Patent Publication Number: US-11040556-B2

Title: Printing apparatus with plurality of sheet feed trays

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application Nos. 2018-161375, filed on Aug. 30, 2018, and 2019-138850, filed on Jul. 29, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a printing apparatus which performs printing on sheets. 
     2. Related Art 
     Japanese Patent Application Publication No. 2015-212028 proposes a printing apparatus including a sheet discharge tray which can hold many sheets subjected to printing and discharged. 
     Moreover, among such printing apparatuses, there is an apparatus including a mechanism which automatically unloads the sheets stacked on the sheet discharge tray from the sheet discharge tray. The printing apparatus including such a mechanism can continuously perform a print operation even when the sheet discharge tray becomes full, by automatically unloading the sheets stacked on the sheet discharge tray. 
     SUMMARY 
     Even in the apparatus including the mechanism which automatically unloads the sheets from the sheet discharge tray as described above, there are cases where the print operation stops due to running-out of sheets in the sheet feed tray and the apparatus has to wait for a user to supply the sheets. This leads to a decrease in printed sheet productivity. 
     The disclosure is directed to a printing apparatus which can reduce a decrease in printed sheet productivity. 
     A printing apparatus in accordance with some embodiments includes: a printer configured to perform printing on sheets; a sheet feeder including sheet feed trays and configured to feed the sheets from a selected one of the sheet feed trays to the printer; a sheet discharger configured to discharge and hold the sheets printed by the printer; and a controller. Upon unloading of the sheets from the sheet discharger, the controller is configured to: determine whether to perform switching of a sheet feed source from a first sheet feed tray which is a latest sheet feed source of the sheet feed trays to a second sheet feed tray which is a sheet feed tray other than the first sheet feed tray of the sheet feed trays, based on a first sheet remaining amount in the first sheet feed tray and a second sheet remaining amount in the second sheet feed tray; and upon determining to perform the switching, control the sheet feeder to switch the sheet feed source from the first sheet feed tray to the second sheet feed tray. 
     According to the aforementioned configuration, a decrease in printed sheet productivity can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a printing apparatus according to a first embodiment. 
         FIG. 2  is a schematic configuration diagram of an external sheet feeder and a printing apparatus main body of the printing apparatus illustrated in  FIG. 1 . 
         FIG. 3  is a schematic configuration diagram of a large-capacity sheet discharger and an unloader of the printing apparatus illustrated in  FIG. 1 . 
         FIG. 4  is a flowchart of processing of switching an external sheet feed tray. 
         FIG. 5  is a flowchart for explaining operations of the external sheet feed tray in the case where sheets are set. 
         FIG. 6  is a flowchart for explaining an unloading operation by the unloader. 
         FIG. 7  is a flowchart of control of the external sheet feeder performed in execution of the unloading operation by the unloader in a first embodiment. 
         FIG. 8  is a flowchart of control of the external sheet feeder performed in the execution of the unloading operation by the unloader in a second embodiment. 
         FIG. 9  is a block diagram illustrating a configuration of a printing apparatus according to a third embodiment. 
         FIG. 10  is a schematic configuration diagram of a large-capacity sheet discharger in the printing apparatus illustrated in  FIG. 9 . 
         FIG. 11  is a flowchart for explaining an operation in switching of a discharge destination of the sheets. 
         FIG. 12  is a flowchart for explaining an operation of the printing apparatus in unloading of the sheets. 
         FIG. 13  is a flowchart of control of the external sheet feeder performed in execution of work of unloading the sheets from the large-capacity sheet discharger in the third embodiment. 
         FIG. 14  is a flowchart of control of the external sheet feeder performed in the execution of work of unloading the sheets from the large-capacity sheet discharger in a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     Description will be hereinbelow provided for an embodiment of the present invention by referring to the drawings. It should be noted that the same or similar parts and components throughout the drawings will be denoted by the same or similar reference signs, and that descriptions for such parts and components will be omitted or simplified. In addition, it should be noted that the drawings are schematic and therefore different from the actual ones. 
       FIG. 1  is a block diagram illustrating a configuration of a printing apparatus  1  according to a first embodiment.  FIG. 2  is a schematic configuration diagram of an external sheet feeder  2  and a printing apparatus main body  3  in the printing apparatus  1  illustrated in  FIG. 1 .  FIG. 3  is a schematic configuration diagram of a large-capacity sheet discharger  4  and a conveyor  5  in the printing apparatus  1  illustrated in  FIG. 1 . In the following description, a direction orthogonal to the sheet surfaces of  FIGS. 2 and 3  are referred to as front-rear direction. Moreover, up, down, left, and right in the sheet surfaces of  FIGS. 2 and 3  are referred to as directions of up, down, left, and right. In  FIGS. 2 and 3 , the directions of right, left, up, and down are denoted by RT, LT, UP, and DN, respectively. 
     Routes illustrated by bold lines in  FIGS. 2 and 3  are conveyance routes through which sheets P being print media are conveyed. Among the conveyance routes, a route illustrated by a solid line is a normal route RC. Routes illustrated by broken lines are a main body side sheet discharge route RD 1  and a large-capacity sheet discharge route RD 2 . Routes illustrated by one-dot chain lines are a sheet reverse refeed route RR 1  and a sheet discharge reverse route RR 2 . Routes illustrated by two-dot chain lines are an external sheet feed route RS 1  and an internal sheet feed route RS 2 . Upstream and downstream in the following description mean upstream and downstream in the conveyance routes. 
     As illustrated in  FIGS. 1 to 3 , the inkjet printing apparatus  1  according to the first embodiment includes the external sheet feeder (sheet feeder)  2 , the printing apparatus main body  3 , the large-capacity sheet discharger (sheet discharger)  4 , the conveyor  5 , and a controller  6 . 
     The external sheet feeder  2  feeds the sheets P to a printer  32  of the printing apparatus main body  3  to be described later. The external sheet feeder  2  includes external sheet feed trays (sheet feed trays)  11 A,  11 B, external sheet feed roller units  12 A,  12 B, lifting-lowering motors  13 A,  13 B, upper limit sensors  14 A,  14 B, sheet presence-absence sensors  15 A,  15 B, remaining amount detectors  16 A,  16 B, sheet set switches  17 A,  17 B, multiple pairs of external sheet feed conveyance rollers  18 , and an external sheet feeder case  19  housing or holding the units of the external sheet feeder  2 . Note that the external sheet feed trays  11 A,  11 B and the like are sometimes collectively referred to by omitting the alphabets attached to the reference numerals. 
     The external sheet feed trays  11  are trays on which the sheets P used for printing are stacked. The external sheet feed trays  11  are configured to be capable of being lifted and lowered. The external sheet feed trays  11 A,  11 B are arranged one on top of another in the up-down direction. The external sheet feed tray  11 B is installed below the external sheet feed tray  11 A. The external sheet feed trays  11 A,  11 B are selectively used as a sheet feed source of the printer  32 . 
     Each external sheet feed tray  11  is provided with a detection target plate  11   a . The detection target plate  11   a  is a member to be detected by remaining amount sensors  26  of a remaining amount detector  16  to be described later. 
     The external sheet feed roller units  12 A,  12 B pick up the sheets P stacked on the external sheet feed trays  11 A,  11 B and send them toward the printing apparatus main body  3  (to the right). Each external sheet feed roller unit  12  includes a pair of external sheet feed rollers  21  and a detection target plate  22 . 
     The upstream (left) external sheet feed roller  21  in each pair of external sheet feed rollers  21  comes into pressure contact with the top (uppermost) sheet P among the sheets P stacked on the external sheet feed tray  11  and picks up the sheet P to send it to the right by means of friction force. The upstream external sheet feed roller  21  is configured to be capable of achieving sheet feeding pressure at which the sheet P can be picked up when a top surface position of the sheets P (height position of the top sheet P) on the external sheet feed tray  11  is at a predetermined sheet feed position. The upstream external sheet feed roller  21  moves up and down within a certain range depending on fluctuation in the position (height) of the top sheet P. 
     The upstream external sheet feed roller  21  is rotationally driven by a not-illustrated motor. A not-illustrated drive force transmission mechanism transmits rotational drive force of the motor from the upstream external sheet feed roller  21  to the downstream external sheet feed roller  21  and the paired external sheet feed rollers  21  thereby rotate. 
     The downstream (right) external sheet feed roller  21  out of the paired external sheet feed rollers  21  guides the sheet P sent by the upper external sheet feed roller  21  between itself and a not-illustrated friction pad and sends the sheet P to the right. 
     The detection target plate  22  is a member provided to be used by the upper limit sensor  14  to detect that the top surface position of the sheets P on the external sheet feed tray  11  is at the sheet feed position. The sheet feed position is a position set as the height position of the top sheet P in the case where the upstream external sheet feed roller  21  picks up the top sheet P on the external sheet feed tray  11 . The detection target plate  22  moves up and down depending on upward and downward movement of the upstream external sheet feed roller  21 . 
     The lifting-lowering motors  13 A,  13 B lift and lower the external sheet feed trays  11 A,  11 B, respectively. 
     The upper limit sensors  14 A,  14 B are sensors which detect that the top surface positions of the sheets P on the external sheet feed trays  11 A,  11 B are at the sheet feed positions. Each upper limit sensor  14  is arranged to detect the detection target plate  22  when the top sheet P on the external sheet feed tray  11  is at the sheet feed position. When the top sheet P on the external sheet feed tray  11  is below the sheet feed position, the detection target plate  22  is moved downward together with the upstream external sheet feed roller  21 . In this case, the upper limit sensor  14  does not detect the detection target plate  22 . 
     A state where the upper limit sensor  14  detects the detection target plate  22  and thus detects that the top surface position of the sheets P on the external sheet feed tray  11  is at the sheet feed position is referred to as on. Moreover, a state where the upper limit sensor  14  does not detect the detection target plate  22  and thus does not detect that the top surface position of the sheets P on the external sheet feed tray  11  is at the sheet feed position is referred to as off. 
     The sheet presence-absence sensors  15 A,  15 B detect presence or absence of the sheets P on the external sheet feed trays  11 A,  11 B, respectively. A state where each sheet presence-absence sensor  15  detects that there are sheets P on the external sheet feed tray  11  is referred to as on. A state where the sheet presence-absence sensor  15  does not detect that there are sheets P on the external sheet feed tray  11  is referred to as off. 
     The remaining amount detectors  16 A,  16 B are sensors used to detect sheet remaining amounts which are amounts of sheets P stacked on the external sheet feed trays  11 A,  11 B, respectively. Each remaining amount detector  16  includes multiple remaining amount sensors  26 . 
     The multiple remaining amount sensors  26  detect the height position of the external sheet feed tray  11  by detecting the detection target plate  11   a . The multiple remaining amount sensors  26  are arranged one on top of the other in the up-down direction. The positions of the respective remaining amount sensors  26  are set such that a range of the sheet remaining amount and a lower limit position (sheet supply position) of the external sheet feed tray  11  can be determined from the states of detection of the detection target plate  11   a  in the respective remaining amount sensors  26  (whether the detection target plate  11   a  is detected). The lower limit position of the external sheet feed tray  11  is a position set as a height position of the external sheet feed tray  11  in the case where a user supplies the sheets P to the external sheet feed tray  11 . 
     The sheet set switches  17  are switches which the user operates when the user supplies the sheets P to the external sheet feed trays  11  and the external sheet feed trays  11  are to be moved to the upper limit positions. The upper limit position of each external sheet feed tray  11  is a height position of the external sheet feed tray  11  in the case where the top surface position of the sheets P on the external sheet feed tray  11  is at the sheet feed position. The sheet set switches  17 A,  17 B correspond respectively to the external sheet feed trays  11 A,  11 B. 
