Patent Publication Number: US-6986511-B2

Title: Finisher for an image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. application Ser. No. 10/175,777 filed Jun. 21, 2002, now U.S. Pat. No. 6,631,896, which is a divisional of U.S. Ser. No. 09/923,947 filed Aug. 8, 2001, now U.S. Pat. No. 6,416,052, which is a divisional application of U.S. Ser. No. 09/777,893 filed Feb. 7, 2001, now U.S. Pat. No. 6,322,070, which is a divisional application of U.S. Ser. No. 09/332,442 filed Jun. 14, 1999, now U.S. Pat. No. 6,231,045, and claims priority to Japanese Application Nos. 10-165219 filed Jun. 12, 1998, 10-170154 filed Jun. 17, 1998, 10-221688 filed Aug. 5, 1998, 10-224356 filed Aug. 7, 1998, 10-245792 filed Aug. 31, 1998, 10-248492 filed Sep. 2, 1998, 10-252274 filed Sep. 7, 1998, 10-252805 filed Sep. 7, 1998, 10-325033 filed Nov. 16, 1998 and 11-077995 filed Mar. 23, 1999. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a finisher for finishing papers driven out of an image forming apparatus. 
   A finisher for the above application is taught in, e.g., Japanese Patent Laid-Open Publication No. 8-26579. The finisher includes a single tray mounted on one side thereof. A staple mode and a shift mode are available with the finisher. In the staple mode, papers sequentially driven out of an image forming apparatus are stacked on a staple tray disposed in the finisher, stapled together, and then discharged to the tray. In the shift mode, papers are directly discharged to the above tray without being stapled. The tray may be constructed to be movable up and down in order to stack a great number of papers, as also proposed in the past. 
   The above finisher has a paper outlet where a drive roller and a driven roller are arranged in a pair. The driven roller is rotatably mounted on one end of a roller support member that is angularly movable about the other end. The driven roller is pressed against the drive roller due to its own weight. In the shift mode, the drive roller and driven roller are held in contact with each other for discharging the papers. In the staple mode, the roller support member is angularly moved to release the driven roller from the drive roller. 
   However, the problem with this type of finisher is that all the groups of papers or all the stacks of papers are loaded on the single tray and cannot be distinguished from each other. This is particularly true when a plurality of persons share the finisher. Moreover, an image forming apparatus with such a finisher is often used as a printer for a computer or an output device for a facsimile apparatus. As a result, copies and printings are apt to exist together on the tray. This makes the distinction between copies produced by different persons and between copies and printings extremely difficult. 
   In light of the above, the finisher may be provided with another paper outlet and another tray or proof tray in addition to the above tray. Even this kind of scheme has a problem that because the outlets and trays are provided in one-to-one correspondence, various functions including a sort mode and a staple mode available with the finisher are limited. Specifically, when the proof tray is selected, stapling or similar advanced function is not available. 
   Japanese Patent Laid-Open Publication Nos. 9-48567 and 9-48559, for example, each disclose a finisher including a plurality of trays arranged one above the other and capable of locating one of them at a paper outlet. This kind of finisher, however, has the following problems left unsolved. The trays selected and the trays not selected each are moved across the outlet while only the tray selected is located at the outlet. Therefore, to prevent papers stacked on any one of trays from returning into the outlet, a shutter or similar sophisticated device must be arranged in the outlet. 
   Moreover, the trays each have an end fence for positioning the trailing edges of papers stacked thereon. Because the end fence is implemented by the wall of the finisher where the outlet is formed, an outlet roller cannot overlap the wall. As a result, although the trailing edge of a paper may successfully move away from the outlet roller, the paper is apt to partly remain between the outlet roller and the wall of the finisher. The finisher therefore fails to surely discharge papers. 
   Japanese Patent Laid-Open Publication No. 9-110259, for example, proposes a finisher addressing the above problems. The finisher taught in this document includes an outlet roller disposed in a paper outlet formed in the wall of the finisher. The outlet roller is movable toward and away from the paper outlet. After the trailing edge of a paper has reached the above wall, the outlet roller is moved away from the wall so as to prevent the trailing edge of the paper from remaining between it and the wall. This, however, complicates the arrangement of the paper outlet. 
   The finisher of Laid-Open Publication No. 9-48559 mentioned earlier has another problem left unsolved. After a tray unit has been moved to locate a designated tray at the single paper outlet, papers are discharged to the tray. As a result, the operation for discharging the papers must be delayed by a period of time necessary for the particular tray to reach the paper outlet. 
   Japanese Patent Laid-Open Publication No. 7-228401, for example, proposes a finisher constructed to reduce the above period of time and adaptive to a high-speed image forming apparatus. This finisher includes two paper outlets and two trays associated one-to-one with the paper outlets. The trays are arranged one above the other and movable up and down independently of each other. When the upper tray is used as a mass paper tray, the lower tray is retracted downward as soon as the upper tray is lowered to a preselected position. However, the paper outlets each being associated with a particular tray are sophisticated. 
   Japanese Patent Laid-Open Publication No. 8-73107 teaches a sorter capable of moving a plurality of trays up and down at the same time and varying the distance between nearby trays. The sorter allows the number of papers to be stacked on each tray to be varied, as desired. Each tray is movable via a paper outlet and is returned to its home position when papers are removed therefrom. However, the problem with this sorter is that all the trays are connected together and limit the stroke available for mass paper discharge, i.e., a sufficient capacity is not available for mass paper discharge. 
   In the finisher of the type locating one of a plurality of trays at a paper outlet by driving it independently of the other trays, when an upper tray should be brought to the paper outlet, a lower tray must be retracted downward away from the paper outlet. Also, when the lower tray should be brought to the paper outlet, the upper tray must be retracted upward away from the paper outlet. When the lower tray is used as a mass paper tray, it should preferably be retracted away from the paper outlet as far as possible from the capacity standpoint. This, however, increases a distance that the lower tray should be brought to the paper outlet when selected later, slowing down the finishing operation. 
   Further, Japanese Patent Laid-Open Publication No. 8-119518 discloses a finisher including a plurality of trays arranged one above the other and at least one of which is movable up and down for mass paper discharge. When the movable tray is selected, it is moved from a stand-by position where papers have been removed to a paper outlet. That is, the finisher taught in the above document recognizes a position where papers have been removed as a stand-by position. In practice, however, the movable tray sometimes reaches its lower limit position in the event of mass paper discharge. It follows that a substantial period of time is necessary for the tray to move from the stand-by position (lower limit position) to the paper outlet. The lower limit position of the mass paper discharge tray is naturally close to the bottom of the finisher, so that the function of the tray can be made most of. This increases the period of time necessary for the tray to move from the lower limit position to the paper outlet and is therefore apt to lower the processing speed of the image forming apparatus. To solve this problem, the moving speed of the tray must be varied by sophisticated control, as needed. 
   When a paper jams the paper outlet in any one of the conventional finishers, the operator must put the operator&#39;s hand in the paper outlet and move outlet rollers provided in a pair away from each other, i.e., rotate a roller support member for removing the paper. At this instant, the tray moving upward via the paper outlet is apt to injure the operator and damage structural elements around the paper outlet. Although the shutter taught in Laid-Open Publication No. 9-48557 or 9-48559 may obviate such an accident, it sophisticates the configuration of the outlet and control. 
   SUMMARY OF THE INVENTION 
   It is therefore a first object of the present invention to provide a finisher capable of moving a lower tray to a paper outlet in a short period of time without resorting to any sophisticated control. 
   It is a second object of the present invention to provide a finisher highly productive and easy to use. 
   It is a third object of the present invention to provide a finisher capable of stacking a great number of papers without scaling up a drive source and moving a tray to a paper outlet in a short constant period of time without resorting to any sophisticated control. 
   It is a fourth object of the present invention to provide a finisher capable of preventing papers from returning from a tray to a paper outlet without complicating the configuration of the outlet, or promoting sure positioning of papers without complicating the configuration of the outlet, or reducing the period of time necessary for a tray to reach the outlet. 
   It is a fifth object of the present invention to provide a finisher capable of reducing a paper discharging time with a plurality of trays sharing a single paper outlet, and implementing mass paper discharge. 
   It is a sixth object of the present invention to provide a finisher capable of receiving, with a relatively simple construction, papers with a plurality of trays without causing the papers from returning from the trays to a paper outlet. 
   It is a seventh object of the present invention to provide a finisher capable of obviating accidents ascribable to the movement of a tray with a relatively simple construction. 
   It is an eighth object of the present invention to provide a finisher capable of preventing trays from colliding with each other, and reducing the distance of movement of a tray to a paper outlet to thereby adapt to a high-speed image forming operation. 
   In accordance with the present invention, a finisher for an image forming apparatus includes a paper outlet for discharging papers. A plurality of trays are capable of being selectively located at the paper outlet and include at least an upper tray and a lower tray movable up and down independently of each other. A controller selectively locates either one of the upper tray and lower tray at the paper outlet. The controller moves the lower tray to a retracted position when locating the upper tray at the paper outlet. A stand-by position sensor senses the stand-by position of the lower tray which is a home position defined between the paper outlet and the retracted position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a side elevation showing a first embodiment of the finisher in accordance with the present invention; 
       FIG. 2  is a block diagram schematically showing a control system included in the first embodiment; 
       FIG. 3  is a front view showing a mechanism included in the first embodiment for moving trays up and down; 
       FIG. 4  is a perspective view showing a mechanism included in the first embodiment for moving lower one of the trays; 
       FIG. 5  is a side elevation showing the construction and operation of a sensor responsive to the stand-by position of the lower tray; 
       FIG. 6  is a side elevation showing the mechanism for moving an upper tray located at a paper outlet; 
       FIG. 7  is a side elevation demonstrating the retraction of the upper tray; 
       FIG. 8  is a side elevation showing the mechanism for moving the lower tray located at the paper outlet; 
       FIG. 9  shows the upper tray and lower tray each being located at the respective home position; 
       FIGS. 10 and 11  are flowcharts representative of initialization which the first embodiment executes with a sensor responsive to the lower retracted position of the lower tray; 
       FIG. 12  is a side elevation showing a specific condition wherein papers are sequentially stacked on the lower tray; 
       FIG. 13  is a side elevation showing another specific condition wherein the lower tray is retracted while the upper tray is located at the paper outlet; 
       FIG. 14  is a side elevation showing still another specific condition wherein the lower tray has reached its full state; 
       FIG. 15  is a side elevation showing a further specific condition wherein the full lower tray is retracted while the upper tray is located at the paper outlet; 
       FIG. 16  is a flowchart showing a procedure which the first embodiment executes for determining a retracted position without using the sensor responsive to the lower retracted position; 
       FIGS. 17 and 18  are flowcharts showing initialization which the first embodiment executes without using the sensor responsive to the lower retracted position; 
       FIGS. 19 and 20  are flowcharts demonstrating initialization representative of a second embodiment of the present invention; 
       FIG. 21  is a flowchart showing a specific procedure that the second embodiment executes for determining whether or not the upper tray is usable; 
       FIGS. 22 and 23  are flowcharts showing another specific procedure which the second embodiment executes for determining whether or not papers have been removed; 
       FIGS. 24 and 25  are flowcharts showing still another specific procedure which the second embodiment executes for canceling inhibition relating to the upper tray; 
       FIGS. 26 and 27  are flowcharts showing initialization representative of a third embodiment of the present invention; 
       FIG. 28  is a side elevation showing the home positions of the trays for executing an alternative control procedure; 
