (NO in step S 2007 ), the processing proceeds to step S 2012 . 
     In step S 2008 , the control unit  205  determines whether the stacker trays A and B are tray-full. The stacker trays A and B are lowered each time sheets are stacked. Therefore, if the tray-full detection sensors illustrated in  FIG. 9  detect that the stacker trays A and B are at the lowest position, the control unit  205  determines that the stacker trays A and B are tray-full. If the control unit  205  determines that the stacker trays A and B are tray-full (YES in step S 2008 ), the processing proceeds to step S 2012 . Otherwise, if the control unit  205  determines that the stacker trays A and B are not tray-full (NO in step S 2008 ), the processing proceeds to step S 2009 . 
     In step S 2009 , the control unit  205  determines that large-size sheets will be discharged onto the large-size sheets already stacked. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. The reason why the control unit  205  performs control in this way is that the size of the sheets already stacked on the mass stacker (large size) is the same as the size of the sheets to be subsequently discharged and therefore the stacked sheets are assumed to remain stable even after subsequent sheets have been discharged thereon. 
     In step S 2012 , the control unit  205  determines whether a dischargeable discharge destination has been searched for from the mass stacker  2 . If the control unit  205  determines that a dischargeable discharge destination has not yet been searched for from the mass stacker  2  (NO in step S 2012 ), the processing proceeds to C (to step S 2028  illustrated in  FIG. 15 ) to search for a dischargeable discharge destination from the mass stacker  2 . 
     Otherwise, if the control unit  205  determines that a dischargeable discharge destination has already been searched for (YES in step S 2012 ) from the mass stacker  2 , the processing proceeds to step S 2013 . 
     In step S 2013 , the control unit  205  saves the job and restricts execution of the job. Since the size (large size) of the sheets already stacked on the mass stacker is different from the size of the sheets to be subsequently discharged, the stacked sheets may become unstable after subsequent sheets have been discharged thereon. Saving the job refers to storing it in a save area of the HDD  209 . At this timing, the control unit  205  displays a message “REMOVE SHEETS FROM MASS STACKER” on the operation unit  204 . If sheets have been removed from the mass stacker by the user, the control unit  205  executes the saved job and then ends the processing. 
     The following describes a case where the processing proceeds to step S 2015  from step S 2005 . 
     In step S 2015 , the control unit  205  determines whether the stacker tray A is tray-full based on a signal from a relevant tray-full detection sensor. If the control unit  205  determines that the stacker tray A is tray-full (YES in step S 2015 ), the processing proceeds to step S 2020 . Otherwise, if the control unit  205  determines that the stacker tray A is not tray-full (NO in step S 2015 ), the processing proceeds to step S 2016 . 
     In step S 2016 , the control unit  205  determines whether sheets have already been stacked on the stacker tray A of the mass stacker  1  based on the discharge status table. The control unit  205  takes priority of the stacker tray A over the stacker tray B as a discharge destination candidate because the stacker tray A is less distant from the sheet discharge slot of the stack path than the stacker tray B. If the control unit  205  determines that sheets have not been stacked on the stacker tray A (NO in step S 2016 ), the processing proceeds to step S 2017 . Otherwise, if the control unit  205  determines that sheets have already been stacked on the stacker tray A (YES in step S 2016 ), the processing proceeds to step S 2018 . 
     In step S 2017 , the control unit  205  determines that sheets will be discharged onto the stacker tray A. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2018 , the control unit  205  determines whether the size of the sheets stacked on the stacker tray A is the same as the size of the sheets to be subsequently discharged. If the control unit  205  determines that the size of the sheets stacked on the stacker tray A is the same as the size of the sheets to be subsequently discharged (YES in step S 2018 ), the processing proceeds to step S 2019 . Otherwise, if the control unit  205  determines that the size of the sheets stacked on the stacker tray A is different from the size of the sheets to be subsequently discharged (NO in step S 2018 ), the processing proceeds to step S 2020 . 
     In step S 2019 , the control unit  205  determines that sheets will be discharged onto the stacker tray A. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. The reason why the control unit  205  performs control in this way is that the size of the sheets already stacked on the mass stacker A is the same as the size of the sheets to be subsequently discharged and therefore the stacked sheets are assumed to remain stable even after subsequent sheets have been discharged thereon. 