     The external sheet feed conveyance rollers  18  convey the sheets P, picked up from the external sheet feed trays  11 A,  11 B, toward registration rollers  46  of the printing apparatus main body  3  to be described later. The external sheet feed conveyance rollers  18  are arranged along the external sheet feed route RS 1 . One pair of the external sheet feed conveyance rollers  18  is arranged in a main body case  39  of the printing apparatus main body  3  to be described later. The external sheet feed conveyance rollers  18  are rotationally driven by a not-illustrated motor. 
     The printing apparatus main body  3  performs printing on the sheets P. The printing apparatus main body  3  includes an internal sheet feeder  31 , the printer  32 , an upper conveyor  33 , a main body side sheet discharger  34 , a reverser  35 , a communicator  36 , a notification lamp (notifier)  37 , an operation panel  38 , and the main body case  39  housing or holding the units of the printing apparatus main body  3 . 
     The internal sheet feeder  31  feeds the sheets P to the printer  32  in the printing apparatus main body  3 . The internal sheet feeder  31  includes multiple internal sheet feed trays  41 , multiple pairs of internal sheet feed rollers  42 , and multiple pairs of internal sheet feed conveyance rollers  43 . 
     The internal sheet feed trays  41  are trays on which the sheets P to be used in printing are stacked. The internal sheet feed trays  41  are arranged in the main body case  39 . 
     The internal sheet feed rollers  42  pick up the sheets P from the internal sheet feed trays  41  and send them to the internal sheet feed route RS 2 . The internal sheet feed rollers  42  are rotationally driven by a not-illustrated motor. 
     The internal sheet feed conveyance rollers  43  convey the sheets P picked up from the internal sheet feed trays  41  toward the registration rollers  46  to be described later. The internal sheet feed conveyance rollers  43  are arranged along the internal sheet feed route RS 2 . The internal sheet feed conveyance rollers  43  are rotationally driven by a not-illustrated motor. 
     The printer  32  prints images on the sheets P while conveying the sheets P. The printer  32  includes a pair of registration rollers  46 , a belt platen portion  47 , and an inkjet head portion  48 . 
     The registration rollers  46  temporarily stops each of the sheets P conveyed from any of the external sheet feeder  2 , the internal sheet feeder  31 , and the reverser  35  to correct skewing and then conveys the sheet P toward the belt platen portion  47 . The registration rollers  46  are arranged on the normal route RC near a merging point of the external sheet feed route RS 1 , the internal sheet feed route RS 2 , and the sheet reverse refeed route RR 1 . The registration rollers  46  are rotationally driven by a not-illustrated motor. 
     The belt platen portion  47  conveys the sheets P conveyed from the registration rollers  46  while sucking and holding the sheets P on a belt. 
     The inkjet head portion  48  includes a line inkjet head (not illustrated) in which multiple nozzles are aligned in a direction (front-rear direction) substantially orthogonal to the conveyance direction of the sheets P. The inkjet head portion  48  is arranged above the belt platen portion  47 . The inkjet head portion  48  prints images on the sheets P conveyed by the belt platen portion  47  by ejecting ink from the inkjet head to the sheets P. 
     The upper conveyor  33  conveys the sheets P conveyed by the belt platen portion  47  such that the sheets P are turned around to be conveyed from right to left. The upper conveyor  33  includes multiple pairs of upward conveyance rollers  51  and multiple pairs of horizontal conveyance rollers  52 . 
     The upward conveyance rollers  51  convey the sheets P, conveyed by the belt platen portion  47 , to the horizontal conveyance rollers  52  above the upward conveyance rollers  51 . The upward conveyance rollers  51  are arranged in an upward conveyance portion in an intermediate section of the normal route RC. The upward conveyance rollers  51  are rotationally driven by a not-illustrated motor. 
     The horizontal conveyance rollers  52  convey the sheets P, conveyed by the upward conveyance rollers  51 , to the main body side sheet discharger  34  or the reverser  35 . The most downstream pair of horizontal conveyance rollers  52  is arranged in an upstream portion of the sheet reverse refeed route RR 1 . The other horizontal conveyance rollers  52  are arranged along a horizontal portion in a downstream section of the normal route RC. The horizontal conveyance rollers  52  are rotationally driven by a not-illustrated motor. 
     The main body side sheet discharger  34  discharges the sheets P subjected to printing in the printer  32 . The main body side sheet discharger  34  includes a switching unit  56 , a pair of sheet discharge rollers  57 , and a sheet discharge tray  58 . 
     The switching unit  56  switches the conveyance route of the sheets P between the main body side sheet discharge route RD 1  and the sheet reverse refeed route RR 1 . The switching unit  56  is arranged at a branching point between the main body side sheet discharge route RD 1  and the sheet reverse refeed route RR 1 . 
     The sheet discharge rollers  57  convey the sheets P guided toward the main body side sheet discharge route RD 1  by the switching unit  56  and discharge the sheets P to the sheet discharge tray  58 . The sheet discharge rollers  57  are arranged between the switching unit  56  and the sheet discharge tray  58  in the main body side sheet discharge route RD 1 . The sheet discharge rollers  57  are rotationally driven by a not-illustrated motor. 
     The sheet discharge tray  58  is a tray on which the sheets P subjected to printing and discharged by the sheet discharge rollers  57  are stacked. 
     The reverser  35  reverses the sheets P subjected to printing on the front side and conveys the sheets P to the registration rollers  46  in duplex printing. The reverser  35  includes a pair of reverse rollers  61 , a pair of sheet refeed rollers  62 , and a switching gate  63 . 
     The reverse rollers  61  switch back the sheets P conveyed by the horizontal conveyance rollers  52  of the upper conveyor  33  and convey the sheets P to the sheet refeed rollers  62 . The reverse rollers  61  are configured to be rotatable in forward and reverse directions to perform switch back conveyance of the sheets P. The reverse rollers  61  are arranged between the most downstream pair of horizontal conveyance rollers  52  and the pair of sheet refeed rollers  62  in the sheet reverse refeed route RR 1 . The reverse rollers  61  are rotationally driven by a not-illustrated motor. 
     The sheet refeed rollers  62  convey the sheets P switched back by the reverse rollers  61  to the registration rollers  46 . The sheet refeed rollers  62  are arranged between the pair of reverse rollers  61  and the pair of registration rollers  46  in the sheet reverse refeed route RR 1 . The sheet refeed rollers  62  are rotationally driven by a not-illustrated motor. 
     The switching gate  63  guides the sheets P, conveyed by the horizontal conveyance rollers  52 , to the reverse rollers  61 . Moreover, the switching gate  63  guides the sheets P, switched back by the reverse rollers  61 , to the sheet refeed rollers  62 . The switching gate  63  is arranged near the center of mass of three portions of the most downstream pair of horizontal conveyance rollers  52 , the pair of reverse rollers  61 , and the pair of sheet refeed rollers  62 . 
     The communicator  36  sends the sheets P subjected to printing in the printer  32  to a large-capacity sheet discharger  4 . The communicator  36  includes a switching unit  66  and a pair of communication rollers  67 . 
     The switching unit  66  switches the conveyance route of the sheets P between the normal route RC and the large-capacity sheet discharge route RD 2 . The switching unit  66  is arranged at a branching point between the normal route RC and the large-capacity sheet discharge route RD 2 . The large-capacity sheet discharge route RD 2  is a route which branches from the normal route RC and extends to a large-capacity sheet discharge tray  73  to be described later. 
     The communication rollers  67  convey the sheets P conveyed from the belt platen portion  47  and sent the sheets P to the large-capacity sheet discharger  4 . The communication rollers  67  are arranged downstream of the switching unit  66  in the large-capacity sheet discharge route RD 2 . The communication rollers  67  are rotationally driven by a not-illustrated motor. 
     The notification lamp  37  notifies the user of time (a timing) to supply the sheets P to the external sheet feed tray  11 . The notification lamp  37  is installed on the main body case  39 . Note that, the notification lamp  37  may be installed in the external sheet feeder case  19  or the like as long as the notification lamp  37  is located at a position visible to the user. 
     The operation panel  38  displays input screens and the like and receives input operations made by the user. The operation panel  38  includes a display unit (not illustrated) including a liquid crystal display panel or the like and an input unit (not illustrated) including various operation keys, a touch panel, and the like. 
     The large-capacity sheet discharger  4  discharges and holds the sheets P subjected to printing in the printer  32  of the printing apparatus main body  3 . The large-capacity sheet discharger  4  includes a sheet discharge conveyor  71 , a sheet discharge reverser  72 , the large-capacity sheet discharge tray  73 , a lifting-lowering motor  74 , a full sensor  75 , a pair of side fences  76 A,  76 B, an end fence  77 , a fence driver  78 , a top surface detection sensor  79 , a sheet discharge conveyance side case  80  housing the sheet discharge conveyor  71  and the like, and a sheet discharge side case  81  housing the large-capacity sheet discharge tray  73  and the like. 
     The sheet discharge conveyor  71  discharges the sheets P subjected to printing and sent out by the communicator  36  and stacks the sheets P on the large-capacity sheet discharge tray  73 . The sheet discharge conveyor  71  includes two pairs of introduction rollers  86 , one pair of upward conveyance rollers  87 , and two pairs of sheet discharge rollers  88 . 
     The introduction rollers  86  introduce the sheets P subjected to printing and sent out from the printing apparatus main body  3  by the communicator  36 , into the large-capacity sheet discharger  4 . The introduction rollers  86  are arranged downstream of the communication rollers  67  along the large-capacity sheet discharge route RD 2 . The introduction rollers  86  are rotationally driven by a not-illustrated motor. 
     The upward conveyance rollers  87  convey the sheets P, conveyed by the introduction rollers  86  or intermediate rollers  92  to be described later, to the sheet discharge rollers  88  above the upward conveyance rollers  87 . The upward conveyance rollers  87  are arranged near and downstream of a merging point of the large-capacity sheet discharge route RD 2  and a downstream portion of the sheet discharge reverse route RR 2 , in the large-capacity sheet discharge route RD 2 . The upward conveyance rollers  87  are rotationally driven by a not-illustrated motor. 
     The sheet discharge rollers  88  convey the sheets P conveyed by the upward conveyance rollers  87  and discharge the sheets P to the large-capacity sheet discharge tray  73 . The sheet discharge rollers  88  are arranged in a most-downstream portion of the large-capacity sheet discharge route RD 2 . The sheet discharge rollers  88  are rotationally driven by a not-illustrated motor. 
     The sheet discharge reverser  72  reverses the sheets P by causing the sheets P received from the large-capacity sheet discharge route RD 2  to move back and forth in the sheet discharge reverse route RR 2  and returning the sheets P to the large-capacity sheet discharge route RD 2 . The sheet discharge reverser  72  includes a switching unit  89 , two pairs of reverse entrance rollers  90 , one pair of reverse rollers  91 , and two pairs of intermediate rollers  92 . 
     The switching unit  89  switches the conveyance route of the sheets P between the large-capacity sheet discharge route RD 2  and the sheet discharge reverse route RR 2 . The switching unit  89  is arranged at a branching point between the large-capacity sheet discharge route RD 2  and the sheet discharge reverse route RR 2 . 