       FIGS. 29 and 30  are flowcharts showing initialization associated with the arrangement of  FIG. 28 ; 
       FIG. 31  is a flowchart showing initialization representative of a fourth embodiment of the present invention; 
       FIG. 32  is a flowchart showing a specific control procedure that the fourth embodiment executes when the upper tray is selected; 
       FIG. 33  is a side elevation showing the retracted position of the lower tray particular to the fourth embodiment; 
       FIG. 34  is a flowchart showing another specific control procedure that the fourth embodiment executes when the upper tray is selected; 
       FIG. 35  is a flowchart showing another specific control procedure that the fourth embodiment executes when the lower tray is executed; 
       FIG. 36  is a flowchart showing another specific control procedure that the fourth embodiment executes when the lower tray is selected; 
       FIG. 37  is a flowchart showing another specific control procedure that the fourth embodiment executes when the lower tray is selected; 
       FIG. 38  is a flowchart showing another specific control procedure that the fourth embodiment executes when the lower tray is selected; 
       FIG. 39  is a side elevation showing a specific condition wherein the lower tray has reaches its full state; 
       FIG. 40  is a side elevation showing another specific condition wherein the full lower tray is further lowered while the upper tray is located at the paper outlet; 
       FIGS. 41 and 42  are flowcharts showing a specific procedure which a fifth embodiment of the present invention executes for moving the trays during finish processing; 
       FIGS. 43 and 44  are flowcharts showing another specific procedure which the fifth embodiment executes for moving each of the trays at a particular timing; 
       FIGS. 45 and 46  are flowcharts showing another specific procedure which the fifth embodiment executes for moving each of the trays at a particular timing; 
       FIGS. 47 and 48  are flowcharts showing another specific procedure which the fifth embodiment executes for moving each of the trays at a particular timing; 
       FIGS. 49 and 50  are flowcharts showing another specific procedure which the fifth embodiment executes for moving each of the trays at a particular timing; 
       FIG. 51  is a flowchart showing initialization representative of a sixth embodiment of the present invention; 
       FIG. 52  is a flowchart showing a specific procedure that the sixth embodiment executes when one of the trays is selected; 
       FIG. 53  is a flowchart showing another specific procedure that the sixth embodiment executes when the other tray is selected; 
       FIG. 54  is a side elevation showing a mechanism included in a seventh embodiment of the present invention for driving the lower tray up and down; 
       FIG. 55  is a plan view showing the mechanism of  FIG. 54 ; 
       FIG. 56  is an enlarged front view of an arrangement around a paper outlet included in the seventh embodiment; 
       FIG. 57  is an enlarged front view showing the arrangement of  FIG. 56  in a condition wherein a stack of papers is discharged via the outlet; 
       FIGS. 58 and 59  are schematic block diagrams each showing a particular condition of switching means included in the seventh embodiment; 
       FIG. 60  is an enlarged front view of the arrangement around the paper outlet in which a pair of outlet rollers are moved away from each other by an unexpected object; 
       FIG. 61  is a front view showing a mechanism included in an eighth embodiment of the present invention for moving the trays up and down in a particular condition; 
       FIGS. 62 and 63  are front views each showing the mechanism of  FIG. 61  in another particular condition; 
       FIGS. 64 and 65  are flowcharts showing a specific procedure which the eighth embodiment executes for locating the upper tray and lower tray at their home positions; 
       FIGS. 66 and 67  are flowcharts showing another specific procedure which the eighth embodiment executes for causing the lower tray to retract when the number of papers stacked thereon is small; 
       FIG. 68  is a front view showing the lower tray retracted to a stand-by position; 
       FIGS. 69 and 70  are flowcharts showing another specific procedure which the eighth embodiment executes for causing the lower tray to retract to the stand-by position when papers are removed therefrom; 
       FIG. 71  is a front view showing the lower tray of the eighth embodiment from which papers have been removed; and 
       FIG. 72  is a front view showing the lower tray retracted to its stand-by position after the removal of papers. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the finisher in accordance with the present invention will be described hereinafter. 
   First Embodiment 
   Referring to  FIG. 1  of the drawings, a finisher embodying the present invention and directed toward the first object stated earlier will be described. As shown, the finisher or paper stacking device, generally labeled F, receives a paper from a copier G at a transfer position J. The copier G belongs to a family of image forming apparatuses. An inlet sensor SN 1  and inlet rollers  4  are arranged around the transfer position J. A proof tray P is provided on the top of the finisher F. A paper received via the inlet rollers  4  is discharged to the proof tray P via an outlet E 1 , or discharged to an upper tray  1  or a lower tray  2  via an outlet E 2  without being stapled or after being stapled, depending on the operation mode. Mainly, the upper tray  1  is used to stack papers. The lower tray  1  is capable of stacking a great number of papers. 
   A path selector  21  is positioned downstream of the inlet rollers  4  in the direction of conveyance of papers and operated by a solenoid  21   a  (see FIG.  2 ). When the solenoid  21   a  is turned off, the path selector  21  is brought to a position indicated by a solid line in FIG.  1 . In this position, the path selector  21  steers a paper being conveyed by the inlet rollers  4  toward the outlet E 1 . At this instant, rollers  5   a  convey the paper toward the outlet E 1  while outlet rollers  7  discharge the paper to the proof tray P. An outlet sensor SN  2  is located between the rollers  5   a  and outlet rollers  7 , as illustrated. It is to be noted that the rollers  5   a  and outlet rollers  7 , as well as other rollers, each are implemented as a drive roller and a driven roller cooperating with each other. 
   When the solenoid  21   a  is turned on, it brings the path selector  21  to a position indicated by a dash-and-dots line in FIG.  1 . In this position, the path selector  21  steers the paper into a horizontal path. Another path selector  20  is positioned on the horizontal path downstream of the path selector  21  and operated by a solenoid  20   a  (see FIG.  2 ). When the solenoid  20   a  is turned on, it switches the path selector  21  to a position indicated by a dash-and-dots line in FIG.  1 . As a result, the path selector  21  steers the paper to a vertical staple route A. When the solenoid  20   a  is turned off, it switches the path selector  21  to a position indicated by a solid line in FIG.  1  and causes it to steer the paper to a non-staple route B. 
   Rollers  5   b  are arranged on the non-staple route B for conveying the paper introduced into the route B. An outlet roller or drive roller  8  cooperates with a driven roller  8   a  for discharging the paper to the upper tray  1  or the lower tray  2 . An outlet sensor SN 4  is positioned between the rollers  5   n  and the outlet roller  8 . The trays  1  and  2  each are driven by a respective drive source. A controller  100  selectively locates either one of the trays  1  and  2  at the outlet E 2 . 
   On the staple route A, rollers  6 C convey the paper to a staple unit  12 . Papers stapled by the staple unit  12  are discharged to the tray  1  or  2  by the outlet roller  8 . An outlet sensor SN 3  is located on the staple route A. 
   Assume that the operator of the copier G selects a staple mode. In the staple mode, papers sequentially guided into the staple route A are stacked on a staple tray disposed in the finisher F by a discharge roller  6 . A tap roller  9  positions every paper in the vertical direction (direction of conveyance) while a jogger fence  11  positions every paper in the horizontal direction (widthwise direction). The controller  100  sends a staple signal to a stapler S between consecutive jobs, i.e., during interval between the last paper of one stack and the first paper of the next stack. A paper stack stapled by the stapler S is immediately conveyed to the outlet roller  8  by a belt  10   a  having a catch  10  and driven out to the tray  1  or  2  located at the outlet E 2  by the roller  8 . 
   The tap roller  9  pivots about a fulcrum  9   a  by being driven by return solenoid  9   b  (see FIG.  2 ). Every time a paper is driven onto the staple tray, the tap roller  9  acts on the paper and causes it to abut against a rear fence  46 . At this instant, a brush roller  6   a  cooperative with the discharge roller  6  prevents the trailing edge of the paper from returning toward the staple path A. The tap roller  9  is rotatable in the counterclockwise direction. A home position sensor SN 8  is responsive to the home position of the catch  10 . 
   As shown in  FIG. 2 , the controller  100  is implemented by a microcomputer including a CPU (Central Processing Unit)  102  and an I/O (Input/Output) interface  104 . A control panel, not shown, is mounted on the top of the finisher body and includes various switches (SW). Signals output from the switches and various sensors are input to the CPU  102  via the I/O interface  104 . In response, the CPU  102  controls a motor  50  assigned to the upper tray  1 , a motor  51  assigned to the lower tray  2 , the solenoids  20   a  and  21   a,  the return solenoid  9   b,  a motor  52  assigned to the rollers  5   a,    5   b  and  5   c,  a motor  53  assigned to the outlet rollers  7  and  8 , motors  54  and  56  assigned to the stapler S, a motor  55  assigned to the belt  10   a,  a motor  57  assigned to the jogger fence  11 , etc. Pulse signals for driving the motor  52  assigned to the rollers  5   c  are input to the CPU  102  and counted thereby. The CPU  102  controls the return solenoid  9   b  in accordance with the number of the pulse signals. Also shown in  FIG. 2  are a DM motor DCM and a stepping motor STMP. 
   Sensors SN 5 , SN 6 , SN 9  and SN 7  are sequentially arranged on the outlet E 2  side of the finisher body from the upper portion to the lower portion. The sensor SN 5  is retracted position sensing means for sensing a position to which the upper tray  1  is retracted when the lower tray  2  should be brought to the outlet E 2 . The sensor SN 6  is discharge position sensing means responsive to the tray  1  or  2  brought to the outlet E 2 . The sensor SN 9  is stand-by position or home position sensing means responsive to the stand-by position or home position of the tray  2 . The sensor SN 7  is retracted position sensing means responsive to the tray  2  brought to its retracted position. The outputs of the sensors SN 5 , SN 6 , SN 9  and SN 7  are input to the CPU  102  via the I/O interface  104 . 
   A desired operation mode and a desired tray are input on an operation panel, not shown, mounted on the copier G or a computer, not shown, connected to the copier G. When the staple mode is input despite that the proof tray P is selected, the staple mode is automatically canceled with priority given to the proof tray P. 
   A mechanism for moving the trays  1  and  2  up and down will be described with reference to FIG.  3 . As shown, the upper tray  1  is mounted on a base  40  affixed to opposite side walls  39   a  and  39   b.  Guide rollers  44  are mounted on the side walls  39   a  and  39   b  via stubs not shown. The guide rollers  44  are rollable on and along the inner peripheries of guide rails  30   a  and  30   b  each having a generally U-shaped section. The guide rollers  44  are positioned by the assembly of the side walls  39   a  and  39   b  and base  40  and prevented from slipping out of the guide rails  30   a  and  30   b  thereby. Two timing belts  37  each are passed over a pair of timing pulleys  36 . The motor  50  drives the timing belts  37  via a drive shaft  33   a  and a driven shaft  33   b  on which the timing pulleys  36  are mounted. The side walls  39   a  and  39   b  each are partly affixed to the adjoining timing belt  37 . In this configuration, the unit including the upper tray  1  is movable up and down. 
   The lower tray  2 , like the upper tray  1 , is mounted on a base  43  affixed to opposite side walls  42   a  and  42   b.  Guide rollers  44  are mounted on the side walls  42   a  and  42   b  via stubs not shown. The guide rollers  44  are rollable on end along the inner peripheries of the guide rails  30   a  and  30   b.  The guide rollers  44  are positioned by the assembly of the side walls  42   a  and  42   b  and base  43  and prevented from slipping out of the guide rails  30   a  and  30   b  thereby. Two timing belts  35  each are passed over a pair of timing pulleys  34 . The motor  51  drives the timing belts  35  via a drive shaft  41   a  and a driven shaft  41   b  on which the timing pulleys  34  are mounted. The side walls  42   a  and  42   b  each are partly affixed to the adjoining timing belt  35 . In this configuration, the unit including the lower tray  2  is movable up and down. 
     FIG. 4  shows a mechanism for driving the lower tray  2 . As shown, the rotation of the motor  51  is transferred via a worm gear  58  to the last gear of a gear train mounted on the drive shaft  41   a.  The worm gear  58  allows the tray  2  to be held at a preselected position. The upper tray  1  is driven by a similar mechanism. The sensor SN 7  mentioned earlier is located between the opposite runs of the timing belt  35  and turned on and off by a part of the side wall  42   a  or  42   b  affixed to one run of the timing belt  35  located at the discharge side. This is also true with the sensor SN 5 . The driven roller  8   a  is not shown in FIG.  4 . 