     In step S 2020 , the control unit  205  determines whether the stacker tray B is tray-full based on a signal from a relevant tray-full detection sensor. If the control unit  205  determines that the stacker tray B is tray-full (YES in step S 2020 ), the processing proceeds to step S 2025 . Otherwise, if the control unit  205  determines that the stacker tray B is not tray-full (NO in step S 2020 ), the processing proceeds to step S 2021 . 
     In step S 2021 , the control unit  205  determines whether sheets have already been stacked on the stacker tray B. If the control unit  205  determines that sheets have not been stacked on the stacker tray B (NO in step S 2021 ), the processing proceeds to step S 2022 . Otherwise, if the control unit  205  determines that sheets have already been stacked on the stacker tray B (YES in step S 2021 ), the processing proceeds to step S 2023 . 
     In step S 2022 , the control unit  205  determines that sheets will be discharged onto the stacker tray B. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2023 , the control unit  205  determines whether the size of the sheets stacked on the stacker tray B is the same as the size of the sheets to be subsequently discharged. If the control unit  205  determines that the size of the sheets stacked on the stacker tray B is the same as the size of the sheets to be subsequently discharged (YES in step S 2023 ), the processing proceeds to step S 2024 . Otherwise, if the control unit  205  determines that the size of the sheets stacked on the stacker tray B is different from the size of the sheets to be subsequently discharged (NO in step S 2023 ), the processing proceeds to step S 2025 . 
     In step S 2024 , the control unit  205  determines that sheets will be discharged onto the stacker tray B. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2025 , the control unit  205  determines whether a dischargeable discharge destination has already been searched for from the mass stacker  2 . If the control unit  205  determines that a dischargeable discharge destination has not yet been searched for from the mass stacker  2  (NO in step S 2025 ), the processing proceeds to C (to step S 2028  illustrated in  FIG. 15 ) to search for a dischargeable discharge destination from the mass stacker  2 . 
     In step S 2026 , since there is no stackable stacker tray, the control unit  205  saves the job and restricts execution of the job. The control unit  205  displays a message “REMOVE SHEETS FROM MASS STACKER” on the operation unit  204 . If sheets have been removed from the mass stacker  2  by the user, the control unit  205  resumes the execution of the job. Thus, the control unit  205  performs control in this way so that a plurality of sheet sizes is not present when stacking small-size sheets onto the stacker tray. Then, the control unit  205  ends the processing. 
     The following describes a case where processing proceeds to step S 2027  from step S 2004  with reference to  FIG. 15 . 
     In step S 2027 , the control unit  205  determines whether the discharge destination of the sheets discharged by executing a job is the mass stacker  2  based on the setting information stored in the RAM  208 . If the control unit  205  determines via the screen illustrated in  FIG. 6  (YES in step S 2027 ) that the received job is set to perform mass stacking (mass stacker  2 ), the processing proceeds to step S 2028 . Otherwise, if the control unit  205  determines that the received job is not set to perform mass stacking (mass stacker  2 ) (NO in step S 2027 ), the processing proceeds to step S 2032 . 
     In step S 2032 , the control unit  205  performs control to discharge sheets onto the discharge destination specified by the job. For example, if the job is set to perform saddle stitch bookbinding, the control unit  205  conveys sheets to the saddle stitch bookbinding machine, and controls the saddle stitch bookbinding machine to perform saddle stitch bookbinding and discharge sheets onto the sheet discharge unit of the saddle stitch bookbinding machine. Otherwise, if the job is not set to performing sheet processing, the control unit  205  conveys sheets to the saddle stitch bookbinding machine, and controls the saddle stitch bookbinding machine to discharge sheets onto the sheet discharge tray  337 . Then, the control unit  205  ends the processing. 
     In step S 2028 , the control unit  205  determines whether the size of the sheets to be discharged by executing the job is the large size or small size. If the sheet size of the received job is the A4 or B5 size, the control unit  205  determines the sheet size as the small size. If the sheet size of the received job is the A3 or B4 size, the control unit  205  determines the sheet size as the large size. If the control unit  205  determines the sheet size of the received job as the large size (LARGE SIZE in step S 2028 ), the processing proceeds to step S 2006 . Otherwise, if the control unit  205  determines the sheet size of the received job as the small size (SMALL SIZE in step S 2028 ), the processing proceeds to step S 2036 . 