     The reverse entrance rollers  90  convey the sheets P, guided from the large-capacity sheet discharge route RD 2  to the sheet discharge reverse route RR 2  by the switching unit  89 , to the reverse rollers  91 . The reverse entrance rollers  90  are arranged along an upstream portion of the sheet discharge reverse route RR 2 . The reverse entrance rollers  90  are rotationally driven by a not-illustrated motor. 
     The reverse rollers  91  switch back the sheets P conveyed by the reverse entrance rollers  90  and convey the sheets P to the intermediate rollers  92 . The reverse rollers  91  are configured to be capable of rotating in the forward and reverse direction to perform switch back conveyance of the sheets P. The reverse rollers  91  are arranged downstream of the reverse entrance rollers  90  in the sheet discharge reverse route RR 2 . The reverse rollers  91  are rotationally driven by a not-illustrated motor. 
     The intermediate rollers  92  convey the sheets P, switched back by the reverse rollers  91 , to the upward conveyance rollers  87 . The intermediate rollers  92  are arranged along a downstream portion of the sheet discharge reverse route RR 2 . The intermediate rollers  92  are rotationally driven by a not-illustrated motor. 
     The large-capacity sheet discharge tray  73  is a tray on which the sheets P subjected to printing and discharged by the sheet discharge conveyor  71  are stacked. The large-capacity sheet discharge tray  73  is arranged below and downstream of the sheet discharge rollers  88 . The large-capacity sheet discharge tray  73  is configured to be capable of being lifted and lowered such that the larger the number of stacked sheets P is, the more the large-capacity sheet discharge tray  73  is lowered. Many sheets P can be stacked on the large-capacity sheet discharge tray  73 . 
     Moreover, the large-capacity sheet discharge tray  73  is formed of a belt conveyor and is capable of sending out the sheets P, stacked on the large-capacity sheet discharge tray  73 , to the conveyor  5 . The large-capacity sheet discharge tray  73  includes an annular belt  96  on which the sheets P are mounted, a drive roller  97  which endlessly moves the belt  96 , and a following roller  98  which rotates by following the belt  96 . The belt  96  is wound around the drive roller  97  and the following roller  98 . A not-illustrated motor rotationally drives the drive roller  97  to endlessly move the belt  96  and the sheets P stacked on an upper surface of the belt  96  are thereby moved. 
     The lifting-lowering motor  74  lifts and lowers the large-capacity sheet discharge tray  73 . 
     The full sensor  75  detects that the large-capacity sheet discharge tray  73  is full of the sheets P. Specifically, the full sensor  75  detects the large-capacity sheet discharge tray  73  at a lower limit position which is a height position in the case where the maximum stackable number of sheets P are stacked on the large-capacity sheet discharge tray  73 . 
     The paired side fences  76 A,  76 B restrict the positions of the sheets P discharged onto the large-capacity sheet discharge tray  73  in a width direction (front-rear direction). The side fences  76 A,  76 B are provided in a state suspended from the fence driver  78  above the large-capacity sheet discharge tray  73 . The side fences  76 A,  76 B are arranged side by side at an interval in the front-rear direction. 
     The end fence  77  restricts the position of leading edges (right edges) of the sheets P discharged onto the large-capacity sheet discharge tray  73 . The end fence  77  is provided in a state suspended from the fence driver  78  above the large-capacity sheet discharge tray  73 . 
     The fence driver  78  moves the paired side fences  76 A,  76 B in directions toward and away from each other. Moreover, the fence driver  78  moves the end fence  77  in the left-right direction. The fence driver  78  is provided above the large-capacity sheet discharge tray  73 . 
     The top surface detection sensor  79  detects the top surface position (height position of the top surface of the top sheet P) of the sheets P stacked on the large-capacity sheet discharge tray  73 . The top surface detection sensor  79  is used to maintain the top surface position of the sheets P stacked on the large-capacity sheet discharge tray  73  around a specified height position in a sheet discharge operation. The specified height position is set in advance as a height position depending on a drop distance appropriate for orderly stacking the sheets P dropped from the sheet discharge rollers  88  onto the large-capacity sheet discharge tray  73 . The top surface detection sensor  79  includes a light emitter  79   a  and a light receiver  79   b.    
     The light emitter  79   a  and the light receiver  79   b  are arranged at an interval in the left-right direction with a region in which the sheets P are stacked on the large-capacity sheet discharge tray  73  located therebetween. The arrangement of the light emitter  79   a  in the height direction is such that an optical axis thereof is located at the specified height position and the light emitter  79   a  emits light rightward toward the light receiver  79   b . The light receiver  79   b  receives light from the light emitter  79   a  when there are no sheets P between the light receiver  79   b  and the light emitter  79   a  and does not receive the light from the light emitter  79   a  when the light is blocked by the sheets P. Specifically, the top surface detection sensor  79  detects the sheets P by being set to a state where the light receiver  79   b  does not receive the light from the light emitter  79   a . In this description, the state where the top surface detection sensor  79  detects the sheets P is referred to as on and the state where the top surface detection sensor  79  does not detect the sheets P is referred to as off. 
     The conveyor  5  unloads the sheets P stacked on the large-capacity sheet discharge tray  73  of the large-capacity sheet discharger  4  from the large-capacity sheet discharge tray  73  and holds the sheets P. The conveyor  5  is arranged near and on the right side of the large-capacity sheet discharger  4 . The conveyor  5  is arranged at substantially the same height as the large-capacity sheet discharge tray  73  at the lower limit position. 
     The conveyor  5  is formed of a belt conveyor. The conveyor  5  includes an annular belt  101  on which the sheets P are stacked, a drive roller  102  which endlessly moves the belt  101 , and a following roller  103  which rotates by following the belt  101 . The belt  101  is wound around the drive roller  102  and the following roller  103 . A not-illustrated motor rotationally drives the drive roller  102  to endlessly move the belt  101  and the belt  101  can thereby receive and unload the sheets P sent from the large-capacity sheet discharge tray  73 . The conveyor  5  has a length capable of holding multiple piles of sheets P. 
     The controller  6  controls operations of the units of the printing apparatus  1 . The controller  6  includes a CPU, a RAM, a ROM, a hard disk drive, and the like. 
     In a print operation in which the sheets P are fed from the external sheet feeder  2  and discharged to the large-capacity sheet discharger  4 , the controller  6  determines whether to perform switching of the external sheet feed tray  11  being the sheet feed source, based on a sheet remaining amount Rp in the external sheet feed tray  11  which is the latest sheet feed source and a sheet remaining amount Rq in another external sheet feed tray  11  which is the other one of the external sheet feed trays  11  in unloading of the sheets P from the large-capacity sheet discharger  4 . When determining to perform the switching of the sheet feed source, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     Specifically, if the sheet remaining amount Rp in the external sheet feed tray  11  which is the latest sheet feed source is a supply reference value L or less and the sheet remaining amount Rq in another external sheet feed tray  11  is a sheet feed reference value A or more in unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  determines to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     In this case, the supply reference value L is a value set in advance as a value of the sheet remaining amount at which the sheets P needs to be supplied. Moreover, the sheet feed reference value A is a value set in advance as a value of the sheet remaining amount sufficient for the sheet feeding. 
     Next, operations of the printing apparatus  1  are described. 
     Since the printing apparatus  1  of the first embodiment is characterized in the control of the external sheet feeder  2  in the case where the sheets P are fed from the external sheet feeder  2  and discharged to the large-capacity sheet discharger  4 , description is given of a print operation of executing a print job in which the sheets P are fed from the external sheet feeder  2  and discharged to the large-capacity sheet discharger  4 . In this print operation, it is assumed that the sheets P of the same type and the same size are stacked on the external sheet feed trays  11 A,  11 B of the external sheet feeder  2 . 
     When the print operation is started, the sheets P not subjected to printing are conveyed one by one from one of the external sheet feed trays  11 A,  11 B of the external sheet feeder  2  to the printer  32 . In the printer  32 , the sheets P are conveyed to the belt platen portion  47  by the registration rollers  46 . The sheets P are subjected to printing by the inkjet head portion  48  while being conveyed by the belt platen portion  47 . 
     In the case of simplex printing, the sheets P subjected to printing on one sides are guided from the normal route RC to the large-capacity sheet discharge route RD 2  by the switching unit  66  of the communicator  36 . The sheets P guided to the large-capacity sheet discharge route RD 2  are conveyed by the communication rollers  67  and the upstream introduction rollers  86  and are guided along the large-capacity sheet discharge route RD 2  toward the downstream introduction rollers  86  by the switching unit  89  of the sheet discharge reverser  72 . Then, the sheets P are conveyed along the large-capacity sheet discharge route RD 2  by the introduction rollers  86 , the upward conveyance rollers  87 , and the sheet discharge rollers  88  and are discharged to the large-capacity sheet discharge tray  73 . 
     When the sheets P printed on one sides are to be discharged to the large-capacity sheet discharge tray  73  with the print sides facing downward, the sheets P guided to the large-capacity sheet discharge route RD 2  by the switching unit  66  are guided to the sheet discharge reverse route RR 2  by the switching unit  89  of the sheet discharge reverser  72  while being conveyed by the communication rollers  67  and the upstream introduction rollers  86 . The sheets P guided to the sheet discharge reverse route RR 2  are conveyed to the reverse rollers  91  by the reverse entrance rollers  90  and are switched back by the reverse rollers  91 . The switched-back sheets P are conveyed by the intermediate rollers  92  and are returned from the sheet discharge reverse route RR 2  to the large-capacity sheet discharge route RD 2 . The sheets P are thereby turned upside down. Then, the sheets P are conveyed along the large-capacity sheet discharge route RD 2  by the upward conveyance rollers  87  and the sheet discharge rollers  88  and are discharged to the large-capacity sheet discharge tray  73 . 
     In the case of duplex printing, the sheets P subjected to printing on the front sides by the printer  32  are guided to the upper conveyor  33  by the switching unit  66  of the communicator  36  to be conveyed by the upper conveyor  33  and are then guided to the sheet reverse refeed route RR 1  by the switching unit  56  of the main body side sheet discharger  34 . In this description, a side subjected to the printing first in the duplex printing is referred to as front side and a side subjected printing later is referred to as back side. The sheets P guided to the sheet reverse refeed route RR 1  are guided to the reverse rollers  61  by the switching gate  63  and are switched back by the reverse rollers  61  in the reverser  35 . Then, the sheets P are guided to the sheet refeed rollers  62  by the switching gate  63  and are conveyed to the printer  32  by the sheet refeed rollers  62 . In the printer  32 , the sheets P are conveyed to the belt platen portion  47  by the registration rollers  46 . In this case, the sheets P are turned upside down (backside front) by the switch back and the back sides not subjected to printing face the inkjet head portion  48 . The sheets P are subjected to printing on the back sides with the ink ejected from the inkjet head portion  48  while being conveyed by the belt platen portion  47 . 
     As in the case of the simplex printing described above, the sheets P subjected to printing on both sides are conveyed by the communication rollers  67 , the introduction rollers  86 , the upward conveyance rollers  87 , and the sheet discharge rollers  88  to be discharged to the large-capacity sheet discharge tray  73 . 
     When the sheets P subjected to printing on both sides are to be discharged to the large-capacity sheet discharge tray  73  with the back sides facing downward, as in the aforementioned case of discharging the sheets P subjected to the simplex printing with the print sides facing downward, the sheets P are turned upside down in the sheet discharge reverser  72  and are discharged to the large-capacity sheet discharge tray  73 . 