   The sensor SN 9  is positioned around the center of a paper discharged and operable on a surface which the rear edge of the paper contacts, i.e., on a side wall or rear fence  32 . More specifically, as shown in  FIG. 5 , the sensor SN 9  implemented by a microswitch includes a portion  62  affixed to a stationary member  60  forming a part of the finisher body, and a movable piece  64  rotatably supported by the portion  62  at its one end. The movable piece  64  partly protrudes from the side wall  32  in the paper discharge direction and is actuated by the rear end of the tray  2  or the top of a paper stack. The other sensors SN 5 , SN 6  and SN 7  each have the same configuration as the sensor SN 9 . All the sensors may be implemented by either one of refection type sensors or transmission type sensors. 
   As shown in  FIG. 1 , in the illustrative embodiment, the outlet roller  8  protrudes from the side wall  32  in order to prevent the side wall  32  from catching a paper being discharged via the outlet E 2 . The outlet roller  8 , however, interferes with the upper tray  1  when the tray  1  is retracted upward. As shown in  FIG. 6 , to obviate such interference, the guide rails  30   a  and  30   b  each include a bent portion  31 .  FIG. 6  shows a condition wherein the upper tray  1  is located at the outlet E 2  while the lower tray  2  is retracted. As shown in  FIG. 7 , as the guide rollers  44  are displaced, the tray  1  is angularly moved and prevented from interfering with the outlet roller  8 . The distance L 1  between the guide rollers  44  of the tray  1  are greater than the length L of the bent portion  31 . 
   The angular movement of the tray  1  causes the tension acting on the timing belt  37  to vary. In light of this, as shown in  FIG. 6 , the lower timing pulley  36  is affixed to a movable bracket  68  to which a spring  66  is anchored.  FIG. 8  shows a condition wherein the lower tray  2  is located at the outlet E 2  while the upper tray  1  is retracted upward. 
   How the controller  100  controls the upper tray  1  and lower tray  2  will be described hereinafter.  FIG. 9  shows home positions at which the trays  1  and  2  are located on the power-up of the copier G. As shown, the sensor SN 5  senses the upper end of an end fence  1   a  included in the tray  1  when the tray  1  is located at its home position. The sensor SN 9  senses the lower end of the tray  2  when the tray  2  is located at its home position. 
   Reference will be made to  FIGS. 10 and 11  for describing the initialization of the trays  1  and  2 , i.e., a procedure for locating them at the home positions. As shown, on the power-up of the copier G, initialization begins (step S 1 ). Specifically, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 2 ). If the answer of the step S 2  is positive (YES), the controller  100  raises the tray  2  (step S 3 ) and then determines whether or not the sensor SN 9  is in an ON state (step S 4 ). If the answer of the step S 4  is YES, the controller  100  stops the elevation of the tray  2  (step S 5 ). As a result, the tray  2  is caused to stop at its home position. To move the trays  1  and  2  up and down, the controller  100  drives the motors  50  and  51 . 
   Subsequently, the controller  100  determines whether or not the sensor SN 5  is in an ON state (step S 6 ). If the answer of the step S 6  is YES, meaning that the tray  1  is located at its home position, the controller  100  ends the initialization. If the answer of the step S 6  is negative (NO), the controller raises the tray  1  (step S 7 ) and determines whether or not the sensor SN 5  is in an ON state (step S 8 ). The controller  100  stops the movement of the tray  1  as soon as the sensor SN 5  senses the upper end of the end fence  1   a  (step S 9 ). 
   If the answer of the step S 2  is NO, meaning that the sensor SN 7  is in an OFF state, the controller  100  lowers the tray  2  (step S 10 ) and then determines whether or not the sensor SN 9  is an ON state (step S 11 ). If the answer of the step S 11  is YES, the controller  100  stops the movement of the tray  2  (step S 12 ). In this case, the tray  2  is moved downward toward the sensor SN 9 . 
   If the answer of the step S 11  is NO, i.e., if the sensor SN 9  is in an OFF state, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 13 ). If the answer of the step S 13  is YES, the controller  100  stops the movement of the tray  2  (step S 14 ). Subsequently, the controller  100  raises the tray  2  (step S 15 ) and then determines whether or not the sensor SN 9  is in an ON state (step S 16 ). As soon as the sensor SN 9  senses the lower end of the tray  2  (YES, step S 16 ), the controller  100  stops the movement of the tray  2  (step S 17 ). In this case, the tray  2  is raised from a position between the sensors SN 9  and SN 7 . This is followed by the step S 6 . 
   The controller  100  may move the two trays  1  and  2  at the same time, if desired. 
   To locate the tray  1  at the outlet E 2 , the controller  100  once stops the movement of the tray  1  when the sensor SN 6  senses the upper end of the end fence  1   a,  then raises the tray  1  by a preselected distance, and then stops it. To locate the other tray  2  at the outlet E 2 , the controller  100  once stops the movement of the tray  2  when the sensor SN 6  senses the upper end of the tray  2 , then lowers the tray  2  by a preselected distance, and then stops it. 
   Because the home position of the tray  2  corresponds to the position of the sensor SN 7 , the tray  2  is moved from the home position to the outlet E 2  when selected. This successfully reduces the distance and time of movement of the tray  2 , compared to a case wherein the above home position corresponds to the position of the sensor SN 7 . 
   The end fence  1   a  of the tray  1  has its intermediate portion notched so as not to interfere with a push roller  70  (see  FIG. 4 ) although not shown specifically. The sensor SN 6  is therefore so positioned as to sense the end fence  1   a,  the rear end of the tray  2  or the top of a paper stack. In this sense, the sensor SN 6  serve as a paper sensor at the same time. 
     FIG. 12  shows a condition wherein the tray  2  is selected and has received a certain number of papers. Assume that the other tray  1  is selected in the condition shown in FIG.  12 . Then, as shown in  FIG. 13 , the tray  2  is retracted until the sensor SN 7  senses it, while the tray  1  is brought to the outlet E 2 . 
     FIG. 14  shows a condition wherein the tray  2  is selected and has received a number of papers great enough to turn on both of the sensors SN 6  and SN 9 . In the illustrative embodiment, the tray  2  reached the condition of  FIG. 14  is determined to be full. More specifically, the sensor SN 9  is capable of detecting the full state of the tray  2  alone. When the tray  1  is selected with the tray  2  being in its full state, the tray  2  must be retracted. However, the tray  2  should only be retracted by a distance equal to the height of a stack that the tray  1  can accommodate, i.e., a dimension H shown in FIG.  14 . 
   In light of the above, the sensors SN 9  and SN 7  are spaced by the distance H from each other. It follows that when the full tray  2  is retracted and the tray  1  is brought to the outlet E 2 , no wasteful space exists between the trays  1  and  2 , as shown in FIG.  15 . 
   As stated above, the optimal distance between the sensors SN 9  and SN 7  can be regarded as the height of a stack that the tray  1  can accommodate (dimension H). Therefore, considering the sensor SN 9  to be a reference, it is possible to determine the position of the sensor SN 7  in terms of the distance of movement of the tray  2 . This obviates the need for the sensor SN 7 . 
   A specific procedure for retracting the tray  2  without using the sensor SN 7  will be described with reference to FIG.  16 . As shown, the controller  100  lowers the tray  2  (step S 1 ) and then determines whether or not the sensor SN 9  is in an ON state (step S 2 ). If the answer of the step S 2  is NO, the controller  100  resets a counter, not shown, for counting the pulses of the motor  51  to zero (step S 3 ). When the sensor SN 9  turns on (YES, step S 2 ), the controller  100  starts counting the pulses of the motor  51  with the above counter (step S 4 ). On counting a preselected number of pulses (YES, step S 5 ), the controller  100  stops the movement of the tray  2 . As a result, the tray  2  is located at its retracted position. 
   A specific initialization procedure not using the sensor SN 7  will be described with reference to  FIGS. 17 and 18 . As shown, on the power-up of the copier G, the controller  100  starts initialization (step S 1 ). The controller  100  determines whether or not the sensor SN 5  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  determines that the tray  1  is located at its home position, and then determines whether or not the sensor SN 6  is in an ON state (step S 3 ). If the answer of the step S 3  is YES, the controller determines that the tray  2  is located at the outlet E 2 , and then lowers the tray  2  (step S 4 ). As soon as the sensor SN 9  turns on (YES, step S 5 ), the controller  100  stops the movement of the tray  2 . As a result, the tray  2  is located at its home position. 
   If the sensor SN 6  is in an OFF state, as determined in the step S 3 , the controller  100  once raises the tray  2  (step S 7 ) and determines whether or not the sensor SN 6  is an ON state (step S 8 ). If the answer of the step S 8  is YES, the controller stops the movement of the tray  2  (step S 9 ). This is followed by the step S 4 . 
   If the answer of the step S 2  is NO, meaning that the sensor SN 5  is in an OFF state, the controller  100  raises the tray  1  (step S 10 ) and then determines whether or not the sensor SN 5  is in an ON state (step S 11 ). If the answer of the step S 11  is YES, the controller  100  stops the movement of the tray  1  (step S 12 ). Consequently, the tray  1  is located at its home position. This is followed by the sequence of steps to be executed when the answer of the step S 2  is YES. 
   The above embodiment achieves various unprecedented advantages, as enumerated below. 
   (1) The stand-by position or home position assigned to the lower tray is higher in level than the retracted position. Therefore, when the lower tray is selected, it can move to the outlet in a short period of time. 
   (2) The home position sensing means bifunctions as means for sensing the full state of the lower tray. This obviates the need for extra means for sensing the full state and thereby reduces the cost of the finisher. 
   (3) The stand-by position or home position sensing means is positioned above the retracted position by the height of a stack that the upper tray can accommodate. This minimizes the distance of retraction of the full lower tray and thereby obviates a wasteful space. 
   (4) Because the retracted position is determined in terms of the distance of movement of the lower tray without using extra means, the cost is further reduced. 
   (5) The discharge position sensing means is so located as to operate on a surface which the trailing edge of a paper on the tray contacts. The sensing means can therefore sense both of the upper tray and lower tray as well as the top of a paper stack. This simplifies the sensing arrangement and reduces the cost of the finisher. 
   Second Embodiment 
   This embodiment is directed mainly toward the second object stated earlier. Because the second embodiment is similar to the first embodiment of  FIGS. 1-8  in construction and operation. The following description will concentrate on differences. This is also true with the other embodiments to be described later. 
   A tray control procedure to be executed by the controller  100  and unique to this embodiment will be described with reference to  FIGS. 19-25  in addition to  FIGS. 1-8 . The sensor SN 5  senses the upper end of the end fence  1   a  of the tray  1  when the tray  1  is located at its home position while the sensor SN 9  senses the upper rear end of the tray  2  when the tray  2  is located at its home position. 
   Reference will be made to  FIGS. 19 and 20  for describing the initialization of the trays  1  and  2 , i.e., a procedure for locating them at the home positions. As shown, on the power-up of the copier G, the controller  100  starts initialization and lowers the tray  2  (step S 1 ). The controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 3 ). If the answer of the step S 3  is YES, the controller  100  determines that the number of papers stacked on the tray  2  is so great, the tray  1  cannot be lowered. In this case, the controller  100  sets a tray  1  inhibition flag in the flag area of a RAM (Random Access Memory), not shown, to thereby inhibit the tray  1  from being used (step S 4 ). Then, the controller  100  stops the movement of the tray  2  (step S 5 ). 
   If the sensor SN 9  is in an OFF state, as determined in the step S 3 , the controller stops of the tray  2 , then raises it (step S 6 ), and again determines whether or not the sensor SN 9  is in an ON state (step S 7 ). If the answer of the step S 7  is YES, the controller  100  stops the tray  2  and again lowers it (step S 8 ). As soon as the sensor SN 9  turns off (YES, step S 9 ), the controller  100  stops the tray  2 . As a result, the upper surface of the tray  2  or that of a paper stack on the tray  2  is located at or below the home position of the tray  2 . Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state. If the sensor SN 6  is in an OFF state, the controller  100  lowers the tray  1  (step S 11 ). As soon as the sensor SN 6  senses the upper end of the end fence  1   a  of the tray  1  (YES, step S 12 ), the controller  100  stops the tray  1  (step S 13 ). 
   During the downward movement of the tray  2  or during the stacking of papers on the tray  2 , the controller  100  determines whether or not the tray  1  can be lowered in accordance with a subroutine program shown in FIG.  21 . For example, while papers are sequentially stacked on the tray  2 , the tray  2  is sequentially lowered for accommodating a great number of papers. However, it sometimes occurs that after the current job, the tray  1  is selected in place of the tray  2  without the stack of papers being removed from the tray  2 . 
   In the above situation, the tray  2  is lowered. Specifically, as shown in  FIG. 21 , the controller  100  determines whether or not the tray  1  inhibition flag is set (step S 1 ). If the answer of the step S 1  is NO, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 3 ). 