     In step S 2029 , the control unit  205  determines whether sheets have already been stacked on the mass stacker  2 . In determining whether sheets have already been stacked on the mass stacker  2 , the control unit  205  uses, for example, the stacking status management table as illustrated in  FIGS. 21A ,  21 B,  21 C, and  21 D. The stacking status management table is stored in the HDD  209 . If the control unit  205  determines that sheets have already been stacked on at least one of the two stacker trays of the mass stacker  2  (YES in step S 2029 ), the processing proceeds to step S 2030 . Otherwise, if the control unit  205  determines that no sheets have been stacked on the two stacker trays of the mass stacker  2  (NO in step S 2029 ), the processing proceeds to step S 2035 . 
     In step S 2035 , the control unit  205  determines that large-size sheets will be discharged over the two stacker trays C and D of the mass stacker  2  by executing the job. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. In this case, sheets are stacked over the two stacker trays, as illustrated in  FIG. 11 . 
     In step S 2030 , the control unit  205  determines whether the size of the stacked sheets is the same as the size of the sheets to be discharged by executing the job. If the control unit  205  determines that the size of the stacked sheets is the same as the size of the sheets to be discharged by executing the job (YES in step S 2030 ), the processing proceeds to step S 2031 . Otherwise, if the control unit  205  determines that the size of the stacked sheets is different from the size of the sheets to be discharged by executing the job (NO in step S 2030 ), the processing proceeds to step S 2033 . 
     In step S 2031 , the control unit  205  determines whether the stacker trays C and D are tray-full based on signals from relevant tray-full detection sensors. The stacker trays C and D are lowered each time sheets are stacked. Therefore, if a tray-full detection sensor illustrated in  FIG. 9  detects that the stacker trays C and D are at the lowest position, the control unit  205  determines that the stacker trays C and D are tray-full. If the control unit  205  determines that the stacker trays C and D are tray-full (YES in step S 2031 ), the processing proceeds to step S 2033 . Otherwise, if the control unit  205  determines that the stacker trays C and D are not tray-full (NO in step S 2031 ), the processing proceeds to step S 2032 . 
     In step S 2032 , the control unit  205  determines that large-size sheets will be discharged onto the large-size sheets already stacked. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. The reason why the control unit  205  performs control in this way is that the size of the sheets already stacked on the mass stacker (large size) is the same as the size of the sheets to be subsequently discharged and therefore the stacked sheets are assumed to remain stable even after subsequent sheets have been discharged thereon. 
     In step S 2033 , the control unit  205  determines whether a dischargeable discharge destination has already been searched for from the mass stacker  1 . If the control unit  205  determines that a dischargeable discharge destination has not yet been searched for from the mass stacker  1  (NO in step S 2033 ), the processing proceeds to D (to step S 2005  illustrated in  FIG. 15 ) to search for a dischargeable discharge destination from the mass stacker  1 . 
     Otherwise, if the control unit  205  determines that the dischargeable discharge destination has already been searched for from the mass stacker  1  (YES in step S 2033 ), the processing proceeds to step S 2034 . 
     In step S 2034 , the control unit  205  saves the job and restricts the execution of the job. Since the size of the sheets already stacked on the mass stacker (large size) is different from the size of the sheets to be subsequently discharged, the stacked sheets may become unstable after subsequent sheets have been discharged thereon. Saving the job refers to storing it in a save area of the HDD  209 . At this timing, the control unit  205  displays a message “REMOVE SHEETS FROM MASS STACKER” on the operation unit  204 . If sheets have been removed from the mass stacker  2  by the user, the control unit  205  executes the saved job and then ends the processing. 
     The following describes a case where processing proceeds to step S 2036  from step S 2028 . 
     In step S 2036 , the control unit  205  determines whether the stacker tray C is tray-full based on a signal from a relevant tray-full detection sensor. If the control unit  205  determines that the stacker tray C is tray-full (YES in step S 2036 ), the processing proceeds to step S 2041 . Otherwise, if the control unit  205  determines that the stacker tray C is not tray-full (NO in step S 2036 ), the processing proceeds to step S 2037 . 