     In the aforementioned print operation, the controller performs control of lowering the large-capacity sheet discharge tray  73  such that the top surface position of the sheets P stacked on the large-capacity sheet discharge tray  73  is maintained at the specified height position. 
     Specifically, when the top surface detection sensor  79  turns on, the controller  6  lowers the large-capacity sheet discharge tray  73  until the top surface detection sensor  79  turns off. 
     Moreover, in the aforementioned print operation, the controller  6  performs control of lifting the external sheet feed tray  11  being the sheet feed source such that the top surface position of the sheets P on the external sheet feed tray  11  is maintained at the sheet feed position. Specifically, when the upper limit sensor  14  for the external sheet feed tray  11  being the sheet feed source turns off due to feeding of the sheets P, the controller  6  lifts the external sheet feed tray  11  until the upper limit sensor  14  turns ON. 
     Moreover, when the external sheet feed tray  11  being the sheet feed source runs out of the sheets P in the aforementioned print operation, the controller  6  switches the sheet feed source to the other external sheet feed tray  11 . This processing of switching the external sheet feed tray  11  is described with reference to the flowchart of  FIG. 4 . The processing of the flowchart of  FIG. 4  starts with the start of the aforementioned print operation. 
     In step S 1  of  FIG. 4 , the controller  6  determines whether there are no sheets P on the external sheet feed tray  11  being the current sheet feed source. When the sheet presence-absence sensor  15  for the external sheet feed tray  11  being the sheet feed source turns off, the controller  6  determines that there are no sheets P on the external sheet feed tray  11 . When the controller  6  determines that there are sheets P on the external sheet feed tray  11  being the sheet feed source (step S 1 : NO) the controller  6  repeats step S 1 . 
     When the controller  6  determines that there are no sheets P on the external sheet feed tray  11  being the sheet feed source (step S 1 : YES), in step S 2 , the controller  6  lowers the external sheet feed tray  11  being the sheet feed source to the lower limit position. The controller  6  can determine whether the external sheet feed tray  11  being the sheet feed source reaches the lower limit position based on the states of detection of the detection target plate  11   a  in the respective remaining amount sensors  26  in the remaining amount detector  16  for this external sheet feed tray  11 . 
     In this case, sheet feeding from the external sheet feeder  2  to the printer  32  is stopped due to running-out of the sheets P on the external sheet feed tray  11  being the sheet feed source. 
     Next, in step S 3 , the controller  6  determines whether there are sheets P on the other external sheet feed tray  11  based on the detection state of the sheet presence-absence sensor  15  for the other external sheet feed tray  11 . 
     When the controller  6  determines that there are sheets P on the other external sheet feed tray  11  (step S 3 , YES), in step S 4 , the controller  6  switches the sheet feed source to the other external sheet feed tray  11 . The sheet feeding is resumed from the external sheet feed tray  11  being the new sheet feed source and the print operation is continued. 
     Next, in step S 5 , the controller  6  notifies the user of the running-out of sheets P in the external sheet feed tray being the sheet feed source before the switching. 
     Specifically, the controller  6  turns on the notification lamp  37  and displays, on the display unit of the operation panel  38 , that the external sheet feed tray  11  being the sheet feed source before the switching has run out of the sheets P and the sheets P need to be supplied. The series of processes is thereby completed. 
     In step S 3 , when the controller  6  determines that there are no sheets P on the other external sheet feed tray  11  (step S 3 : NO), in step S 6 , the controller  6  stops the print operation. 
     Then, in step S 7 , the controller  6  lowers the other external sheet feed tray  11  to the lower limit position. 
     Next, in step S 8 , the controller  6  notifies the user of the running-out of sheets P in both of the external sheet feed trays  11 A,  11 B. Specifically, the controller  6  turns on the notification lamp  37  and displays, on the display unit of the operation panel  38 , that both of the external sheet feed trays  11 A,  11 B have run out of the sheets P. The series of processes is thereby completed. 
     The user sets the sheets P in the external sheet feed tray  11  having run out of sheets. An operation of the external sheet feed tray  11  in the case where the sheets P are set in the external sheet feed tray  11  having run out of sheets is described with reference to the flowchart of  FIG. 5 . 
     The processing of the flowchart of  FIG. 5  starts when the sheet presence-absence sensor  15  for one of the external sheet feed trays  11 A,  11 B which is a processing target turns off. Here, the case where the external sheet feed tray  11 A is the processing target is described. However, the processing of the flowchart of  FIG. 5  is performed for each of the external sheet feed trays  11 A,  11 B. 
     In step S 11  of  FIG. 5 , the controller  6  determines whether the sheet presence-absence sensor  15 A for the external sheet feed tray  11 A turns on from the off state. The sheet presence-absence sensor  15 A turns on from the off state when the sheets P are set in the external sheet feed tray  11 A having run out of sheets. When the controller  6  determines that the sheet presence-absence sensor  15 A does not turn on (step S 11 : NO), the controller  6  repeats step S 11 . 
     When the controller  6  determines that the sheet presence-absence sensor  15 A turns on (step S 11 : YES), in step S 12 , the controller  6  determines whether the sheet set switch  17 A turns on. In this case, the sheet set switch  17 A turns on when the user sets the sheets P in the external sheet feed tray  11 A and performs operation of turning on the sheet set switch  17 A. 
     When the controller  6  determines that the sheet set switch  17 A turns on (step S 12 : YES), in step S 13 , the controller  6  lifts the external sheet feed tray  11 A to the upper limit position. Specifically, the controller  6  lifts the external sheet feed tray  11 A and stops the lifting of the external sheet feed tray  11 A when the upper limit sensor turns on. A state where the sheet feeding from the external sheet feed tray  11 A is possible is thereby achieved and the series of processes is completed. 
     When the controller  6  determines that the sheet set switch  17 A is not on in step S 12  (step S 12 : NO), in step S 14 , the controller  6  determines whether the sheet feeding from the external sheet feed tray  11 A is to be started. In this case, for example, when the sheet feeding is to be resumed with the external sheet feed tray  11 A set as the sheet feed source after the switching in the aforementioned switching processing of the external sheet feed tray  11 , the controller  6  determines to start the sheet feeding from the external sheet feed tray  11 A. When the controller  6  determines not to start the sheet feeding from the external sheet feed tray  11 A (step S 14 : NO), the controller  6  returns to step S 12 . 
     When the controller  6  determines to start the sheet feeding from the external sheet feed tray  11 A (step S 14 : YES), the controller  6  proceeds to step S 13  and lifts the external sheet feed tray  11 A to the upper limit position. A state where the sheet feeding from the external sheet feed tray  11 A is possible is thereby achieved and the series of processes is completed. 
     Next, an unloading operation in which the conveyor  5  unloads the sheets P from the large-capacity sheet discharger  4  is described with reference to the flowchart of  FIG. 6 . The processing of the flowchart in  FIG. 6  starts with the start of the aforementioned print operation. 
     In step S 21  of  FIG. 6 , the controller  6  determines whether the large-capacity sheet discharger  4  is full. In this case, the controller  6  determines that the large-capacity sheet discharger  4  is full when the full sensor  75  detects the large-capacity sheet discharge tray  73 . 
     When the controller  6  determines that the large-capacity sheet discharger  4  is not full (step S 21 : NO), in step S 22 , the controller  6  determines whether the print job is completed. When the controller  6  determines that the print job is not completed (step S 22 : NO), the controller  6  returns to step S 21 . 
     When the controller  6  determines that the large-capacity sheet discharger  4  is full (step S 21 : YES) or that the print job is completed (step S 22 : YES), in step S 23 , the controller  6  starts the unloading operation by the conveyor  5 . 
     Specifically, first, the controller  6  lowers the large-capacity sheet discharge tray  73  to the lower limit position when the large-capacity sheet discharger  4  is not full. Then, the controller  6  starts to rotate the drive roller  102  of the conveyor  5  clockwise in  FIG. 3  with the large-capacity sheet discharge tray  73  located at the lower limit position. The belt  101  of the conveyor  5  thereby starts to rotate clockwise in  FIG. 3 . Along with this rotation, the controller  6  starts to rotate the drive roller  97  of the large-capacity sheet discharge tray  73  clockwise in  FIG. 3 . The belt  96  of the large-capacity sheet discharge tray  73  thereby starts to rotate clockwise in  FIG. 3 . 
     The sheets P on the large-capacity sheet discharge tray  73  thereby start to move to the right. When right end portions of the sheets P on the large-capacity sheet discharge tray  73  reach the conveyor  5 , the sheets P are unloaded from the large-capacity sheet discharge tray  73  by the conveyor  5  while being sent out by the large-capacity sheet discharge tray  73 . 
     When the large-capacity sheet discharger  4  becomes full or when the large-capacity sheet discharger  4  is not full but the print job is completed, the unloading operation by the conveyor  5  is started by the aforementioned processing in steps S 21  to S 23 . When the unloading operation by the conveyor  5  is to be started, the controller  6  temporarily stops the sheet feeding to the printer  32  by the external sheet feeder  2 . In other words, the controller  6  temporarily stops the print operation. 
     When the unloading operation by the conveyor  5  is started, in step S 24 , the controller  6  turns on an unloading operation execution flag. The unloading operation execution flag indicates whether the conveyor  5  is executing the unloading operation. The unloading operation execution flag is turned on when the conveyor  5  is performing the unloading operation. 
     Next, in step S 25 , the controller  6  determines whether the unloading operation by the conveyor  5  is completed. In this case, the controller  6  can determine the time (timing) at which the unloading from the large-capacity sheet discharge tray  73  to the conveyor  5  is completed based on the size of the sheets P discharged to the large-capacity sheet discharge tray  73 . When the controller  6  determines that the unloading operation by the conveyor  5  is not completed (step S 25 : NO), the controller  6  repeats step S 25 . 
     When the controller  6  determines that the unloading operation by the conveyor  5  is completed (step S 25 : YES), in step S 26 , the controller  6  turns off the unloading operation execution flag. 
     Next, in step S 27 , the controller  6  resumes the print operation. The print operation (sheet feeding operation) is thereby continued with suspension due to the unloading operation by the conveyor  5  in the middle. However, when the unloading operation by the conveyor  5  is executed due to completion of the print job, the controller  6  does not resume the print operation unless the next print job is inputted. 
     The sheets P unloaded from the large-capacity sheet discharge tray  73  and stacked on the conveyor  5  by the unloading operation of the conveyor  5  are removed by the user. 
     Next, control of the external sheet feeder  2  performed in execution of the unloading operation by the conveyor  5  is described with reference to the flowchart of  FIG. 7 . 
     In step S 31  of  FIG. 7 , the controller  6  determines whether the unloading operation execution flag is turned on. When the controller  6  determines that the unloading operation execution flag is not turned on (step S 31 : NO), the controller  6  repeats step S 31 . 
     When the controller  6  determines that the unloading operation execution flag is turned on (step S 31 : YES), in step S 32 , the controller  6  determines whether the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source is the supply reference value L or less. The external sheet feed tray  11  being the latest sheet feed source is one of the external sheet feed trays  11 A,  11 B which is used as the sheet feed source right up to the moment when the sheet feeding to the printer  32  by the external sheet feeder  2  is temporarily stopped due to the start of the unloading operation by the conveyor  5 . 