   Assume that the sensors SN 9  and SN 6  both turn on while the tray  2  is in downward movement. Then, the controller  100  determines that the top of the stack on the tray  2  may lie in the range to which the tray  1  should be lowered. In this case, the controller  100  sets the tray  1  inhibition flag (step S 4 ) and sends a signal indicative of the inhibition to a controller, not shown, included in the copier G. In response, the controller of the copier G urges the operator to remove the stack from the tray  2  via, e.g., the operation panel. 
   Assume that the sensor SN 9  senses a paper during stacking of papers on the tray  2  and then turns off. Then, the controller  100  raises the tray  2 , determining that the operator has removed the stack from the tray  2 . This will be described specifically with reference to  FIGS. 22 and 23 . As shown, the controller  100  determines whether or not papers are being stacked on the tray  2  (step S 1 ). If the answer of the step S 1  is YES, the controller  100  determines whether or not a sense flag relating to the sensor SN 9  is set (step S 2 ). If the answer of the step S 2  is YES, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 3 ). If the answer of the step S 3  is YES, the controller  100  sets the sense flag relating to the sensor SN 9  (step S 4 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 9  is in an OFF state (step S 5 ). If the answer of the step S 5  is YES, the controller  100  raises the tray  2  and clears the sense flag relating to the sensor SN 9  (step S 6 ). As soon as the sensor SN 6  senses the tray  2  being raised (YES, step S 7 ), the controller  100  stops the tray  2  (step S 8 ). Thereafter, the controller  100  lowers the tray  2  (step S 9 ) and then stops it as soon as the sensor SN 6  turns off (step S 11 ). This successfully locates the tray  2  at the adequate position for receiving papers via the outlet  2 . Even when some papers are left on the tray  2 , the top of the papers is located at the adequate position. 
     FIGS. 24 and 25  demonstrate a procedure for canceling the inhibition of the tray  1 . As shown, the controller  100  determines whether or not the tray  1  inhibition flag is set (step S 1 ). If the answer of the step S 1  is YES and if the sensor SN 9  turns off later (YES, step S 2 ), the controller raises the tray (step S 3 ) and then stops it (step S 5 ) as soon as the sensor SN 9  turns on (YES, step S 4 ). As a result, the tray  2  is located at the home position or stand-by position. The controller  100  again determines whether or not the sensor SN 7  is in an OFF state (step S 6 ). If the answer of the step S 6  is YES, the controller  100  clears the tray  1  inhibition flag (step S 7 ) while sending a signal indicative of the cancellation to the controller of the copier G. In response, the controller of the copier G cancels the inhibition relating to the tray  1 . 
   As stated above, the second embodiment achieves the following advantages. 
   (1) The stand-by position or home position assigned to the lower tray is higher in level than the retracted position. Therefore, when the lower tray is selected, it can move to the outlet in a short period of time. 
   (2) The retracted position sensing means senses the upper surface of the lower tray or the top of papers stacked on the lower tray. The lower tray can therefore wait at a preselected position without regard to the number of papers stacked thereon. This is successful to render the removal of the paper stack stable and the period of time necessary for the lower tray to reach the outlet constant. 
   (3) Because the stand-by position sensing means is located in the range of movement of a paper of minimum size available with the lower tray, the above advantages (1) and (2) are achievable with papers of all sizes. 
   (4) By simply adding the stand-by position sensing means, it is possible to prevent the upper tray from interfering with the lower tray when moved downward. This reduces the down time of the entire system including the finisher while promoting safety operation. 
   (5) Even when the paper stack is abruptly removed, the stand-by position sensing means allows the lower tray to be located at the adequate discharge position without fail. It follows that a wasteful space above the lower tray is obviated. 
   Third Embodiment 
   This embodiment is directed mainly toward the third embodiment stated earlier. This embodiment is also similar to the first embodiment except for the tray control procedure to be executed by the control means. A first tray control procedure available with the third embodiment will be described with reference to  FIGS. 26-30  in addition to  FIGS. 1-8 . Again, the sensor SN 6  senses the end fence  1   a  of the tray  1  when the tray  1  is in its home position while the sensor SN 9  senses the upper rear end of the tray  2  when the tray  2  is in its home position. 
   As shown in  FIGS. 26 and 27 , on the power-up of the copier G, the control means  100  starts initialization (step S 1 ). Specifically, the control means  100  determines whether or not the sensor SN 7  is in an ON state (step S 2 ). If the sensor SN 7  is in an OFF state (NO, step S 2 ), the controller lowers the tray  2 , determining that the tray  2  is positioned above the sensor SN 7  (step S 3 ). Then, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 4 ). If the answer of the step S 4  is YES, the controller  100  stops the movement of the tray  2  (step S 5 ). As a result, the tray  2  is located at its home position. 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 6 ). If the answer of the step S 6  is NO, the controller  100  lowers the tray  1 , determining that the tray  1  is positioned above the sensor SN 6  (step S 7 ). The controller  100  again determines whether or not the sensor SN 6  is in an ON state (step S 8 ). As soon as the sensor SN 6  senses the upper end of the end fence  1   a  (YES, step S 8 ), the controller  100  stops the movement of the tray  1  (step S 9 ). 
   If the answer of the step S 2  is YES, the controller  100  raises the tray  2  (step S 10 ) until the sensor SN 9  turns on (YES, step S 11 ). Then, the controller  100  stops the movement of the tray  2  (step S 12 ). 
   When the tray  2  is located between the sensors SN 7  and SN 9 , the sensor SN 9  remains in an OFF state, as determined in the step S 4 . In this case, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 13 ). If the answer of the step S 13  is YES, the controller  100  stops the movement of the tray  2  (step S 14 ) and then raises the tray  2  (step S 15 ). As soon as the sensor SN 9  turns on (YES, step S 16 ), the controller  100  stops the movement of the tray  2  (step S 17 ). 
   As stated above, in the first tray control procedure, the tray  1  is located at the paper discharge position. Therefore, when the tray  1  is selected, neither the tray  1  nor the tray  2  is moved. This promotes the efficient use of the tray  1  when the tray  1  is frequently used. 
   When the tray  2  is selected, the tray  1  is elevated until the sensor SN 5  senses it. The tray  2  is raised from its stand-by position until the sensor SN 6  senses it. The elevation of the tray  1  and that of the tray  2  may be effected at the same time, if desired. 
   Because the tray  2  is moved from its stand-by position to the outlet E 2 , a period of time necessary for the tray  2  to reach the outlet  2  is shorter than when the tray  2  is moved from its retracted position (lower limit position) defined by the sensor SN 7 . 
   A second tray control procedure available with the illustrative embodiment is as follows. As shown in  FIG. 28 , the sensor SN 5  senses the upper end of the end fence  1   a  of the tray  1  when the tray  1  is in its home position while the sensor SN 9  senses the upper rear end of the tray  2  when the tray  2  is in its home position. Initialization of the trays  1  and  2  will be described with reference to  FIGS. 29 and 30 . 
   As shown, on the power-up of the copier G, the controller  100  starts initialization (step S 1 ). Specifically, the control means  100  determines whether or not the sensor SN 7  is in an ON state (step S 2 ). If the sensor SN 7  is in an OFF state (NO, step S 2 ), the controller lowers the tray  2 , determining that the tray  2  is positioned above the sensor SN 7  (step S 3 ). Then, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 4 ). If the answer of the step S 4  is YES, the controller  100  stops the movement of the tray  2  (step S 5 ). As a result, the tray  2  is located at its home position. 
   Subsequently, the controller  100  determines whether or not the sensor SN 5  is in an ON state (step S 6 ). If the answer of the step S 6  is NO, the controller  100  raises the tray (step S 7 ) and then determines whether or not the sensor SN 5  is in an ON state (step S 8 ). If the answer of the step S 8  is YES, meaning that the sensor SN 5  has sensed the upper end of the end fence  1   a,  the controller  100  stops the movement of the tray  1  (step S 9 ). If the answer of the step S 6  is YES, the controller  100  ends the initialization, determining that the tray  1  is held its home position. 
   If the sensor SN 7  is in an ON state, as determined in the step S 2 , the controller  100  raises the tray  2  (step S 10 ) and then determines whether or not the sensor SN 9  is in an ON state (step S 11 ). If the answer of the step S 11  is YES, the controller  100  stops the movement of the tray  2  (step S 12 ). If the sensor SN 9  is in an ON state, as determined in the step S 4 , the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 13 ). If the answer of the step S 13  is YES, the controller  100  stops the movement of the tray  2  (step S 14 ). Subsequently, the controller  100  raises the tray  2  (step S 15 ). As soon as the sensor SN 9  senses the upper rear end of the tray  2  (YES, step S 16 ), the controller  100  stops the movement of the tray  2  (step S 17 ). In this case, the tray  2  has been positioned between the sensors SN 9  and SN 7  before. 
   While the above specific procedure moves the tray  1  after the tray  2 , the trays  1  and  2  may be moved at the same time, if desired. 
   To bring the tray  1  to the outlet E 2 , the tray  1  is stopped when the sensor SN 6  senses the upper end of its end fence  1   a.  To bring the tray  2  to the outlet E 2 , the tray  2  is once stopped when the sensor SN 6  senses its upper rear end, then lowered by a preselected distance, and then brought to a stop. 
   Because the home position of the tray  2  corresponds to the position of the sensor SN 9 , the tray  2  is moved from the home position to the outlet E 2  when selected. This successfully reduces the distance and time of movement of the tray  2 , compared to the case wherein the home position is located below the position of the sensor SN 7 . 
   The end fence  1   a  of the tray  1  has its intermediate portion notched so as not to interfere with the push roller  70 ,  FIG. 4 , although not shown specifically. The sensor SN 6  is therefore so positioned as to sense the end fence  1   a,  the rear end of the tray  2  or the top of a paper stack. In this sense, the sensor SN 6  serves as a paper sensor at the same time. 
   As stated above, in the finisher shown in  FIG. 28 , the distance between the trays  1  and  2  held in their stand-by positions is greater than in the finisher of  FIG. 1 , facilitating the removal of a paper stack from the tray  2 . It follows that the stand-by position of the tray  2  defined by the sensor SN 9  can be selected in consideration of easy removal of a paper stack also. 
   The third embodiment shown and described has the following advantages. 
   (1) The stand-by position of the lower tray is located above the lower limit position of the same. This reduces a period of time necessary for the lower tray to reach the paper discharge position when selected. 
   (2) Because the home position of the upper tray corresponds to the paper discharge position, the finisher can be efficiently used when the upper tray is frequently used. 
   (3) Because the stand-by position of the upper tray is coincident with the retracted position above the outlet, the distance between the upper tray and the lower tray can be increased to facilitate the removal of a paper stack from the lower tray. 
   Fourth Embodiment 
   This embodiment is directed mainly toward the fourth object stated earlier. This embodiment differs from the previous embodiments in that the sensor SN 9  responsive to the retracted position or home position of the tray  2  is absent and in that the controller  100  controls the trays  1  and  2  in a unique way. The fourth embodiment will be described with reference to  FIGS. 31-40  in addition to  FIGS. 1-8 . 
   The sensor SN 5  senses the upper end of the end fence  1   a  of the tray  1  when the tray  1  is in its home position. The home position of the tray  2  is lower than the position where the sensor SN 6  senses it by a preselected distance. 
     FIG. 31  demonstrates initialization for locating the trays  1  and  2  at their home positions. As shown, on the power-up of the copier G, initialization begins (step S 1 ). The controller  100  determines whether or not the sensor SN 5  is in an ON state (step S 2 ). If the answer of the step S 2  is NO, the controller  100  raises the tray  1  (S 3 ), determining that the tray  1  is positioned below the sensor SN 5 . As soon as the sensor SN 5  senses the tray  1  (YES, step S 2 ), the controller  100  stops the movement of the tray  1  (step S 4 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 5 ). If the answer of the step S 5  is NO, the controller  100  raises the tray  2  (step S 6 ). When the sensor SN 6  senses the tray  2 , the controller  100  stops the tray  2  at a position where the sensor SN 6  has not sense it. If the answer of the step S 5  is YES, the controller  100  lowers the tray  2  (step S 7 ), determining that the tray  2  has overrun. Then, the controller determines whether or not the sensor SN 6  is in an ON state (step S 8 ), and stops the tray  2  when the sensor SN 6  stops sensing the tray  2  (step S 9 ). 
   Assume that the operator selects the tray  1  on the operation panel of the copier G or the computer connected thereto. Then, the controller  100  first determines whether or not the sensors SN 5  and SN 6  each are in an ON state in order to see the positions of the trays  1  and  2 . Patterns A-D shown below are representative of the possible combinations of the ON/OFF states of the sensors SN 5  and SN 6  and the positions of the trays  1  and  2 . 
   