     In step S 2037 , the control unit  205  determines whether sheets have already been stacked on the stacker tray C of the mass stacker  2  based on the discharge status table. The control unit  205  takes priority of the stacker tray C over the stacker tray D as a discharge destination candidate because the stacker tray C is less distant from the sheet discharge slot of the stack path than the stacker tray D. If the control unit  205  determines that sheets have not been stacked on the stacker tray C (NO in step S 2037 ), the processing proceeds to step S 2038 . Otherwise, if C when the control unit  205  determines that sheets have already been stacked on the stacker tray C (YES in step S 2037 ), the processing proceeds to step S 2039 . 
     In step S 2038 , the control unit  205  determines that sheets will be discharged onto the stacker tray C. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2039 , the control unit  205  determines whether the size of the sheets stacked on the stacker tray Cis the same as the size of the sheets to be subsequently discharged. If the control unit  205  determines that the size of the sheets stacked on the stacker tray C is the same as the size of the sheets to be subsequently discharged (YES in step S 2039 ), the processing proceeds to step S 2040 . Otherwise, if the control unit  205  determines that the size of the sheets stacked on the stacker tray C is different from the size of the sheets to be subsequently discharged (NO in step S 2039 ), the processing proceeds to step S 2041 . 
     In step S 2040 , the control unit  205  determines that sheets will be discharged onto the stacker tray C. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. The reason why the control unit  205  performs control in this way is that the size of the sheets already stacked on the mass stacker C is the same as the size of the sheets to be subsequently discharged and therefore the stacked sheets are assumed to remain stable even after subsequent sheets have been discharged thereon. 
     In step S 2041 , the control unit  205  determines whether the stacker tray D is tray-full based on a signal from a tray-full detection sensor. If the control unit  205  determines that the stacker tray D is tray-full (YES in step S 2041 ), the processing proceeds to step S 2046 . Otherwise, if the control unit  205  determines that the stacker tray D is not tray-full (NO in step S 2041 ), the processing proceeds to step S 2042 . 
     In step S 2042 , the control unit  205  determines whether sheets have already been stacked on the stacker tray D. If the control unit  205  determines that sheets have not been stacked on the stacker tray D (NO in step S 2042 ), the processing proceeds to step S 2043 . Otherwise, if the control unit  205  determines that sheets have already been stacked on the stacker tray D (YES in step S 2042 ), the processing proceeds to step S 2044 . 
     In step S 2043 , the control unit  205  determines that sheets will be discharged onto the stacker tray D. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2044 , the control unit  205  determines whether the size of the sheets stacked on the stacker tray D is the same as the size of the sheets to be subsequently discharged. If the control unit  205  determines that the size of the sheets stacked on the stacker tray D is the same as the size of the sheets to be subsequently discharged (YES in step S 2044 ), the processing proceeds to step S 2045 . Otherwise, if the control unit  205  determines that the size of the sheets stacked on the stacker tray A is different from the size of the sheets to be subsequently discharged (NO in step S 2044 ), the processing proceeds to step S 2046 . 
     In step S 2045 , the control unit  205  determines that sheets will be discharged onto the stacker tray D. Then, the processing proceeds to step S 2011 , and the control unit  205  performs print processing. 
     In step S 2046 , the control unit  205  determines whether a dischargeable discharge destination has already been searched for from the mass stacker  1 . If the control unit  205  determines that a dischargeable discharge destination has not yet been searched for from the mass stacker  1  (NO in step S 2046 ), the processing proceeds to D (to step S 2005  illustrated in  FIG. 14 ) to search for a dischargeable discharge destination from the mass stacker  1 . 
     In step S 2047 , since there is no stackable stacker tray, the control unit  205  saves the job and restricts the execution of the job. The control unit  205  displays a message “REMOVE SHEETS FROM MASS STACKER” on the operation unit  204 . If sheets have been removed from the mass stacker  1  by the user, the control unit  205  resumes the execution of the job. Thus, the control unit  205  performs control in this way so that a plurality of sheet sizes is not present when stacking small-size sheets onto a stacker tray. Then, the control unit  205  ends the processing. The control unit  205  performs control in this way to determine a discharge destination of sheets printed by executing a job. 
     The following describes the print processing performed in step S 2011  with reference to  FIG. 16 . Processing in each step illustrated in the flowchart in  FIG. 16  is implemented by the control unit  205  reading a program stored in the ROM  207  and then executes it. 
     In step S 3001 , based on the setting information received in step S 2001 , the control unit  205  determines a sheet cassette from which sheets will be supplied, and supplies one sheet from the determined sheet cassette. Then, the control unit  205  prints an image on the supplied sheet. 