     In this case, the remaining amount detector  16  is a detector which does not detect the exact number of sheets P remaining on the external sheet feed tray  11  but is a detector which can detect a certain range of the sheet remaining amount from the states of detection of the detection target plate  11   a  in the respective remaining amount sensors  26 . Accordingly, the controller  6  determines the range of the sheet remaining amount including the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source, based on the states of detection of the detection target plate  11   a  in the respective remaining amount sensors  26  in the remaining amount detector  16  for the external sheet feed tray  11  being the latest sheet feed source. When the upper limit value of the range of the sheet remaining amount including the sheet remaining amount Rp is the supply reference value L or less, the controller  6  determines that Rp≤L. 
     When the controller  6  determines that Rp≤L (step S 32 : YES), in step S 33 , the controller  6  determines whether the sheet remaining amount Rq in the other external sheet feed tray  11  is equal or more than the sheet feed reference value A. Specifically, the controller  6  determines the range of the sheet remaining amount including the sheet remaining amount Rq in the other external sheet feed tray  11 , based on the states of detection of the detection target plate  11   a  in the respective remaining amount sensors  26  of the remaining amount detector  16  for the other external sheet feed tray  11 . When the lower limit value of the range of the sheet remaining amount including the sheet remaining amount Rq is the sheet feed reference value A or more, the controller  6  determines that Rq≥A. 
     When the controller  6  determines that Rq≥A (step S 33 : YES), in step S 34 , the controller  6  lowers the external sheet feed tray  11  being the latest sheet feed source to the lower limit position. 
     Next, in step S 35 , the controller  6  notifies the user that it is time to supply the sheets P to the external sheet feed tray  11 . Specifically, the controller  6  turns on the notification lamp  37  and displays, on the display unit of the operation panel  38 , a message prompting the user to supply the sheets P to one of the external sheet feed trays  11 A,  11 B which is the latest sheet feed source (which is lowered to the lower limit position). Moreover, in this case, the controller  6  displays, on the display unit, a message instructing the user to remove the sheets P unloaded from the large-capacity sheet discharge tray  73  and stacked on the conveyor  5 , from the conveyor  5 . 
     Next, in step S 36 , the controller  6  switches the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . When the unloading operation by the conveyor  5  is completed and then the print operation is resumed, the sheets P are fed from the external sheet feed tray  11  being the new sheet feed source. 
     When the controller  6  determines that Rp&gt;L in step S 32  (step S 32 : NO) or when the controller  6  determines that Rq&lt;A in step S 33  (step S 33 : NO), the controller  6  terminates the series of control processes. 
     As described above, in the first embodiment, in the execution of the unloading operation by the conveyor  5 , that is in the unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  determines whether to perform the switching of the external sheet feed tray  11  being the sheet feed source, based on the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source and the sheet remaining amount Rq in the other external sheet feed tray  11 . When determining to perform the switching of the sheet feed source, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     This allows the user to perform work of removing the discharged sheets P and supply the sheets P to the external sheet feed tray  11  with a small sheet remaining amount without stopping the print operation to supply the sheets P to the external sheet feed tray  11 . As a result, it is possible to reduce occurrence of a situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P. Accordingly, a decrease in printed sheet productivity can be reduced. 
     Specifically, if the sheet remaining amount Rp in the external sheet feed tray  11  which is the latest sheet feed source is the supply reference value L or less and the sheet remaining amount Rq in the other external sheet feed tray  11  is the sheet feed reference value A or more in unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  determines to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . As a result, it is possible to properly determine whether to perform the switching of the sheet feed source for supply of sheets depending on the sheet remaining amounts of the respective external sheet feed trays  11 . Accordingly, a frequency of supply of sheets can be decreased. 
     Moreover, when the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 , the controller  6  lowers the external sheet feed tray  11  being the sheet feed source before the switching to the lower limit position. This facilitates the work of supplying the sheets P to the external sheet feed tray  11  being the sheet feed source before the switching. 
     Furthermore, when the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 , the controller  6  controls the notification lamp  37  such that the notification lamp  37  notifies the user that it is time to supply the sheets P to the external sheet feed tray  11 . This can reduce occurrence of the case where the user forgets supplying the sheets P to the external sheet feed tray  11 . As a result, it is possible to further reduce occurrence of the situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P. Accordingly, the decrease in printed sheet productivity can be further reduced. 
     Moreover, since the printing apparatus  1  includes the conveyor  5 , the unloading of the sheets P from large-capacity sheet discharger  4  can be automatically performed. Accordingly, the print operation can be continuously performed even when the large-capacity sheet discharger  4  becomes full. 
     Next, a second embodiment is described in which the control of the external sheet feeder  2  performed in the execution of the unloading operation by the conveyor  5  is changed from that in the first embodiment. 
       FIG. 8  is a flowchart of the control of the external sheet feeder  2  performed in the execution of the unloading operation by the conveyor  5  in the second embodiment. 
     Processing of step S 41  in  FIG. 8  is the same as the processing of step S 31  in  FIG. 7  described in the first embodiment. 
     When the controller  6  determines that the unloading operation execution flag is turned on in step S 41  (step S 41 : YES), in step S 42 , the controller  6  lowers one of the external sheet feed trays  11 A,  11 B with the smaller sheet remaining amount to the lower limit position. Note that, when the sheet remaining amounts of the external sheet feed trays  11 A,  11 B are included in the same range among ranges of the sheet remaining amounts detectable by the remaining amount detectors  16 , the controller  6  may assume either of the external sheet feed trays  11  as the external sheet feed tray  11  with the smaller sheet remaining amount and lower this external sheet feed tray  11 . 
     Next, in step S 43 , the controller  6  notifies the user that it is time to supply the sheets P to the external sheet feed tray  11 . Specifically, the controller  6  turns on the notification lamp  37  and displays, on the display unit of the operation panel  38 , a message prompting the user to supply the sheets P to one of the external sheet feed trays  11 A,  11 B lowered to the lower limit position. Moreover, in this case, the controller  6  displays, on the display unit, a message instructing the user to remove the sheets P unloaded from the large-capacity sheet discharge tray  73  and stacked on the conveyor  5 , from the conveyor  5 . 
     Next, in step S 44 , the controller  6  determines whether the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount. In other words, the controller  6  determines whether the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  lowered to the lower limit position in step S 42 . 
     When the controller  6  determines that the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount (step S 44 : YES), in step S 45 , the controller  6  switches the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . When the unloading operation by the conveyor  5  is completed and then the print operation is resumed, the sheets P are fed from the external sheet feed tray  11  being the new sheet feed source. 
     When the controller  6  determines that the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the larger sheet remaining amount (step S 44 : NO), the controller  6  terminates the series of control processes. 
     As described above, in the second embodiment, in the execution of the unloading operation by the conveyor  5 , that is in the unloading of the sheets P from the large-capacity sheet discharger  4 , if the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount, the controller  6  determines to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     The controller  6  determines whether the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount and, based on the determination, determines whether to perform the switching of the sheet feed source. That is, the controller  6  determines whether to perform the switching of the sheet feed source based on the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source and the sheet remaining amount Rq in the other external sheet feed tray  11 . When determining to perform the switching, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     This allows the user to perform work of removing the discharged sheets P and supply the sheets P to the external sheet feed tray  11  without stopping the print operation to supply the sheets P to the external sheet feed tray  11  in the second embodiment, as in the first embodiment. As a result, it is possible to reduce the occurrence of the situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P. Accordingly, the decrease in printed sheet productivity can be further reduced. 
     The controller  6  determines to perform the switching of the sheet feed source when the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount in the unloading of the sheets P from the large-capacity sheet discharger  4 . This allows the user to supply the sheets P to the external sheet feed tray  11  with the smaller sheet remaining amount at every performance of work of removing the discharged sheets P. Accordingly, a frequency of occurrence of running-out of sheets P can be decreased. 
     Moreover, in the unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  lowers the external sheet feed tray  11  with the smaller sheet remaining amount to the lower limit position. This facilitates the work of supplying the sheets P to the external sheet feed tray  11  with the smaller sheet remaining amount. 
     Furthermore, in the unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  controls the notification lamp  37  such that the notification lamp  37  notifies the user that it is time to supply the sheets P to the external sheet feed tray  11 . This can reduce occurrence of the case where the user forgets supplying the sheets P to the external sheet feed tray  11 . As a result, it is possible to further reduce occurrence of the situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P. Accordingly, the decrease in printed sheet productivity can be further reduced. 
     Next, a third embodiment is described in which part of the printing apparatus of the first embodiment is changed. In  FIG. 10 , the directions of right, left, up, and down are denoted by RT, LT, UP, and DN, respectively. 
       FIG. 9  is a block diagram illustrating a configuration of a printing apparatus according to the third embodiment.  FIG. 10  is a schematic configuration diagram of a large-capacity sheet discharger in the printing apparatus illustrated in  FIG. 9 . 
     As illustrated in  FIGS. 9 and 10 , the printing apparatus LA according to the third embodiment has a configuration in which the large-capacity sheet discharger  4  in the printing apparatus  1  of the first embodiment is replaced by a large-capacity sheet discharger  4 A and the conveyor  5  is omitted. In the printing apparatus LA, the user performs work of unloading the sheets P from the large-capacity sheet discharger  4 A. 
     The large-capacity sheet discharger  4 A includes a sheet discharge conveyor  71 A, a sheet discharge reverser  72 , stackers  111 A,  111 B, a sheet discharge conveyance side case  80  housing part of the sheet discharge conveyor  71 A and the like, stacker cases  112 A,  112 B housing the stackers  111 A,  111 B, respectively, and an intermediate case  113  housing a portion of the sheet discharge conveyor  71 A between the stackers  111 A,  111 B. 
     The sheet discharge conveyor  71 A has a configuration in which a switching unit  116  and multiple pairs of downstream sheet discharge conveyance rollers  117  are added to the sheet discharge conveyor  71  of the first embodiment. 
     The switching unit  116  switches the conveyance route of the sheets P between a downstream portion of the large-capacity sheet discharge route RD 2  and a large-capacity downstream sheet discharge route RD 3 . The large-capacity downstream sheet discharge route RD 3  is a route branching from the large-capacity sheet discharge route RD 2  at a position between the pair of upward conveyance rollers  87  and the upstream pair of the sheet discharge rollers  88  and extending toward the downstream stacker  111 B. The switching unit  116  is arranged at the point where the large-capacity downstream sheet discharge route RD 3  branches from the large-capacity sheet discharge route RD 2 . 
     The downstream sheet discharge conveyance rollers  117  conveys and discharges the sheets P, guided to the large-capacity downstream sheet discharge route RD 3  by the switching unit  116 , to the downstream stacker  111 B. The downstream sheet discharge conveyance rollers  117  are arranged between the switching unit  116  and the stacker  111 B along the large-capacity downstream sheet discharge route RD 3 . The downstream sheet discharge conveyance rollers  117  are rotationally driven by a not-illustrated motor. 
     The sheet discharge reverser  72  of the large-capacity sheet discharger  4 A is the same as the sheet discharge reverser  72  of the large-capacity sheet discharger  4  in the first embodiment. 
     The stackers  111  are portions where the sheets P subjected printing and discharged by the sheet discharge conveyor  71 A are stacked and held. The sheets P discharged by the sheet discharge rollers  88  are stacked on the upstream stacker  111 A. The sheets P discharged by the downstream sheet discharge conveyance rollers  117  are stacked on the downstream stacker  111 B. Each stacker  111  includes a sheet discharge tray  121 , a lifting-lowering motor  122 , a full sensor  123 , one pair of side fences  124 A,  124 B, an end fence  125 , a fence driver  126 , a top surface detection sensor  127 , a sheet presence-absence sensor  128 , an unloading button  129 , an open-close sensor  130 , and a wheeled platform  131 . 