     
       
         
             
             
             
             
           
             
                 
                 
             
             
                 
               SN5 
               SN6 
               Positions of Trays 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               A: 
               ON 
               ON 
               tray 1 at retracted position 
             
             
                 
                 
                 
                 
               tray 2 overrun 
             
             
                 
               B: 
               ON 
               OFF 
               tray 1 at retracted position 
             
             
                 
                 
                 
                 
               tray 2 below SN6 
             
             
                 
               C: 
               OFF 
               ON 
               tray 1 at discharge position 
             
             
                 
                 
                 
                 
               tray 2 below SN6 
             
             
                 
               D: 
               OFF 
               OFF 
               tray 1 between retracted position 
             
             
                 
                 
                 
                 
               and 2 discharge position 
             
             
                 
                 
                 
                 
               tray 2 below SN6 
             
             
                 
                 
             
          
         
       
     
   
   As shown in  FIG. 32 , as for the above pattern A, the controller  100  first lowers the tray  2  (step S 1 ) while determining whether or not the sensor SN 6  is in an ON state (step S 2 ). When the sensor SN 6  stops sensing the tray  2  (NO, step S 2 ), the controller  100  further lowers the tray  2  by a preselected distance of (L 3 +L 4 ) (step S 3 ) and then stops its movement. As shown in  FIG. 33 , the distance (L 3 +L 4 ) is great enough for the tray  1  to move to the paper discharge position and for the operator to pick up a paper stack from the tray  2 . Specifically, the distance L 3  is a height that the tray  1  occupies when brought to the paper discharge position. The distance L 4  is a height for implementing a tray gap L 5  necessary for the operator to pick up a paper stack from the tray  2 . To set the distance (L 3 +L 4 ), a pulse counter, not shown, counts pulses for driving the motor  51  assigned to the tray  2  after the sensor SN 6  has stopped sensing the tray  2 . 
   Subsequently, the controller  100  lowers the tray  1  (step S 5 ) and determines whether or not the sensor SN 6  has sensed the upper end of the end fence  1   a  (step S 6 ). If the answer of the step S 6  is YES, the controller  100  stops the movement of the tray  1  (step S 7 ). 
   As shown in  FIG. 34 , as for the pattern B, the controller  100  once raises the tray  2  (S 1 ) and determines whether or not the sensor SN 6  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  stops the movement of the tray  2  (step S 3 ). Subsequently, the controller  100  lowers the tray  2  (step S 4 ) and determines whether or not the sensor SN 6  is in an ON state (step S 5 ). If the answer of the step S 5  is NO, the controller  100  lowers the tray  2  by the distance (L 3 +L 4 ) (step S 6 ) and then stops it (step S 7 ). Thereafter, the controller  100  lowers the tray  1  (step S 8 ) and determines whether or not the sensor SN 6  has sensed the upper end of the end fence  1   a  (step S 9 ). If the answer of the step S 8  is YES, the controller  100  stops the movement of the tray  1 . 
   As for the pattern C, the controller  100  does not execute any tray control and allows a job to be executed immediately. 
   As for the pattern D, the controller  100  determines that the position of the tray  1  is unusual, executes initialization, and then sets up the pattern B. 
   When the operator selects the tray  2  on the operation panel of the copier G or the computer connected thereto, the controller  100  also determines the statuses of the sensors SN 5  and SN 6  first in order to see the positions of the trays  1  and  2 . 
   Specifically, as shown in  FIG. 35 , as for the pattern A, the controller  100  first lowers the tray  2  (step S 1 ) and determines whether or not the sensor SN 6  is in an ON state (step S 2 ). When the sensor SN 6  stops sensing the tray  2  (NO step S 2 ), the controller  100  stops the movement of the tray  2  (step S 3 ). 
   As shown in  FIG. 36 , as for the pattern  8 , the controller  100  once raises the tray  2  (step S 1 ) and determines whether or not the sensor SN 6  is in an ON state (step S 2 ). As soon as the sensor SN 6  senses the tray  2  (YES, step S 2 ), the controller  100  stops the movement of the tray  2  (step S 3 ). Subsequently, the controller  100  lowers the tray  2  (step S 4 ) and then stops it (step S 6 ) as soon as the sensor SN 6  stops sensing it (NO, step S 5 ). 
   As shown in  FIG. 37 , as for the pattern C, the controller  100  first raises the tray  1  (step S 1 ) and determines whether or not the sensor SN 5  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  stops the movement of the tray  1 . Subsequently, the controller  100  lowers the tray  2  (step S 4 ) and then stops the tray  2  (step S 6 ) as soon as the sensor SN 6  stops sensing it (NO, step S 5 ). 
   As shown in  FIG. 38 , as for the pattern D, the controller  100  first raises the tray  1  (step S 1 ) and determines whether or not the sensor SN 5  is in an ON state (step S 2 ). If the answer of the step S 2  is YES, the controller  100  stops the movement of the tray  1  (step S 3 ). Subsequently, the controller  100  raises the tray  2  (step S 4 ) and determines whether or not the sensor SN 6  is in an ON state (step S 5 ). If the answer of the step S 5  is YES, the controller  100  stops the movement of the tray  2  (step S 6 ). Thereafter, the controller  100  lowers the tray  2  and then stops the tray  2  (step S 9 ) as soon as the sensor SN 6  stops sensing it (NO, step S 8 ). 
   As shown in  FIG. 39 , the sensor SN 7  is positioned such that the tray  2  having been sensed by the sensor SN 7  can further move downward by a preselected distance. Stated another way, a preselected distance is available between the bottom of the finisher and the sensor SN 7  responsive to the lower limit position.  FIG. 39  shows the tray  2  in its full state. Specifically, as a great number of papers are stacked on the tray  2 , the tray  2  is sequentially lowered. When the sensors SN 7  and SN 6  sense the tray  2  and the top of the paper stack on the tray  2 , respectively, the controller  100  determines that the tray  2  is full. 
   When the tray  2  is full, the controller  100  lowers it to a position below the lower limit position by a preselected distance L 6  (see FIG.  40 ). The distance L 6  is selected to be greater than the distance (L 3 +L 4 ), FIG.  33 . Therefore, even when the tray  2  is left in its full state, the tray  1  can be located at the paper discharge position. The distance L 6 , like the distance (L 3 +L 4 ), is determined in terms of the number of pulses for driving the motor  51 . 
   As stated above, the above embodiment achieves the following advantages. 
   (1) The end fence of the tray movable via the outlet is capable of preventing papers stacked on the tray from returning to the outlet without complicating the configuration of the outlet. The papers do not contact the structural elements of the outlet and are therefore free from disturbance and contamination. 
   (2) The upper tray is movable outward in the paper discharge direction via the outlet. This allows the papers to be neatly positioned without complicating the configuration of the outlet. 
   (3) The retracted position of the lower tray can be determined without resorting to extra sensing means which would increase the cost of the finisher. Because the retracted position is located above the lower limit position and because the lower tray can be moved from the retracted position, a period of time necessary for the lower tray to move to the paper discharge position is reduced when the lower tray is selected. 
   (4) The preselected distance is such that the upper tray can move to the paper discharge position and a paper stack can be picked up from the lower tray. This not only guarantees easy removal of a paper stack, but also reduces the tray switching time (moving time). 
   (5) Even when the lower tray is full, it can be lowered to allow extra papers to be stacked. 
   (6) Even when the lower tray is held in its full state, the upper tray can be lowered to the paper discharge position. This promotes the effective use of a paper discharge space available at the side of the finisher. 
   Fifth Embodiment 
   This embodiment is directed mainly toward the fifth object stated earlier. This embodiment also differs from the previous embodiments in that the sensor SN 9  responsive to the retracted position or home position of the tray  2  is absent and in that the controller  100  controls the trays  1  and  2  in a unique way. The fourth embodiment will be described with reference to  FIGS. 41-50  in addition to  FIGS. 1-8 . 
   In the illustrative embodiment, the home position of the tray  1  is a position which the upper end of the end fence  1   a  reaches when raised by a preselected distance (amount) after being sensed by the sensor SN 6 . The home position of the tray  2  is a position where the tray  2  is sensed by the sensor SN 7 . 
   On the power-up of the copier G, the controller  100  determines whether or not the sensor SN 7  responsive to the lower limit position is in an ON state. If the sensor SN 7  is in an OFF state, the controller  100  lowers the tray  2  via the motor  51 , determining that the tray  2  is positioned above the sensor SN 7 . As soon as the sensor SN 7  senses the tray  2 , the controller  100  stops lowering the tray  2 . Subsequently, the controller  100  determines whether or not the sensor SN 5  is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  once raises the tray  1  via the motor  50  and then lowers the tray  1  as soon as the sensor SN 5  senses it. When the sensor SN 6  senses the upper end of the end fence  1   a,  the controller  100  raises the tray  1  by a preselected distance and then stops it. Further, if the sensor SN 5  is in an ON state, the controller  100  lowers the tray  1 , then raises it by the preselected distance when the sensor SN 6  senses the tray  1 , and then stops the tray  1 . 
   A first tray control procedure available with the illustrative embodiment will be described with reference to  FIGS. 41 and 42 . As shown, the discharge of papers from the copier G begins in the staple mode input on the copier G or the computer connected thereto (step S 1 ), the controller  100  determines whether or not the tray  1  is selected by the operator (step S 2 ). At the same time, the controller  100  determines whether or not the sensor SN 7  is in an ON state. If the sensor SN 7  is in an OFF state, the controller  100  lowers the tray  2  (step S 3 ), determining that the tray  2  is positioned above the sensor SN 7 . The controller  100  again determines whether or not the sensor SN 7  is in an ON state (step S 4 ) and then stops the movement of the tray  2  (step S 5 ). Thereafter, the controller  100  lowers the tray  1  (step S 6 ) and determines whether or not the sensor SN 6  is in an ON state (step S 7 ). If the answer of the step S 7  is YES, the controller  100  stops the movement of the tray  1  (step S 8 ). Then, the controller  100  raises the tray  1  by a preselected distance (amount) (step S 9 ) and then stops it (step S 10 ). As a result, the tray  1  is located at the paper discharge position. 
   When a paper is discharged to the staple tray of the finisher, the controller  100  determines whether or not stapling has ended (step S 12 ). If the answer of the step S 12  is YES, the controller  100  causes a stapled paper stack to be driven out to the tray  1  (step S 13 ). 
   Assume that the tray  2  is selected via the copier G or the computer connected thereto. Then, the controller  100  first determines whether or not the sensor SN 5  responsive to the retracted position is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  raises the tray  1  (step S 14 ) and again checks the sensor SN 5  (step S 15 ). When the sensor SN 5  turns on (YES, step S 15 ), the controller  100  stops the movement of the tray  1  (step S 16 ), then raises the tray  2  (step S 17 ), and then determines whether or not the sensor SN 6  is in an ON state (step S 18 ). If the answer of the step S 18  is YES, the controller  100  once stops the movement of the tray  2 , then lowers the tray  2  by a preselected distance (amount) (step S 20 ), and then stops it (step S 21 ). As a result, the tray  2  is located at the proper discharge position. 
   When a paper is discharged to the staple tray of the finisher (S 22 ), the controller  100  determines whether or not stapling has ended (step S 23 ). If the answer of the step S 23  is YES, the controller  100  causes a stapled paper stack to be driven out to the tray  2  (step S 24 ). 
   The tray  1  or  2  is located at the outlet E 2  beforehand in response to information received from the copier G or the computer connected thereto. This successfully reduces a period of time relating to the movement of the tray  1  or  2 . Stated another way, the trays  1  and  2  are not moved relative to the outlet E 2  independently of each other, but are moved in parallel by staple processing within a necessary period of time. The finisher can therefore complete its operation in a shorter period of time than the conventional finishers. 