     In step S 3002 , the control unit  205  discharges the sheet having the image printed thereon to the discharge destination determined by the above-described processing. 
     In step S 3003 , the control unit  205  determines whether the discharged sheet is the first sheet discharged by executing a job. Specifically, the control unit  205  determines whether the discharged sheet is the first sheet by referring to the value of a counter, provided in the RAM  208 , for counting the number of sheets discharged for each job. This counter is reset to zero by the control unit  205  when a job is started. If the control unit  205  determines that the discharged sheet is the first sheet discharged by executing a job (YES in step S 3003 ), the processing proceeds to step S 3005 . Otherwise, if the control unit  205  determines that the discharged sheet is not the first sheet discharged by executing a job (NO in step S 3003 ), the processing proceeds to step S 3004 . 
     In step S 3004 , the control unit  205  determines whether the discharge destination of the discharged sheet is different from the discharge destination of the sheets previously discharged. If the control unit  205  determines that the discharge destination of the discharged sheet is different from the discharge destination of the sheets previously discharged (YES in step S 3004 ), the processing proceeds to step S 3005 . Otherwise, if the control unit  205  determines that the discharge destination of the discharged sheet is not different from the discharge destination of sheets previously discharged (NO in step S 3004 ), the processing proceeds to step S 3006 . 
     In step S 3005 , the control unit  205  stores the discharge destination in the discharge destination information block prepared in step S 2002 . The control unit  205  stores discharge destinations from the top downward in the discharge destination information block to enable recognizing the discharge order for each job. 
     In step S 3006 , the control unit  205  determines whether tray-full has occurred in the stacker tray of the discharge destination based on a signal from a relevant tray-full detection sensor. If the control unit  205  determines that tray-full has occurred in the stacker tray of the discharge destination (YES in step S 3006 ), the processing proceeds to D (step S 2005  illustrated in  FIG. 14 ) to predetermine a discharge destination of the following sheet. In step S 2005 , the control unit  205  performs the subsequent processing to determine a discharge destination of the following sheet. 
     In step S 3007 , the control unit  205  determines whether there is an unprinted page. If the control unit  205  determines that there is an unprinted page (YES in step S 3007 ), the processing returns to step S 3001 . Otherwise, if the control unit  205  determines that there is no unprinted page (NO in step S 3007 ), the control unit  205  ends the processing. 
       FIG. 17B  illustrates the discharge destination information block when the control unit  205  has separately discharged sheets onto a plurality of stacker trays for each job through the above-described processing. 
     Referring to  FIG. 17B , for a job having a job ID “6542”, print products have been discharged onto the stacker tray C, the stacker tray D, and the stacker tray A in this order. 
     After the job having the job ID “6542”, a job having a job ID “6543” has been executed. For the job having the job ID “6543”, print products have been discharged onto the stacker tray B. 
     The following describes processing for taking out sheets with reference to  FIG. 18 . Processing in each step illustrated in the flowchart in  FIG. 18  is implemented by the control unit  205  reading a program stored in the ROM  207  and then executes it. 
     In step S 4000 , if the user requests to display a job history screen, the control unit  205  displays the job history screen illustrated in  FIG. 19  on the operation unit  204 . When executing a job, the control unit  205  stores the job ID (reception number) issued for the job, the date and time of execution of the job, the job name, the user name, and the execution result of the job in the HDD  209 , and displays the screen illustrated in  FIG. 19  based on the stored information. 
     The control unit  205  displays a “PRESENT” button in the DISCHARGE INFORMATION column for jobs whose discharge destination information block is stored in the HDD  209 . 
     In step S 4001 , the control unit  205  determines whether an instruction for displaying the discharge status screen is received from the user. Specifically, the control unit  205  determines whether the “PRESENT” button illustrated in  FIG. 19  is pressed. If the control unit  205  determines that the “PRESENT” button illustrated in  FIG. 19  is pressed (YES instep S 4001 ), the processing proceeds to step S 4002 . Otherwise, if the control unit  205  determines that the “PRESENT” button illustrated in  FIG. 19  is not pressed (NO in step S 4001 ), the processing returns to step S 4000 . 