     The sheet discharge tray  121  is a tray on which the sheets P subjected to printing and discharged by the sheet discharge conveyor  71 A are stacked. The sheet discharge tray  121  of the upstream stacker  111 A is arranged below and downstream of the sheet discharge rollers  88 . The sheet discharge tray  121  of the downstream stacker  111 B is arranged below and downstream of the most downstream pair of downstream sheet discharge conveyance rollers  117 . Each sheet discharge tray  121  is configured to be capable of being lifted and lowered such that the larger the number of the stacked sheets P is, the more the sheet discharge tray  121  is lowered. Many sheets P can be stacked on the sheet discharge tray  121 . The sheet discharge tray  121  of the stacker  111 A and the sheet discharge tray  121  of the stacker  111 B are selectively used as the sheet discharge destination of the sheets P subjected to printing in the printer  32 . 
     The lifting-lowering motor  122  lowers and lifts the sheet discharge tray  121 . 
     The full sensor  123  detects that the sheet discharge tray  121  is full of the sheets P. Specifically, the full sensor  123  detects the sheet discharge tray  121  at a lower limit position which is a height position in the case where the maximum stackable number of sheets P are stacked on the sheet discharge tray  121 . 
     The side fences  124 A,  124 B, the end fence  125 , and the fence driver  126  are the same as the side fences  76 A,  76 B, the end fence  77 , and the fence driver  78  of the large-capacity sheet discharger  4  in the first embodiment, respectively. 
     The top surface detection sensor  127  is the same as the top surface detection sensor  79  of the large-capacity sheet discharger  4  in the first embodiment and includes a light emitter  127   a  and a light receiver  127   b  which are the same as the light emitter  79   a  and the light receiver  79   b  of the top surface detection sensor  79 . 
     The sheet presence-absence sensor  128  detects presence or absence of the sheets P on the sheet discharge tray  121 . In this case, a state where the sheet presence-absence sensor  128  detects that there are sheets P on the sheet discharge tray  121  is referred to as on and a state where the sheet presence-absence sensor  128  does not detect that there are sheets P on the sheet discharge tray  121  is referred to as off. 
     The unloading button  129  is a button to be pressed when the user performs work of unloading the sheets P from the stacker  111 . When the unloading button  129  is pressed, the sheet discharge tray  121  is placed on the wheeled platform  131 . 
     The open-close sensor  130  detects whether a door (not illustrated) in the stacker case  112  is opened or closed. The door in the stacker case  112  is opened when the user unloads the sheets P from the stacker  111 . 
     The wheeled platform  131  is used by the user to pull out the sheet discharge tray  121  on which the sheets P are stacked from the stacker case  112  in work of unloading the sheets P from the stacker  111 . 
     Next, description is given of a print operation of the printing apparatus  1 A in the case of executing a print job in which the sheets P are fed from the external sheet feeder  2  and discharged to the large-capacity sheet discharger  4 A. In this print operation, it is assumed that the sheets P of the same type and the same size are stacked on the external sheet feed trays  11 A,  11 B of the external sheet feeder  2 . 
     The sheet feeding operation of the external sheet feeder  2  and the print operation of the printing apparatus main body  3  are the same as the corresponding operations in the printing apparatus  1  described in the first embodiment. 
     The sheets P subjected to the printing are discharged to the sheet discharge tray  121  in one of the stackers  111 A,  111 B in the large-capacity sheet discharger  4 A. 
     When the sheet discharge destination is the sheet discharge tray  121  in the stacker  111 A, the sheets P are guided from the upward conveyance rollers  87  to the sheet discharge rollers  88  by the switching unit  116  and discharged to the sheet discharge tray  121  in the stacker  111 A by the sheet discharge rollers  88 . 
     When the sheet discharge destination is the sheet discharge tray  121  in the stacker  111 B, the sheets P are guided from the upward conveyance rollers  87  to the downstream sheet discharge conveyance rollers  117  by the switching unit  116 . Then, the sheets P are conveyed along the large-capacity downstream sheet discharge route RD 3  by the downstream sheet discharge conveyance rollers  117  and discharged to the sheet discharge tray  121  in the stacker  111 B. 
     In the aforementioned print operation, the controller  6  performs control of lowering the sheet discharge tray  121  such that the top surface position of the sheets P stacked on the sheet discharge tray  121  of the stacker  111  being the sheet discharge destination is maintained at the specified height position. Specifically, when the top surface detection sensor  127  of the stacker  111  being the sheet discharge destination turns on, the controller  6  lowers the sheet discharge tray  121  until the top surface detection sensor  127  turns off. 
     Moreover, in the aforementioned print operation, the controller  6  performs control of lifting the external sheet feed tray  11  such that the top surface position of the sheets P on the external sheet feed tray  11  being the sheet feed source is maintained at the sheet feed position as in the first embodiment. 
     Processing of switching the external sheet feed tray  11  in the case were the external sheet feed tray  11  being the sheet feed source runs out of sheets P is the same as that in the first embodiment and is performed in the steps in the flowchart of  FIG. 4 . 
     Moreover, an operation of the external sheet feed tray  11  in the case where the sheets P are set in the external sheet feed tray  11  having run out of sheets is the same as that in the first embodiment and is performed in the steps in the flowchart of  FIG. 5 . 
     Next, an operation in the case where the sheet discharge destination of the sheets P is changed during the aforementioned print operation is described with reference to the flowchart of  FIG. 11 . The processing of the flowchart of  FIG. 11  starts with the start of the aforementioned print operation. 
     In step S 51  of  FIG. 11 , the controller  6  determines whether the sheet discharge tray  121  being the current sheet discharge destination is full. When the full sensor  123  for the sheet discharge tray  121  being the current sheet discharge destination detects the sheet discharge tray  121  in this case, the controller  6  determines that this sheet discharge tray  121  is full. 
     When the controller  6  determines that the sheet discharge tray  121  being the sheet discharge destination is not full (step S 51 : NO), in step S 52 , the controller  6  determines whether the print job is completed. When the controller  6  determines that the print job is not completed (step S 52 : NO), the controller  6  returns to step S 51 . 
     When the controller  6  determines that the sheet discharge tray  121  being the sheet discharge destination is full (step S 51 : YES) or that the print job is completed (step S 52 : YES), in step S 53 , the controller  6  determines whether the sheet discharging to the other sheet discharge tray  121  is possible. 
     In this case, when there are no sheets P on the other sheet discharge tray  121 , the controller  6  determines that the sheet discharging to the other sheet discharge tray  121  is possible. Note that, even if there are sheets P on the other sheet discharge tray  121 , the controller  6  may determine that the sheet discharging is possible as long as the other sheet discharge tray  121  is not full. 
     When the controller  6  determines that the sheet discharging to the other sheet discharge tray  121  is possible (step S 53 : YES), in step S 54 , the controller  6  controls the switching unit  116  to switch the sheet discharge tray  121  being the sheet discharge destination. 
     Next, in step S 55 , the controller  6  notifies the user that unloading of the sheets P from the large-capacity sheet discharger  4 A is possible. Specifically, the controller  6  displays, on the display unit of the operation panel  38 , a message indicating that the unloading of the sheets P from the large-capacity sheet discharger  4 A is possible. The series of processes is thereby completed. Note that the configuration may be such that a lamp configured to notify the user that the unloading of the sheets P from the large-capacity sheet discharger  4 A is possible is installed in each stacker case  112  and this lamp is turned on to notify the user that the unloading of the sheets P is possible. 
     When the controller  6  determines that the sheet discharging to the other sheet discharge tray  121  is not possible (step S 53 : NO), the controller  6  terminates the series of processes. In this case, since the sheet discharging cannot be performed, the print operation is stopped. 
     When the user is notified that the unloading of the sheets P from the large-capacity sheet discharger  4 A is possible, the user performs work of unloading the sheets P from the large-capacity sheet discharger  4 A. An operation of the printing apparatus  1 A in this unloading of the sheets P is described with reference to the flowchart of  FIG. 12 . The processing of the flowchart of  FIG. 12  starts in response to the switching of the sheet discharge destination. 
     In step S 61  of  FIG. 12 , the controller  6  determines whether the unloading button  129  of the stacker  111  being the sheet discharge destination before the switching (stacker  111  not being the current sheet discharge destination) is pressed or not. When the controller  6  determines the unloading button  129  is not pressed (step S 61 : NO), the controller  6  repeats step S 61 . 
     When the controller  6  determines that the unloading button  129  is pressed (step S 61 : YES), in step S 62 , the controller  6  lowers the sheet discharge tray  121  of the stacker  111  not being the current sheet discharge destination to place the sheet discharge tray  121  on the wheeled platform  131 . Lowering the sheet discharge tray  121  beyond the lower limit position in this case allows the sheet discharge tray  121  to be placed on the wheeled platform  131 . 
     Next, in step S 63 , the controller  6  determines whether the door of the stacker case  112  of the stacker  111  not being the current sheet discharge destination is opened based on an output signal of the open-close sensor  130  of this stacker  111 . When the controller  6  determines that the door is not opened (step S 63 : NO), the controller  6  repeats step S 63 . 
     When the controller  6  determines that the door is opened (step S 63 : YES), in step S 64 , the controller  6  determines whether the opened door is closed based on the output signal of the open-close sensor  130 . When the controller  6  determines that the door is not closed (step S 64 : NO), the controller  6  repeats step S 64 . 
     When the controller  6  determines that the door is closed (step S 64 : YES), in step S 65 , the controller  6  determines whether there are sheets P on the sheet discharge tray  121  of the stacker  111  not being the current sheet discharge destination. When the sheet presence-absence sensor  128  of the stacker  111  not being the current sheet discharge destination is on, the controller  6  determines that there are sheets P on the sheet discharge tray  121  of this stacker  111 . 
     In the work of unloading the sheets P from the stacker  111 , the user presses the unloading button  129  and then opens the door of the stacker case  112 . Next, the user pulls out the wheeled platform  131  on which the sheet discharge tray  121  is placed, from the stacker case  112  and removes the sheets P on the sheet discharge tray  121 . Then, the user returns the wheeled platform  131  on which the sheet discharge tray  121  without sheets P is placed, into the stacker case  112  and closes the door. 
     Accordingly, when there are no sheets P on the sheet discharge tray  121  in the stacker case  112  after the opening and closing of the door of the stacker case  112 , the controller  6  can determine that the user has performed the work of unloading the sheets P and removed the sheets P on the sheet discharge tray  121 . Meanwhile, when there are sheets P on the sheet discharge tray  121  in the stacker case  112  after the opening and closing of the door of the stacker case  112 , the controller  6  can determine that the user has returned the wheeled platform  131  into the stacker case  112  and closed the door with the sheets P left on the sheet discharge tray  121 . 
     Note that, in the printing apparatus LA, since the sheet discharging to the other stacker  111  is possible also during the work of unloading the sheets P from the stacker  111 , the print operation can be continued. 
     When the controller  6  determines that there are sheets P on the sheet discharge tray  121  not being the current sheet discharge destination in step S 65  (step S 65 : YES), the controller  6  returns to step S 61 . 
     When the controller  6  determines that there are no sheets P on the sheet discharge tray  121  not being the current sheet discharge destination (step S 65 : NO), in step S 66 , the controller  6  lifts this sheet discharge tray  121  to the upper limit position. Specifically, the controller  6  lifts the sheet discharge tray  121  not being the current sheet discharge destination and stops the lifting of the sheet discharge tray  121  when the top surface detection sensor  127  for this sheet discharge tray  121  turns on. 