   After the tray  1  or  2  has been located at the outlet E 2 , papers are sequentially stacked on the tray  1  or  2 . When the sensor SN 6  senses the top of a paper stack on the tray  1  or  2  held at the outlet E 2 , the tray  1  or  2  is lowered by a preselected distance. Such a procedure is repeated to allow a great number of papers to be stacked on the tray  1  or  2 . This is also true with the other embodiments to be described later. 
   In the non-staple mode, the operator is allowed to select desired one of the proof tray P, upper tray  1  and lower tray  2 ; the trays  1  and  2  each are capable of accommodating a great number of papers. The proof tray P, upper tray  1  and lower tray  2  may be respectively assigned to a facsimile apparatus, a copier or a printer, and a printer or a copier, as desired. The finisher is therefore adaptive to a multifunction image forming apparatus. 
   A second tray control procedure available with the illustrative embodiment will be described with reference to  FIGS. 43-46 . The procedure to be described prevents the trays  1  and  2  from interfering with each other when moved independently of each other. 
   As shown in  FIGS. 43 and 44 , when the controller  100  receives a paper output request from the copier G or the computer connected thereto (step S 1 ), it sends an answer representative of a stand-by state to the copier G or the computer (step S 2 ). The controller  100  determines whether or not the tray  1  is selected (step S 3 ) and determines whether or not the sensor SN 7  is in an ON state. If the sensor SN 7  is in an OFF state, the controller  100  determines that the tray  2  is positioned above the sensor SN 7 , and lowers the tray  2  which would obstruct the positioning of the tray  1  at the outlet E 2  (step S 4 ). Then, on the elapse of a preselected period of time (about 0.1 second to 0.5 second in the illustrative embodiment), the controller  100  lowers the tray  1  (step S 6 ). As soon as the sensor SN 7  turns on (YES, step S 7 ), the controller  100  stops the movement of the tray  2 . 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 9 ). When the sensor SN 6  turns on (YES, step S 9 ), the controller  100  once stops the movement of the tray  1  (step S 10 ), then raises the tray  1  by a preselected distance (amount) (step S 11 ), and then stops it (step S 12 ). The tray  1  is now ready to receive papers via the outlet E 2 . Thereafter, the controller  100  sends a signal representative of the cancellation of the stand-by state to the copier G or the computer (step S 13 ). In response, a paper is transferred from the copier G to the finisher (step S 14 ) and therefrom to the tray  1  (step S 15 ). 
   If the answer of the step S 7  is NO, meaning that the sensor SN 7  is in an OFF state, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 16 ). When the sensor SN 6  turns on (YES, step S 16 ), the controller once stops the movement of the tray  1  (step S 17 ), then relates the tray  1  by a preselected distance (amount) (step S 18 ), and then stops it (step S 19 ). As a result, the tray  1  brought to the outlet E 2 . Further, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 20 ). If the answer of the step S 20  is YES, the controller stops the movement of the tray  2  (step S 21 ). This is followed by the step S 13 . 
   Assume that the tray  2  is selected. Then, as shown in  FIGS. 45 and 46 , the controller  100  determines whether or not the sensor SN 5  is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  raises the tray  1  which would obstruct the positioning of the tray  2  at the outlet E 2  (step S 1 ). Then, on the elapse of a preselected period of time (about 0.1 second to 0.5 second in the illustrative embodiment) (step S 2 ), the controller  100  raises the tray  2  (step S 3 ). As soon as the sensor S 57  turns on (YES, step S 4 ), the controller  100  stops the movement of the tray  1  (step S 5 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 6 ). When the sensor SN 6  turns on (YES, step S 6 ), the controller  100  once stops the movement of the tray  2  (step S 7 ), then lowers the tray  2  by a preselected distance (amount) (step S 8 ), and then stops it (step S 9 ). The tray  2  is now ready to receive papers via the outlet E 2 . Thereafter, the controller  100  sends a signal representative of the cancellation of the stand-by state to the copier G or the computer (step S 10 ). In response, a paper is transferred from the copier G to the finisher (step  11 ) and therefrom to the tray  2  (step  12 ). 
   If the answer of the step S 4  is NO, meaning that the sensor SN 5  is in an OFF state, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 13 ). When the sensor SN 6  turns on (YES, step S 13 ), the controller once stops the movement of the tray  2  (step S 14 ), then lowers the tray  2  by a preselected distance (amount) (step S 15 ), and then stops it (step S 16 ). Thereafter, the controller determined whether or not the sensor SN 5  is in an ON state (step S 17 ). If the answer of the step S 17  is YES, the controller  100  stops the movement of the tray  1  (step S 18 ). As a result, the tray  2  brought to the outlet E 2 . This is followed by the step S 10 . 
   A third tray control procedure available with the illustrative embodiment will be described with reference to  FIGS. 47-50 . Should the trays  1  and  2  each be moved at a particularly timing in order to avoid collision, the total period of time necessary for the movement of the trays  1  and  2  would be increased. This embodiment is capable of solving this problem. 
   As shown in  FIGS. 47 and 48 , when the controller  100  receives a paper output request from the copier G or the computer connected thereto (step S 1 ), it sends an answer representative of a stand-by state to the copier G or the computer (step S 2 ). The controller  100  determines whether or not the tray  1  is selected (step S 3 ) and determines whether or not the sensor SN 7  is in an ON state. If the answer of the step S 3  is YES and if the sensor SN 7  is in an OFF state, the controller  100  determines that the tray  2  is positioned above the sensor SN 7 , and lowers the tray  2  at a first speed  1  (step S 4 ). At the same time, the controller  100  lowers the tray  1  at a second speed  2  (step S 5 ). The speed  1  is selected to be higher than the speed  2 , i.e., the tray to be retracted is moved at a higher speed than the tray to be brought to the outlet E 2 . This is done by controlling the motors  50  and  51  that are implemented by stepping motors. 
   Subsequently, the controller determines whether or not the sensor SN 7  is in an ON state (step S 8 ), and stops the movement of the tray  2  as soon as the sensor SN 7  turns on (step S 7 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 8 ). When the sensor SN 6  turn on (YES, step S 6 ), the controller  100  once stops the movement of the tray  1  (step S 9 ), then raises the tray  1  by a preselected distance (amount) at the speed  2  (step S 10 ), and then stops it (step S 11 ). The tray  1  is now ready to receive papers via the outlet E 2 . Thereafter, the controller  100  sends a signal representative of the cancellation of the stand-by state to the copier G or the computer (step S 12 ). In response, a paper is transferred from the copier G to the finisher (step S 13 ) and therefrom to the tray  1  (step S 14 ). 
   If the answer of the step S 6  is NO, meaning that the sensor SN 7  is in an OFF state, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 15 ). When the sensor SN 6  turns on (YES, step S 15 ), the controller once stops the movement of the tray  1  (step S 16 ), then raises the tray  1  at the speed  2  by a preselected distance (amount) (step S 17 ), and then stops it (step S 18 ). Further, the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 19 ). If the answer of the step S 19  is YES, the controller stops the movement of the tray  2  (step S 20 ). This is followed by the step S 12 . 
   Assume that the tray  2  is selected. Then, as shown in  FIGS. 49 and 50 , the controller  100  determines whether or not the sensor SN 5  is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  raises the tray  1  at the speed  1  (step S 1 ) while raising the tray  2  at the speed  2  (step S 2 ). Then, the controller  100  determines whether or not the sensor SN 5  is in an OFF state (step S 3 ), and stops the movement of the tray  1  when the sensor SN 5  turns on (YES, step S 5 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 5 ). When the sensor SN 6  turns on (YES, step S 5 ), the controller  100  once stops the movement of the tray  2  (step S 6 ), then lowers the tray  2  at the speed  2  by a preselected distance (amount) (step S 7 ), and then stops it (step S 8 ). The tray  2  is now ready to receive papers via the outlet E 2 . Thereafter, the controller  100  sends a signal representative of the cancellation of the stand-by state to the copier G or the computer (step S 9 ). In response, a paper is transferred from the copier G to the finisher (step  10 ) and therefrom to the tray  2  (step  11 ). 
   If the answer of the step S 3  is NO, meaning that the sensor SN 5  is in an OFF state, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 12 ). When the sensor SN 6  turns on (YES, step S 12 ), the controller once stops the movement of the tray  2  (step S 13 ), then lowers the tray  2  at the speed  2  by a preselected distance (amount) (step S 14 ), and then stops it (step S 15 ). Thereafter, the controller determined whether or not the sensor SN 5  is in an ON state (step S 16 ). If the answer of the step S 16  is YES, the controller  100  stops the movement of the tray  1  (step S 17 ). As a result, the tray  2  brought to the outlet E 2 . This is followed by the step S 9 . 
   As stated above, the fifth embodiment achieves the following advantages. 
   (1) A tray selected is brought to the outlet during finish processing. This reduces a period of time relating to the movement of the trays end therefore the entire finishing time. 
   (2) A plurality of trays each are brought to the outlet independently of each other. This implements a mass paper discharge function with a single outlet. 
   (3) The tray that would obstruct the tray selected is retracted first. The trays are therefore prevented from colliding with each other. 
   (4) The tray to be retracted is moved at a higher speed than the tray to be located at the outlet. This, coupled with the fact that the two trays start moving at the same time, obviates a wasteful time otherwise required to prevent the trays from colliding with each other. 
   Sixth Embodiment 
   This embodiment is directed toward the sixth object sated earlier. The operation of the control means  100  unique to this embodiment will be described with reference to  FIGS. 51-53  in addition to  FIGS. 1-8 . 
   Referring again to  FIG. 1 , the controller  100 , i.e., CPU  102  of the illustrative embodiment sets up any one of the following four different paper conveyance modes: 
   (1) conveyance along a first route A 1  (corresponding to the non-staple route B) 
   (2) conveyance along a second route A 2  (corresponding to the staple route A) 
   (3) conveyance along a third route A 3   
   (4) paper discharge to either one of first and second trays selected 
   In the conveyance mode (1), when the sensor S 1  senses a paper, the path selectors  20  and  21  are switched to steer the paper to the outlet E 2 . 
   In the conveyance mode (2), when the sensor SN 1  senses a paper, the path selector  21  is switched to steer the paper to the second route A 2 . The roller  6 , brush roller  6   a  and tap roller  9  are caused to operate. As soon as the sensor SN 3  senses a number of papers expected to be stapled together, the jogger fence  11  positions the edges of the papers, and then the stapler S staples the papers. Subsequently, the belt  10   a  with the catch  10  is driven to convey the stapled paper stack to the outlet E 2 . 
   In the conveyance mode (4), one of the first and second trays is selected on the basis of a command received from, e.g., the computer connected to the copier G. The tray selected is brought to the outlet E 2 . Specifically, in response to the above command, the CPU  102  determines the current positions of the trays  1  and  2 , returns the trays  1  and  2  to their home positions, and then locates the tray selected at the outlet E 2 . 
   As shown in  FIG. 51 , on the power-up of the copier G, the controller  100  moves each of the trays  1  and  2  to the respective home position. The home position of the tray  1  is a position that the end fence  1   a  reaches when raised by a preselected distance after being sensed by the sensor SN 5 . The home position of the tray  2  is a position where the sensor SN 7  senses the tray  2 . 
   In  FIG. 51 , on the power-up of the copier G, the controller  100  determines whether or not the sensor SN 7  is in an ON state, i.e., whether or not it has sensed the tray  2  (step S 1 ). If the answer of the step S 1  is NO, the controller  100  lowers the tray  2  via the motor  51  (step S 2 ), determining that the tray  2  is located above the sensor SN 7 . The controller  100  turns off the motor  51  as soon as the sensor SN 7  senses the tray  2 , thereby stopping the movement of the tray  2  (step S 3 ). As a result, the tray  2  is brought to its home position. 