     In step S 4002 , the control unit  205  displays the discharge status screen illustrated in  FIG. 20  on the operation unit  204 . The discharge status screen illustrated in  FIG. 20  displays the sheet discharge status of print products of a job whose “PRESENT” button illustrated in  FIG. 19  is pressed. The discharge status screen illustrated in  FIG. 20  includes a “TAKE-OUT IN ORDER OF DISCHARGE” button  701 , an appearance  702  illustrating the appearance of the mass stackers  1  and  2 , tray specification buttons  703  to  706 , and a CLOSE button  707 . The control unit  205  obtains configuration information indicating the configuration of the mass stackers  1  and  2  from memories (not illustrated) included in the mass stackers  1  and  2 , respectively, and displays the appearance  702  and the tray specification buttons  703  to  706 . The tray specification buttons  703  to  706  correspond to the stacker trays A to D, respectively. The control unit  205  displays the discharge order of print products discharged by executing a job in association with each stacker tray.  FIG. 20  illustrate an example screen displayed if the user presses the “PRESENT” button for a job X on the screen illustrated in  FIG. 19 . If the user presses the “PRESENT” button for the job X on the screen illustrated in  FIG. 19 , the control unit  205  searches for a discharge destination information block having a job ID “6542” of the job X out of the discharge destination information blocks stored in the HDD  209 . Then, based on the information indicating the discharge order stored in the discharge destination information block having the job ID “6542”, the control unit  205  displays a number indicating the discharge order on each stacker tray. 
     In step S 4003 , the control unit  205  determines whether the user presses the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  on the discharge status screen. If the control unit  205  determines that the user presses the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  (YES in step S 4003 ), the processing proceeds to step S 4004 . Otherwise, if the control unit  205  determines that the user does not press the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  (NO in step S 4003 ), the processing proceeds to step S 4007 . 
     In step S 4004 , for the job currently displayed on the screen illustrated in  FIG. 20 , the control unit  205  identifies a stacker tray having the earliest discharge order in the discharge destination information block. Then, the control unit  205  lowers the relevant stacker tray onto the dolly, and opens the front door corresponding to the stacker tray, allowing the user to take out the sheets stacked onto the stacker tray. Then, the processing proceeds to step S 4005 . 
     Otherwise, if the processing proceeds to step S 4007  from step S 4003 , the control unit  205  determines whether a take-out instruction is received for a specific stacker tray. Specifically, in step S 4007 , the control unit  205  determines whether the user presses any one of the tray specification buttons  703  to  706  illustrated in  FIG. 20 . If the control unit  205  determines that the user presses any one of the tray specification buttons  703  to  706  illustrated in  FIG. 20  (YES in step S 4007 ), the processing proceeds to step S 4008 . Otherwise, if the control unit  205  determines that the user presses none of the tray specification buttons  703  to  706  illustrated in  FIG. 20  (NO in step S 4007 ), the processing proceeds to step S 4006 . 
     In step S 4008 , the control unit  205  lowers the stacker tray corresponding to the pressed button onto the dolly, and opens the front door corresponding to the stacker tray, allowing the user take out the sheets stacked on the stacker tray. Then, the processing proceeds to step S 4005 . 
     In step S 4005 , for the job currently displayed on the screen illustrated in  FIG. 20 , the control unit  205  erases from the discharge destination information block of the job the registration order of the stacker tray from which print products of the job have been taken out. 
     In step S 4006 , the control unit  205  determines whether an instruction for canceling display of the discharge status screen is received. Specifically, the control unit  205  determines whether the user presses the “CLOSE” button  707  illustrated in  FIG. 20 . If the control unit  205  determines that an instruction for canceling display of the discharge status screen is received (YES in step S 4006 ), the control unit  205  ends the processing. Otherwise, if the control unit  205  determines that an instruction for canceling display of the discharge status screen is not received (NO in step S 4006 ), the processing returns to step S 4002 . 
     As described above, the control unit  205  stores the discharge order for each stacker tray during execution of print processing. If an instruction for taking out sheets in the discharge order is made, the control unit  205  performs an operation for allowing the user to take out print products in the discharge order. Thus, even without outputting notification sheets, such as inserting sheets indicating the discharge order, the user can take out print products in the discharge order. 
     Although, in above-described exemplary embodiments, the control unit  205  opens the front door corresponding to a relevant stacker tray in steps S 4004  and S 4008 , the present invention is not limited thereto. For example, the front door may be locked, and, in steps S 4004  and S 4008 , the control unit  205  may unlock the front door so as to be opened and closed. Thus, the user can manually open the front door and take out print products. 