     Next, in step S 67 , the controller  6  turns on an unloading execution flag. The series of processes is thereby completed. The unloading execution flag indicates whether the user has executed the work of unloading the sheets P from the large-capacity sheet discharger  4 A. The unloading execution flag is turned on when the user executes the work of unloading the sheets P from the large-capacity sheet discharger  4 A. 
     Next, control of the external sheet feeder  2  in the case where the work of unloading the sheets P from the large-capacity sheet discharger  4 A is performed is described with reference to the flowchart of  FIG. 13 . 
     In step S 71  of  FIG. 13 , the controller  6  determines whether the unloading execution flag turns on. When the controller  6  determines that the unloading execution flag is not on (step S 71 : NO), the controller  6  repeats step S 71 . 
     When the controller  6  determines that the unloading execution flag is turned on (step S 71 : YES), the controller  6  executes processing of step S 72 . The processing of step S 72  is the same as the processing of step S 32  in  FIG. 7  described in the first embodiment. In this case, in the printing apparatus  1 A, the external sheet feed tray  11  being the current sheet feed source corresponds to the external sheet feed tray  11  being the latest sheet feed source. 
     When the controller  6  determines that Rp≤L in step S 72  (step S 72 : YES), the controller  6  executes processing of step S 73 . The processing of step S 73  is the same as the processing of step S 33  in  FIG. 7  described in the first embodiment. 
     When the controller  6  determines that Rq≥A in step S 73  (step S 73 : YES), in step S 74 , the controller  6  displays, on the display unit of the operation panel  38 , a sheet supply execution confirmation screen (not illustrated). 
     The sheet supply execution confirmation screen is a screen confirming with the user about whether the sheets P are to be supplied to the external sheet feeder  2 . One of the external sheet feed trays  11 A,  11 B to which the sheets P are to be supplied is displayed in the sheet supply execution confirmation screen. The user performs an operation on the operation panel  38  according to an instruction displayed on the sheet supply execution confirmation screen to select whether to execute the supplying of the sheets P to the external sheet feeder  2 . 
     Next, in step S 75 , the controller  6  determines whether execution of the supplying of the sheets P to the external sheet feeder  2  is selected. 
     When the controller  6  determines that the execution of the supplying of the sheets P is selected (step S 75 : YES), in step S 76 , the controller  6  switches the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source (current sheet feed source) to the other external sheet feed tray  11 . 
     Next, in step S 77 , the controller  6  lowers the external sheet feed tray  11  being the sheet feed source before the switching to the lower limit position. The user supplies the sheets P to the external sheet feed tray  11  lowered to the lower limit position. 
     Then, in step S 78 , the controller  6  displays, on the display unit of the operation panel  38 , a sheet supply completion confirmation screen (not illustrated). 
     The sheet supply completion confirmation screen is a screen to which the user having completed the supplying of the sheets P inputs the completion of the supplying so that the printing apparatus  1 A can confirm about the supplying of the sheets P to the external sheet feed tray  11 . The user performs an input operation for the confirmation about the completion of the supplying of the sheets P on the operation panel  38  according to an instruction displayed on the sheet supply completion confirmation screen. 
     Next, in step S 79 , the controller  6  determines whether the input operation for the confirmation about the completion of the supplying of the sheets P is performed. When the controller  6  determines that the input operation for the confirmation is not performed (step S 79 : NO), the controller  6  repeats step S 79 . 
     When the controller  6  determines that the input operation for the confirmation is performed (step S 79 : YES), in step S 80 , the controller  6  lifts the external sheet feed tray  11  to which the sheets P are supplied to the upper limit position. 
     Then, in step S 81 , the controller  6  turns off the unloading execution flag. The series of processes is thereby completed. 
     When the controller  6  determines that: Rp&gt;L in step S 72  (step S 72 : NO); Rq&lt;A in step S 73  (step S 73 : NO); or non-execution of the supplying of the sheets P is selected in step S 75  (step S 75 : NO), the controller  6  proceeds to step S 81  and turns off the unloading execution flag. The series of processes is thereby completed. 
     As described above, in the third embodiment, in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A, that is in the unloading of the sheets P from the large-capacity sheet discharger  4 A, the controller  6  determines whether to perform the switching of the external sheet feed tray  11  being the sheet feed source, based on the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source (current sheet feed source) and the sheet remaining amount Rq in the other external sheet feed tray  11 . When determining to perform the switching of the sheet feed source, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11  if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2 . 
     This allows the user to perform work of removing the discharged sheets P and supply the sheets P to the external sheet feed tray  11  with a small sheet remaining amount without stopping the print operation to supply the sheets P to the external sheet feed tray  11  as in the first embodiment. As a result, it is possible to reduce the occurrence of the situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P, as in the first embodiment. Accordingly, a decrease in printed sheet productivity can be reduced. 
     Specifically, if the sheet remaining amount Rp in the external sheet feed tray  11  which is the latest sheet feed source (current sheet feed source) is the supply reference value L or less and the sheet remaining amount Rq in the other external sheet feed tray  11  is the sheet feed reference value A or more in unloading of the sheets P from the large-capacity sheet discharger  4 A, the controller  6  determines to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 , as in the first embodiment. As a result, it is possible to properly determine whether to perform the switching of the sheet feed source for supply of sheets depending on the sheet remaining amounts of the respective external sheet feed trays  11 , as in the first embodiment. Accordingly, a frequency of supply of sheets can be decreased. 
     Moreover, when the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 , the controller  6  lowers the external sheet feed tray  11  being the sheet feed source before the switching to the lower limit position, as in the first embodiment. This facilitates the work of supplying the sheets P to the external sheet feed tray  11  being the sheet feed source before the switching as in the first embodiment. 
     Furthermore, since the printing apparatus  1 A includes two sheet discharge trays  121  and the two sheet discharge trays  121  are selectively used as the sheet discharge destination, the print operation can be continuously performed. 
     Next, a fourth embodiment is described in which the control of the external sheet feeder  2  performed in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A is changed from that in the third embodiment. 
       FIG. 14  is a flowchart of the control of the external sheet feeder  2  performed in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A in the fourth embodiment. 
     Processing of step S 91  in  FIG. 14  is the same as the processing of step S 71  in  FIG. 13  described in the third embodiment. 
     In step S 91 , when the controller  6  determines that the unloading execution flag is turned on in step S 91  (step S 91 : YES), the controller  6  executes processing of steps S 92 , S 93 . The processing of steps S 92 , S 93  are the same as the processing of steps S 74 , S 75  in  FIG. 13  described in the third embodiment. 
     In step S 93 , when the controller  6  determines that the execution of the supplying of the sheets P is selected (step S 93 : YES), in step S 94 , the controller  6  determines whether the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount. In this case, as described above, in the printing apparatus LA, the external sheet feed tray  11  being the current sheet feed source corresponds to the external sheet feed tray  11  being the latest sheet feed source. 
     When the controller  6  determines that the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount (step S 94 : YES), in step S 95 , the controller  6  switches the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source (current sheet feed source) to the other external sheet feed tray  11 . 
     Next, in step S 96 , the controller  6  lowers one of the external sheet feed trays  11 A,  11 B with the smaller sheet remaining amount to the lower limit position. The user supplies the sheets P to the external sheet feed tray  11  lowered to the lower limit position. 
     When the controller  6  determines that the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the larger sheet remaining amount in step S 94  (step S 94 : NO), the controller  6  skips step S 95  and executes processing of step S 96 . 
     After step S 96 , the controller  6  executes processing of steps S 97  to S 100 . The series of processes is thereby completed. The processing of steps S 97  to S 100  is the same as the processing of steps S 78  to S 81  in  FIG. 13  described in the third embodiment. 
     When the controller  6  determines that non-execution of the supplying of the sheets P is selected in step S 93  (step S 93 : NO), the controller  6  proceeds to step S 100  and turns off the unloading execution flag. The series of processes is thereby completed. 
     As described above, in the fourth embodiment, in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A, that is in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2  and the external sheet feed tray  11  being the latest sheet feed source (current sheet feed source) is the external sheet feed tray  11  with the smaller sheet remaining amount, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     Also in the fourth embodiment as in the second embodiment, the controller  6  determines whether the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount and, based on the determination, determines whether to perform the switching of the sheet feed source. That is, the controller  6  determines whether to perform the switching of the sheet feed source based on the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source and the sheet remaining amount Rq in the other external sheet feed tray  11 . When determining to perform the switching, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 . 
     This allows the user to perform work of removing the discharged sheets P and supply the sheets P to the external sheet feed tray  11  without stopping the print operation to supply the sheets P to the external sheet feed tray  11  in the fourth embodiment, as in the first embodiment. As a result, it is possible to reduce the occurrence of the situation where the print operation is stopped due to the running-out of sheets P in the external sheet feed tray  11  and the printing apparatus  1  waits to be supplied with the sheets P. Accordingly, the decrease in printed sheet productivity can be reduced. 
     As described above, the controller  6  determines to perform the switching of the sheet feed source when the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2  in the unloading of the sheets P from the large-capacity sheet discharger  4 A. This allows the user to supply the sheets P to the external sheet feed tray  11  with the smaller sheet remaining amount at every performance of work of removing the discharged sheets P in case where the user selects the execution of the supplying of the sheets P to the external sheet feeder  2 . Accordingly, a frequency of occurrence of running-out of sheets P can be decreased. 
     Moreover, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2 , the controller  6  lowers the external sheet feed tray  11  with the smaller sheet remaining amount to the lower limit position. This facilitates the work of supplying the sheets P to the external sheet feed tray  11  with the smaller sheet remaining amount as in the second embodiment. 
     Although the present invention is described above by using the first to fourth embodiments, the present invention is not limited by these embodiments. 
     In the first and third embodiment, the supply reference value L is a value set in advance (fixed value). However, assume a case where, in the unloading of the sheets P from the large-capacity sheet discharger  4 ,  4 A, the controller  6  knows the number of sheets to be subjected to printing before the next unloading of the sheets P from the large-capacity sheet discharger  4 ,  4 A. In this case, this number of sheets to be subjected to printing may be used as the supply reference value L. 
     For example, assume a case where, in the first embodiment, the unloading operation is executed by the conveyor  5  in response to the completion of the print job. In this case, when the next print job is inputted into the controller  6 , the number of sheets to be subjected to printing in the next print job or the capacity of the large-capacity sheet discharger  4  is the number of sheets to be subjected to printing before the next unloading of the sheets P. Since the controller  6  knows this number of sheets to be subjected to printing from the print job, the number of sheets to be subjected to printing may be used as the supply reference value L. 
     Moreover, for example, assume a case where, in the third embodiment, the sheet discharge destination is switched in response to the completion of the print job and the user unloads the sheets P on the sheet discharge tray  121  being the sheet discharge destination before the switching. In this case, when the next print job is inputted into the controller  6 , the number of sheets to be subjected to printing in the next print job or the capacity of the stacker  111  is the number of sheets to be subjected to printing before the next unloading of the sheets P. Since the controller  6  can know this number of sheets to be subjected to printing from the print job, the number of sheets to be subjected to printing may be used as the supply reference value L. 
     Furthermore, for example, assume a case where, in the first embodiment, the conveyor  5  executes the unloading operation in response to the large-capacity sheet discharger  4  becoming full. In this case, when the next unloading operation is to be performed in response to the completion of the currently-executed print job, the controller  6  can know the remaining number of sheets to be subjected to printing before the next unloading operation, from the print job. Accordingly, the controller  6  may use this number of sheets to be subjected to printing as the supply reference value L. 