   Subsequently, the controller  100  determines whether or not the sensor SN 5  responsive to the tray  1  is in an ON state (step S 4 ). If the answer of the step S 4  is NO, the controller  100  raises the tray  1  via the motor  50  (step S 5 ). As soon as the sensor SN 5  senses the tray  1  (YES, step S 4 ), the controller  100  stops the movement of the tray  1  and then lowers it (step S 6 ). When the sensor SN 6  senses the tray  1  being lowered (YES, step S 7 ), the controller  100  stops the movement of the tray  1 , then raises the tray  1  by a preselected distance, and then stops it (step S 8 ). As a result, the tray  1  is located at its home position and ready to stack papers thereon. In this manner, in the conveyance modes (1) and (2), the tray  1  serves as a main tray for stacking papers sequentially driven out of the copier G. 
   When the tray  1  is selected via, e.g., the computer, the controller  100  executes a sequence of steps shown in FIG.  52 . As shown, the tray  1  is brought to the outlet E 2  by a procedure similar to the procedure described with reference to FIG.  51 . 
   On the other hand, when the tray  2  is selected, the controller  100  executes a sequence of steps shown in FIG.  53 . As shown, the controller  100  determines whether or not the sensor SN 5  is in an ON state, i.e., whether or not it has sensed the tray  1  (step S 1 ). If the answer of the step S 1  is NO, the controller  100  raises the tray  1  via the motor  50  (step S 2 ). As soon as the sensor SN 5  senses the tray  1 , the controller  100  turns off the motor  50  and thereby stops the movement of the tray  1  (step S 3 ). 
   Subsequently, the controller  100  raises the tray  2  via the motor  51  (step S 51 ). When the sensor SN 6  senses the tray  2  (YES, step S 5 ), the controller  100  turns off the motor  51 , then lowers the tray  2  by a preselected distance (step S 6 ), and then causes papers to be stacked on the tray  2 . Every time the sensor SN 6  senses a paper (step S 7 ), the controller  100  repeatedly lowers the tray  2  by a preselected amount (step S 8 ). In this manner, the top of a paper stack on the tray  2  is constantly held at a height where other papers sequentially coming out via the outlet E 2  can be stacked on the tray  2 . 
   As stated above, when either one of the two layers  1  and  2  is selected, papers can be sequentially stacked on the tray selected. Moreover, because the tray  2  has its end fence implemented the wall of the finisher, it allows the trailing edges of papers to be positioned over a broader range than the tray  1  and can therefore accommodate a great number of papers. The procedure shown in  FIG. 52  or  53  is continuously executed until the number of papers indicated by, e.g., the computer have been stacked, although not shown specifically. 
   In the above embodiment, the tray  1  or  2  is selected in accordance with a command received from, e.g., a computer. Alternatively, an arrangement may be made such that when an interrupt mode, for example, is selected in a copy mode, the controller  100  selects a tray other than one being used and causes papers output in the interrupt mode to be stacked; the tray may even by the proof tray. 
   As stated above, the sixth embodiment achieves the following advantages. 
   (1) At least one of a plurality of trays has an end fence and a stacking surface movable up and down in synchronism with each other. In addition, one tray has an end fence implemented by the wall of the finisher body. The trays can therefore be selectively located at the paper discharge position. The tray whose end fence is implemented by the side wall of the finisher is capable of accommodating a great number of papers with a simple configuration. 
   (2) The trays each are driven by a respective drive source and can therefore be freely arranged. This successfully prevents the finisher from increasing in size. 
   (3) The trays share common guide rails. This reduces the cost and size of the finisher while simplifying the construction of the finisher. 
   (4) The tray having the end fence and stacking surface movable up and down in synchronism has its capacity determined by the end fence. Such trays can be arranged at a constant pitch. Therefore, by driving a plurality of trays with exclusive drive sources, it is possible to divide the trays into a group that can be arranged at the above constant pitch and the other group. This obviates an increase in cost ascribable to an increase in the number of drive sources. 
   (5) The guide rails each include a bent portion for preventing the end fence of the tray from interfering with the paper discharging means. This allows the paper discharging means to overlap the wall of the finisher and therefore to prevent the trailing edges of papers from returning to between the discharging means and the wall of the finisher. 
   (6) The bent portion of each guide rail has a length smaller than the pitch of guide means arranged on the tray. This reduces the tilting angle of the tray moving along the guide portion and thereby prevents a paper stack from dropping from the tray. 
   (7) Because the drive means for up-and-down movement are so located as not to interfere with each other, the belts forming part of the drive means can be arranged in parallel to each other. It follows that the finisher body can be reduced in size in the direction perpendicular to the parallel belts. 
   (8) The tray whose end fence is implemented by the wall of the finisher is located below the other trays. Therefore, a space below the lowermost tray can be used with priority, so that a great number of papers can be stacked on the lowermost stray. 
   (9) Because the tray selected is brought to the paper discharge position independently of the other trays, it allows papers to be stacked thereon without effecting the other trays. 
   Seventh Embodiment 
   This embodiment is directed mainly toward the seventh object stated earlier. This embodiment also differs from the previous embodiments in that the sensor SN 9  responsive to the stand-by position of the tray  2  is absent. 
   As shown in  FIG. 54 , a roller support member  84  is supported at its rear end in the paper discharge direction and angularly movable up and down. The driven roller  8   a  cooperative with the drive roller  8  is rotatably supported by the other or free end of the roller support member  84 . A microswitch or limit switch  86  (see  FIGS. 56-60 ) is mounted on a bracket, not shown, above the roller support member  84  and turned on or turned off by the displacement of the roller support member  84 . Such an arrangement will be described more specifically later. 
   A shift mode is available with the illustrative embodiment. In the shift mode, papers are directly discharged to the tray  1  or  2  by way of the non-staple route B,  FIG. 1. A  shift signal is generated between consecutive jobs, i.e., between the last paper of a stack and the first paper of the next stack. In response, a shift motor  88  (see  FIG. 2 ) is energized to shift the tray  1  or  2  in the direction of thrust, i.e., the direction perpendicular to the direction of paper discharge in a horizontal plane, preparing the tray for the next stack of papers. Consequently, consecutive paper stacks are offset from each other on the tray  1  or  2 . 
   The essential part of the mechanism for moving the tray  2  up and down in the illustrative embodiment will be described with reference to  FIGS. 4 and 55 . As shown in  FIG. 4 , the output power of the motor  51  is transferred to a gear  64  mounted on the drive shaft  41   a  via a worm wheel  60  and an intermediate gear  62 . The mechanism includes a safety measure for coping with the unusual downward movement of the tray  2 , as follows. As shown in  FIG. 55 , a gear  66  coaxial with the worm wheel  60  is positioned at the rear of the worm wheel  60 . The worm wheel  60  is held in mesh with the worm gear  58  by a spring  68 . One of the worm wheel  60  and gear  66  is formed with a recess while the other of them is formed with a lug. The recess and lug are capable of meshing with each other in the direction of rotation, but capable of separating from each other in the axial direction. The recess and lug allow the worm wheel  60  and gear  66  to mesh with each other and rotate in synchronism so long as the torque remains in a preselected range. 
   When the tray  2  moves downward in an unusual manner or loaded with an excessive number of papers, the above recess and lug move away from each other with the result that the worm wheel  60  moves to a position indicated by a dash-and-dots line in  FIG. 55  against the action of the spring  68 . Consequently, the worm gear  58  and worm wheel  60  are released from each other, causing the tray  2  to stop moving. Such a mechanism is also applied to the other tray  1 . 
   A tray control procedure particular to this embodiment will be described hereinafter. In the illustrative embodiment, the home position of the tray  1  is a position that the upper end of the end fence  1   a  reaches when raised by a preselected distance after being sensed by the sensor SN 6 . The home position of the tray  2  is a position where the sensor SN 7  senses the tray  2 . On the power-up of the copier G, the controller  100  determines whether or not the sensor SN 7  is in an ON state. If the sensor SN 7  is in an OFF state, meaning that it has not sensed the tray  2 , the controller  100  lowers the tray  2  via the motor  51 , determining that the tray  2  is positioned above the lower limit position. The controller  100  stops lowering the tray  2  when the sensor SN 7  senses the tray  2 . 
   Subsequently, the controller  100  determines whether or not the sensor SN 5  responsive to the retracted position is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  once raises the tray  1  via the motor  50  and then stops the tray  1  as soon as the sensor SN 5  senses it. The controller  100  again lowers the tray  1  until the sensor SN 6  senses the upper end of the end fence  1   a,  then raises the tray  1  by a preselected distance, and then stops it. 
   Assume that the operator selects the tray  1  on the copier G or the computer connected thereto. Then, the controller  100  first determines whether or not the sensor SN 7  is in an ON state. If the sensor SN 7  is in an OFF state, the controller  100  lowers the tray  2  via the motor  51 , determining that the tray  2  is positioned above the lower limit position. The controller  100  stops the movement of the tray  2  when the sensor SN 7  senses the tray  2 . Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  once raises the tray  1  via the motor  60  and then stops the tray  1  when the sensor SN 5  senses it. The controller again lowers the tray  1  until the sensor SN 6  senses the upper end of the end fence  1   a.  Thereafter, the controller  100  raises the tray  1  by a preselected amount. 
   When the sensor SN 5  is in an ON state, the controller  100  lowers the tray  1  until the sensor SN 6  senses the upper end of the end fence  1   a.  Subsequently, the controller  100  raises the tray  1  by a preselected distance and then stops it. In this condition, papers are sequentially stacked on the tray  1 . 
   When the tray  2  is selected on the copier G or the computer connected thereto, the controller  100  first determines whether or not the sensor SN 5  is in an ON state. If the sensor SN 5  is in an OFF state, the controller  100  raises the tray  1  until the sensor SN 5  senses it. Subsequently, the controller  100  raises the tray  2  until the sensor SN 6  senses it, and then lowers the tray  2  by a preselected distance. In this condition, papers are sequentially stacked on the tray  2 . Every time the top of the stack on the tray  2  is sensed by the sensor SN 6 , the controller  100  lowers the tray  2  by a preselected distance in order to stack a great number of papers on the tray  2 . The controller  100  determines that the tray  2  is full when the sensor SN 7  senses the tray  2  and when the sensor SN 6  senses the top of the stack. 
   The arrangement including the roller support member  84 ,  FIG. 54 , and the safety measure unique to the illustrative embodiment will be described more specifically. As shown in  FIG. 56 , the roller support member  84  is rotatable up and down about a fulcrum  84   a.  The driven roller  8   a  is pressed against the drive roller or outlet roller  8  due to its own weight and the weight of the roller support member  84 . The underside  84  of the roller support member  84  serves as a paper guide and forms an outlet path  92  in cooperation with a guide  90  associated with the drive roller  8 . 
   As shown in  FIG. 57 , when papers are discharged in the form of a stack, the roller support member  84  is angularly moved in accordance with the thickness t of the stack. As a result, the driven roller  8   a  is moved away from the drive roller  8 . 
   When, the roller support member  84  moves upward over an angle slightly greater than one corresponding to the maximum thickness t of papers or paper stack, the roller support member  84  contacts the microswitch  86  and turns it off. In the illustrative embodiment, the maximum thickness t is assumed to be the thickness of a stapled stack of fifty papers. It follows that when the operator&#39;s hand or similar object whose thickness is greater than the thickness t is put between the roller  8  and  8   a,  the microswitch  86  turns off. 
   Specifically, as shown in  FIG. 58 , a diode  92  is connected in parallel between the motor  50  assigned to the tray  1  and the microswitch  86 . When the thickness of the papers or paper stack discharged is less than t, the microswitch  86  remains in its ON state. In this condition, the tray  1  is movable up and down, as needed. 