     Other Exemplary Embodiments 
     The user can take out print products by pressing the take-out button  2001  or  2002  provided on the mass stacker  1 , or the take-out button  2003  or  2004  provided on the mass stacker  2 . Also if the user presses any one of these take-out buttons, the control unit  205  lowers the stacker tray corresponding to the pressed take-out button to the dolly. When the control unit  205  opens the front door corresponding to the relevant stacker tray, the user may take out the print products stacked on the stacker tray. In this case, the control unit  205  may or may not erase from each discharge destination information block the registration information of the stacker tray from which sheets have been taken out. 
     In the above-described exemplary embodiments, upon each reception of the depression of the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  once, the control unit  205  lowers one stacker tray, and opens the front door corresponding to the stacker tray. Specifically, upon reception of the depression of the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  illustrated in  FIG. 20  once, the control unit  205  lowers the first stacker tray (a first sheet discharge tray). Upon reception of the depression twice, the control unit  205  lowers the second stacker tray (a second sheet discharge tray), and opens the front door corresponding to each stacker tray. However, the present invention is not limited thereto. Upon reception of the depression of the “TAKE-OUT IN ORDER OF DISCHARGE” button  701  once in the discharge order illustrated in  FIG. 20 , it may lower two or more stacker trays in which sheets have been discharged by executing the job displayed on the screen illustrated in  FIG. 20 , and open the front doors corresponding to the two or more stacker trays. 
     In the above-described exemplary embodiments, sheets are sequentially printed from the first page. When printing sheets in reverse order, i.e., when starting printing from the last page, it is only necessary to sequentially store the discharge order for each job in the discharge destination information block from the bottom upward. Thus, even if reverse order printing is performed, the user can easily take out print products in the page order afterwards. 
     Further, in the exemplary embodiments, sheets are taken out in the discharge order. However, there may be provided a button for taking out sheets in reverse order from the discharge order for each job. If the user presses the relevant button, the control unit  205  sequentially reads the information indicating the discharge order in the discharge destination information block for a relevant job from the bottom upward, and sequentially lowers the stacker trays and opens the front doors corresponding to the relevant information from the bottom upward. Thus, the user can take out print products in reverse order from the discharge order. 
     In the above-described exemplary embodiments, if a print product remains in any stacker tray for which a take-out instruction is made, the page order of print products sequentially took out and combined by the user may be out of sequence. Therefore, if the control unit  205  detects that a front door of amass stacker is closed via a front door open/close detection sensor (not illustrated), the control unit  205  determines whether sheets are present in the stacker tray corresponding to the front door based on a signal from a sheet detection sensor (not illustrated). If the control unit  205  determines that sheets are present, the control unit  205  displays a message as illustrated in  FIG. 22  on the operation unit  204 . With the screen illustrated in  FIG. 22  displayed, the user can easily recognize that a print product remains in a stacker tray and which stacker tray the remaining print product is in. 
     If the user presses the “TAKE-OUT FROM NEXT TRAY” button  901 , the control unit  205  lowers the next stacker tray, ignoring the remaining print product. Otherwise, if the user presses the “LOWER AGAIN THE TRAY IN WHICH SHEET REMAINS” button  902 , the control unit  205  lowers again the stacker tray having the remaining print product. 
     Although, in the above-described exemplary embodiments, each mass stacker is provided with a plurality of stacker trays, the present invention is not limited thereto. The present invention is also applicable to a case where each mass stacker is provided with one stacker tray, and sheets are separately discharged onto these stacker trays. 
     Although, in the above-described exemplary embodiments, a housing-type apparatus whose inside cannot be seen, such as a mass stacker, is used, the present invention is not limited thereto. For example, the present invention is also applicable to a housing-type apparatus whose inside can be seen, or to an ordinary sheet discharge tray. When applying the present invention to an ordinary sheet discharge tray, it is desirable that, when the user takes out sheets stacked on the sheet discharge tray, the sheet discharge tray is movable, and that sheets discharged on a plurality of sheet discharge trays are moved in the discharge order to a position where the user can easily take out the sheets. 
     Other Embodiments 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment (s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     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 priority from Japanese Patent Application No. 2012-134841 filed Jun. 14, 2012, which is hereby incorporated by reference herein in its entirety.