     Moreover, for example, assume a case where, in the third embodiment, the sheet discharge destination is switched in response to the sheet discharge tray  121  being the current sheet discharge destination becoming full and the user unloads the sheets P on the sheet discharge tray  121  being the sheet discharge destination before the switching. In this case, when the next switching of the sheet discharge destination and the next unloading of the sheets P are to be performed in response to the completion of the currently-executed print job, the controller  6  can know the remaining number of sheets to be subjected to printing before the next unloading of the sheets P, from the print job. Accordingly, the controller  6  may use this number of sheets to be subjected to printing as the supply reference value L. 
     Using the number of sheets to be subjected to printing before the next unloading of the sheets P from the large-capacity sheet discharger  4 ,  4 A as the supply reference value L as described above can reduce the unnecessary lowering operation of the external sheet feed tray  11  and switching of the sheet feed source. 
     Moreover, although the configuration in which the external sheet feeder  2  has two external sheet feed trays  11  is described in the first to fourth embodiments, the external sheet feeder  2  may have three or more external sheet feed trays  11 . 
     In this case, the controller  6  determines whether to perform the switching of the external sheet feed tray  11  being the sheet feed source, based on the sheet remaining amount in the external sheet feed tray  11  being the latest sheet feed source and the sheet remaining amounts in the other external sheet feed trays  11  in unloading of the sheets P from the large-capacity sheet discharger  4 ,  4 A. When determining to perform the switching of the sheet feed source, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to any one of the other external sheet feed trays  11 . 
     Specifically, in the first embodiment, in a case where the external sheet feeder  2  has a configuration including three or more external sheet feed trays  11 , in the execution of the unloading operation by the conveyor  5 , that is in the unloading of the sheets P from the large-capacity sheet discharger  4 , if the sheet remaining amount in the external sheet feed tray  11  being the latest sheet feed source is the supply reference value L or less and there is an external sheet feed tray  11  with a sheet remaining amount equal to or more than the sheet feed reference value A among the other external sheet feed trays  11 , the controller  6  determines to perform the switching of the sheet feed source and lowers the external sheet feed tray  11  being the latest sheet feed source to the lower limit position. Then, the controller  6  controls the external sheet feeder  2  such that the sheet feed source after the completion of the unloading operation is switched from the external sheet feed tray  11  being the latest sheet feed source to the external sheet feed tray  11  with the sheet remaining amount equal to or more than the sheet feed reference value A. 
     Moreover, assume a case where, in the second embodiment, the external sheet feeder  2  has a configuration including three or more external sheet feed trays  11 . In this case, in the execution of the unloading operation by the conveyor  5 , that is in the unloading of the sheets P from the large-capacity sheet discharger  4 , the controller  6  lowers the external sheet feed tray  11  with the smallest sheet remaining amount to the lower limit position. Then, if the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smallest sheet remaining amount, the controller  6  controls the external sheet feeder  2  such that the sheet feed source after the completion of the unloading operation is switched from the external sheet feed tray  11  being the latest sheet feed source to another external sheet feed tray  11 . 
     Furthermore, in the third embodiment, in a case where the external sheet feeder  2  has a configuration including three or more external sheet feed trays  11 , in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A, that is in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the sheet remaining amount in the external sheet feed tray  11  being the latest sheet feed source is the supply reference value L or less and there is an external sheet feed tray  11  with a sheet remaining amount equal to or more than the sheet feed reference value A among the other external sheet feed trays  11 , the controller  6  determines to perform the switching of the sheet feed source. And then, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the external sheet feed tray  11  with the sheet remaining amount equal to or more than the sheet feed reference value A if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2 . Moreover, the controller  6  lowers the external sheet feed tray  11  being the sheet feed source before the switching to the lower limit position. 
     Moreover, assume a case where, in the fourth embodiment, the external sheet feeder  2  has a configuration including three or more external sheet feed trays  11 . In this case, in the execution of the work of unloading the sheets P from the large-capacity sheet discharger  4 A, that is in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the user selects the execution of the supplying of the sheets P to the external sheet feeder  2  and the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smallest sheet remaining amount, the controller  6  controls the external sheet feeder  2  such that the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to another external sheet feed tray  11 . Moreover, the controller  6  lowers the external sheet feed tray  11  with the smallest sheet remaining amount to the lower limit position irrespective of whether the sheet feed source is switched or not. 
     Furthermore, in the third and fourth embodiments, operations of displaying the sheet supply execution confirmation screen on the operation panel  38  and making the user select whether to execute the supplying of the sheets P can be omitted. 
     Specifically, in the third embodiment, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the sheet remaining amount Rp in the external sheet feed tray  11  being the latest sheet feed source is the supply reference value L or less and the sheet remaining amount Rq in the other external sheet feed tray  11  is the sheet feed reference value A or more, the controller  6  may switch the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11  without condition. Moreover, in this case, as in the first embodiment, when the sheet feed source is switched from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11 , the notification lamp  37  may notify the user that it is time to supply the sheets P to the external sheet feed tray  11 . 
     Moreover, assume the case where, in the third embodiment, the external sheet feeder  2  has the configuration including three or more external sheet feed trays  11 . In this case, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the sheet remaining amount in the external sheet feed tray  11  being the latest sheet feed source is the supply reference value L or less and there is an external sheet feed tray  11  with a sheet remaining amount equal to or more than the sheet feed reference value A among the other external sheet feed trays  11 , the controller  6  may switch the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the external sheet feed tray  11  with the sheet remaining amount equal to or more than the sheet feed reference value A without condition. 
     Furthermore, in the fourth embodiment, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smaller sheet remaining amount, the controller  6  may switch the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11  without condition. Moreover, in this case, as in the second embodiment, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, the notification lamp  37  may notify the user that it is time to supply the sheets P to the external sheet feed tray  11 . 
     Moreover, assume the case where, in the fourth embodiment, the external sheet feeder  2  has the configuration including three or more external sheet feed trays  11 . In this case, in the unloading of the sheets P from the large-capacity sheet discharger  4 A, if the external sheet feed tray  11  being the latest sheet feed source is the external sheet feed tray  11  with the smallest sheet remaining amount, the controller  6  may switch the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11  without condition. 
     Furthermore, in the first and second embodiments, the sheet supply execution confirmation screen may be displayed on the operation panel  38  to make the user select whether to execute the supplying of the sheets P as in the third and fourth embodiments. 
     Furthermore, in the first embodiment, the controller  6  may determine to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the other external sheet feed tray  11  when the sheet remaining amount Rq in the other external sheet feed tray  11  is greater than the sheet remaining amount Rp in the external sheet feed tray  11  which is the latest sheet feed source regardless of how much the sheet remaining amount Rp is. The same applies to the third embodiments. As a result, it is still possible to properly determine whether to perform the switching of the sheet feed source for supply of sheets depending on the sheet remaining amounts of the respective external sheet feed trays  11 . Accordingly, a frequency of supply of sheets can be decreased. 
     Furthermore, in a case where the external sheet feeder  2  includes three or more external sheet feed trays  11  in the first embodiment, the controller  6  may determine to perform the switching of the sheet feed source from the external sheet feed tray  11  being the latest sheet feed source to the external sheet feed tray  11  having the sheet remaining amount greater than the sheet remaining amount of the external sheet feed tray  11  being the latest sheet feed source when the external sheet feed tray  11  having the sheet remaining amount greater than the sheet remaining amount of the external sheet feed tray  11  being the latest sheet feed source exists. The same applies to a case where the external sheet feeder  2  includes three or more external sheet feed trays  11  in the third embodiment. 
     Moreover, in the first to fourth embodiments, the external sheet feeder  2  may be configured such that supplying of the sheets P is possible without lowering the external sheet feed trays  11  and the operation of lowering the external sheet feed tray  11  to the lower limit position to supply the sheets P is omitted. 
     Furthermore, in third and fourth embodiments, there may be provided an unloader which unloads the sheets P from the stackers  111  of the large-capacity sheet discharger  4 A. For example, the configuration may be such that the sheet discharge trays  121  are formed of belt conveyors and each sheet discharge tray  121  is provided with an unloader formed of a belt conveyor configured to unload the sheets P from the sheet discharge tray  121 . 
     Moreover, although the configuration in which one printing apparatus main body  3  is provided between the external sheet feeder  2  and the large-capacity sheet discharger  4  is described in the first and second embodiments, a configuration in which multiple printing apparatus main bodies  3  are connected to one another may be employed. This also applies to the third and fourth embodiments. 
     The embodiments of the present disclosure have, for example, the following configurations. 
     A printing apparatus in accordance with some embodiments includes: a printer configured to perform printing on sheets; a sheet feeder including sheet feed trays and configured to feed the sheets from a selected one of the sheet feed trays to the printer; a sheet discharger configured to discharge and hold the sheets printed by the printer; and a controller. Upon unloading of the sheets from the sheet discharger, the controller is configured to: determine whether to perform switching of a sheet feed source from a first sheet feed tray which is a latest sheet feed source of the sheet feed trays to a second sheet feed tray which is a sheet feed tray other than the first sheet feed tray of the sheet feed trays, based on a first sheet remaining amount in the first sheet feed tray and a second sheet remaining amount in the second sheet feed tray; and upon determining to perform the switching, control the sheet feeder to switch the sheet feed source from the first sheet feed tray to the second sheet feed tray. 
     The controller may be configured to determine to perform the switching upon the first sheet remaining amount being a supply reference value or less and the second sheet remaining amount being a sheet feed reference value or more. 
     Upon the unloading of the sheets from the sheet discharger and upon the controller knowing a number of sheets to be subjected to printing prior to next unloading of the sheets from the sheet discharger, the controller may be configured to use the number of sheets to be subjected to printing as the supply reference value. 
     The controller may be configured to determine to perform the switching when the first sheet remaining amount is smaller than the second sheet remaining amount. 
     Each of the sheet feed trays may be capable of being lifted and lowered, and the controller may be configured to lower the first sheet feed tray to a sheet supply position upon performing the switching. 
     The printing apparatus may further include a notifier configured to notify a user of a timing to supply sheets to the sheet feed trays. Upon the unloading of the sheets from the sheet discharger and upon performing the switching, the controller may be configured to control the notifier to notify the user that it is a timing to supply the sheets to the first sheet feed tray. 
     The controller may be configured to determine to perform the switching upon the first sheet remaining amount of the first sheet feed tray being smallest among sheet remaining amounts of the sheet feed trays. 
     Each of the sheet feed trays may be capable of being lifted and lowered, and the controller may be configured to lower a sheet feed tray of the sheet feed trays with a smallest sheet remaining amount to a sheet supply position upon the unloading of the sheets from the sheet discharger. 
     The printing apparatus may further include a notifier configured to notify a user of a timing to supply sheets to the sheet feeder. Upon the unloading of the sheets from the sheet discharger, the controller may be configured to control the notifier to notify the user that it is the timing to supply the sheets to the sheet feeder. 
     The sheet discharger may include sheet discharge trays on which the discharged sheets are stacked and is configured to selectively use one of the sheet discharge trays as a sheet discharge destination. 
     The printing apparatus may further include an unloader configured to unload the sheets from the sheet discharger. 
     Embodiments of the present invention have been described above. However, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     Moreover, the effects described in the embodiments of the present invention are only a list of optimum effects achieved by the present invention. Hence, the effects of the present invention are not limited to those described in the embodiment of the present invention.