   Assume that an object having a thickness greater than the thickness t is put between the rollers  8  and  8   a,  moving the roller support member  84  by more than the preselected angle corresponding to the thickness t. Then, as shown in  FIG. 59 , the upper surface of the roller support member  84  presses the contact of the microswitch  86  and thereby turns off the microswitch  86 . As a result, a current stops flowing through the elevation side of a motor driver, causing the tray  1  to stop rising. 
   More specifically, as shown in  FIG. 60 , when the operator&#39;s hand or similar object  94  is put between the rollers  8  and  8   a  while the tray  1  is retracting upward from the paper discharge position, the tray  1  stops rising. This protects the operator from injury and protects the finisher from damage ascribable to the object and tray  1  otherwise hitting against each other. 
   As stated above, the illustrative embodiment achieves the following unprecedented advantages. 
   (1) When the thickness of papers or paper stack discharged by the outlet roller pair is greater than the preselected thickness, the tray is inhibited from moving, e.g., upward. This protects the operator from injury and protects the finisher from damage. 
   (2) Because the roller support member has a paper guide surface, an extra paper guide is not necessary. 
   (3) The switch means is actuated at a position exceeding the thickness that the finisher itself can discharge. It follows that optimal safety matching with the finisher is achievable. 
   Eighth Embodiment 
   This embodiment is directed mainly toward the eighth object stated earlier. This embodiment is identical with the seventh embodiment as to the shift mode operation and the construction and movement of the tray  2 . As shown in  FIGS. 61-63 , this embodiment differs from the previous embodiments as to the positions of the sensors SN 5  and SN 7 . Again, the tray  1  expected to retract upward away from the outlet  2  includes the end fence  1   a  in order to obviate the need for a sophisticated shutter mechanism otherwise arranged in the outlet E 2 . 
   A tray control procedure unique to the eighth embodiment will be described hereinafter. As shown in  FIG. 9 , the home position of the tray  1  is a position where the sensor SN 5  responsive to the retracted position senses the upper end of the end fence  1   a.  The home position of the tray  2  is a position where the sensor SN 9  responsive to the retracted position senses the lower rear end (lower end hereinafter) of the tray  2  in the direction of paper discharge. 
     FIGS. 64 and 65  demonstrate how the controller  100  locates the trays  1  and  2  at their home positions. The controller  100  may cause the trays  1  and  2  to start moving at the same time, if desired. As shown, on the power-up of the copier G, initialization begins (step S 1 ). The controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 2 ). If the sensor SN 7  is in an OFF state (NO, step S 2 ), the controller  100  lowers the tray  2  via the motor  51  (step S 3 ), determining that the tray  2  is positioned above the lower limit position. Then, the controller  100  determines whether or not the sensor SN 9  is in an ON state (step S 4 ). If the sensor of the step S 4  is YES, the controller  100  stops moving the tray  2  (step S 5 ) and again raises it (step S 6 ). Subsequently, the controller  100  determines whether or not the sensor SN 9  is in an OFF state (step S 7 ), and stops the tray  2  (step S 8 ) when the sensor SN 9  turns off (YES, step S 7 ). As a result, the lower end of the tray  2  is located at the stand-by position or home position to which the sensor SN 9  is responsive. 
   If the answer of the step S 2  is YES, meaning that the tray  2  is located at the lower limit position, the controller  100  raises the tray  2  (step S 9 ) and determines whether or not the sensor SN 9  turns on (step S 10 ). When the sensor SN 9  turns on (YES, step S 10 ), meaning that it senses the upper end of the tray  2 ), the controller  100  continuously determines the status of the sensor SN 9  (step S 11 ). As soon as the sensor SN 9  turns off (YES, step S 11 ), the controller  100  stops raising the tray  2 . Consequently, the lower end of the tray  2  is located at the stand-by position. 
   When the tray  2  is located between the lower limit position and the stand-by position, i.e., if the sensor SN 9  is in an OFF state in the step S 4 , the controller  100  determines whether or not the sensor SN 7  is in an ON state (step S 13 ). If the answer of the step S 13  is YES, the controller stops the tray  2  (step S 14 ) and then raises it (step S 15 ). Subsequently, the controller  199  determines whether or not the sensor SN 9  is in an ON state (step S 16 ). If the answer of the step S 16  is YES, meaning that the sensor S 16  has sensed the upper end of the tray  2 , the controller determines the status of the sensor SN 9  (step S 17 ). When the sensor SN 9  turns off (YES, step S 17 ), the controller  100  stops moving the tray  2  (step S 18 ). Consequently, the lower end of the tray  2  is located at the stand-by position. 
   After the step S 18 , the controller  100  determines whether or not the sensor SN 5  is in an ON state (step S 19 ). If the answer of the step S 19  is NO, the controller  100  raises the tray  1  via the motor  50  (step S 20 ) and continuously determines the status of the sensor SN 5  (step S 21 ). When the sensor SN 5  turns on (YES, step S 21 ), the controller  100  stops moving the tray  1  (step S 22 ). 
   Reference will again be made to  FIG. 12  showing a specific condition wherein papers are sequentially stacked on the tray  2  while the tray  1  is held in its retracted position. The position of the tray  2  for receiving the papers via the outlet E 2  is coincident with the position where the sensor SN 6  secures the upper end of the tray  2  or the top of papers stacked thereon. As shown in  FIG. 13 , when papers are sequentially stacked on the tray  1  while the tray  2  is held in its retracted position, the tray  2  is held in its lower limit position in order to prevent papers stacked thereon from contacting the tray  1 . 
   In the condition shown in  FIG. 12 , when the sensor SN 6  senses the top of sheets stacked on the tray  2 , the controller  100  lowers the tray  2  by a preselected distance. The controller  100  repeats this operation when a great number of papers are stacked on the tray  2 . The controller  100  determines that the tray  2  is full when the sensor SN 9  senses the lower end of the tray  2  and when the sensor SN 6  senses the top of papers stacked on the tray  2 , as shown in FIG.  14 . 
   The controller  100  detects the full state of the tray  2  when the tray  2  is positioned above the lower limit position, so that the tray can be switched from the tray  2  to the tray  1  without the papers being removed from the tray  2 . In the illustrated embodiment, the sensor SN 9  responsive to the stand-by position serves to sense the full state of the tray  2  at the same time.  FIG. 15  shows a specific condition wherein the full tray  2  is retracted to its lower limit position while the tray  1  is brought to the outlet E 2 . 
   The full tray  2  must be retracted by an amount great enough for the tray  1  to be located at the position for receiving papers from the outlet E 2 . Therefore, as shown in  FIG. 14 , the above amount is determined by the amount of papers that can be stacked on the tray  1 , i.e., the height H 1  of the end fence  1   a.  More specifically, if the sensor SN 9  is positioned above the sensor SN 7  by a distance H (between the stand-by position and the lower limit position) greater than the height H 1 , the sensor SN 9  can play the role of a tray  2  full sensor (full sensing means) and a stand-by position sensor at the same time. However, the prerequisite is that the distance H 1  between the sensors SN 6  and SN 9  (overall height of the full tray  2  including papers) be greater than or equal to the distance H. 
   Assume that papers should be discharged to the tray  1  when the trays  1  and  2  each are held in the respective home position. Then, as shown in  FIGS. 66 and 67 , the controller  100  lowers or retracts the tray  2  (step S 1 ). When the sensor SN 7  responsive to the lower limit position turns on (YES, step S 2 ), the controller  100  stops lowering the tray  2  (step S 3 ) and lowers the tray  1  (step S 4 ). Subsequently, when the sensor SN 6  turns on (YES, step S 5 ), meaning that it has sensed the lower end of the tray  1 , the controller  100  continuously determines the status of the sensor SN 6 . When the sensor SN 6  turns off (YES, Step S 6 ), the controller  100  stops lowering the tray  1  (step S 7 ). 
   After the step S 7 , the controller  100  determines whether or not the sensor SN 9  is in an OFF state (step S 8 ). If the answer of the step S 8  is YES, the controller  100  raises the tray  2  to the retracted position (step S 9 ), determining that the number of papers on the tray  2  is small. As soon as the sensor SN 9  turns on, i.e., senses the upper end of the tray  2  (YES, step S 10 ), the controller  100  continuously determines the status of the sensor SN 9  (step S 11 ). When the sensor SN 9  turns off (YES, step S 11 ), the controller stops raising the tray  2  (step S 12 ). As a result, the lower end of the tray  2  is located at the retracted position. 
   When the tray  2  is selected in place of the tray  1  later, the tray  2  moves from the above stand-by position closer to the outlet E 2  than the lower limit position, or original retracted position, to the outlet E 2 . This reduces a period of time necessary for the tray  2  to reach the outlet E 2 . 
   Assume that the number of papers stacked on the tray  2  is small when papers are being discharged to the tray  1 . In this condition, the trays  1  and  2  must be prevented from colliding with each other even when the tray  2  is raised to the position where the sensor SN 9  senses the lower end of the tray  2  (stand-by position). To meet this requirement, the illustrative embodiment is so configured as to satisfy relations of H 3 ≧H 1  and H 1 ≧H 2 +H 3 , as shown in FIG.  68 . 
   Reference will be made to  FIGS. 69 and 70  for describing a tray control procedure to be executed when the trays  1  and  2  each are held at the respective home position, when papers should be discharged to the tray  1 , and when papers are removed from the tray  2 . As shown, the controller  100  lowers or retracts the tray  2  (step S 1 ). As soon as the sensor SN 7  senses the tray  2  and turns on (YES, step S 2 ), the controller  100  stops lowering the tray  2  (step S 3 ) and lowers the tray  1  (step S 4 ). Subsequently, the controller  100  determines whether or not the sensor SN 6  is in an ON state (step S 5 ). If the answer of the step S 5  is YES, the controller  100  continuously determines the status of the sensor SN 6  (step S 6 ). When the sensor SN 6  turns off (YES, step S 6 ), the controller  100  stops lowering the tray  1  (step S 7 ). 
   Subsequently, the controller  100  determines whether or not the sensor SN 9  is in an OFF state (step S 8 ). As shown in  FIG. 71 , when the papers are removed from the tray  2 , the sensor SN 9  turns off. If the answer of the step S 8  is YES, the controller  100  raises the tray  2  so as to use the stand-by position as the retracted position (step S 9 ), determining that the number of papers on the sheet  2  is small. When the sensor SN 9  turns on (YES, step S 10 ), the controller continuously determines the status of the sensor SN 9  (step S 11 ). When the sensor SN 9  turns off (YES, step S 11 ), the controller  100  stops raising the tray  2  (step S 12 ). Consequently, the lower end of the tray  2  is located at the stand-by position, as shown in FIG.  72 . 
   When the tray  2  is selected in place of the tray  1  later, the tray  2  moves from the above stand-by position closer to the outlet E 2  than the lower limit position, or original retracted position, to the outlet E 2 . This reduces a period of time necessary for the tray  2  to reach the outlet E 2 . 
   While the above embodiment includes a single tray  1 , it is similarly practicable with a plurality of trays  1 . The finisher may, of course, be constructed integrally with the copier G or similar image forming apparatus. If desired, the number of papers stacked on the tray  2  may be calculated by using the thickness of each paper and the number of papers. 
   As stated above, the illustrative embodiment achieves various advantages, as enumerated below. 
   (1) When the number of papers stacked on the lower tray is small, the stand-by position of the lower tray above the lower limit position is used as the retracted position. This reduces the period of time necessary for the lower tray to move to the outlet and thereby enhances rapid operation. 
   (2) The stand-by position sensing means determines the number of papers stacked on the lower tray. The decision is therefore easy and accurate. 
   (3) When the stand-by position sensing means assigned to the lower tray turns off due to the removal of papers from the lower tray, the stand-by position is used as the retracted position. This also reduces the period of time necessary for the lower tray to reach the outlet. 
   (4) When the retracted position of the lower tray is used as the stand-by position, the upper and lower trays are surely prevented from colliding with each other. 
   (5) The stand-by position sensing means plays the role of the full sensing means at the same time and therefore eliminates the need for extra full sensing means which would sophisticate the construction and increase the cost. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.