Abstract:
An image forming apparatus including: an image forming unit; plural trays; a sheet feed unit that feeds a sheet held in one of the trays; a correspondence storing unit that stores correspondence between trays and kinds of sheets; a priority setting unit that sets priority to a designation of the tray or to a designation of the kind; a sheet feed control unit that feeds the sheet from one of the trays when the priority is given to the designation of the tray and the one of the trays is designated, and searches one of the trays that holds the sheet of the designated kind based on the correspondence to feed the sheet from the searched tray when the priority is given to the designation of the kind and the kind of the sheet is designated; and a setting change unit that changes a setting of the image forming unit correspondingly to the kind of sheet.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2005-088261, filed on Mar. 25, 2005, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relate to an image forming apparatus including a plurality of trays, for forming an image on a sheet held in any one of the trays, an image forming system including the image forming apparatus and an upper apparatus and a program product for controlling the image forming apparatus or the image forming system. 
     BACKGROUND 
     An image forming apparatus including an image forming unit for forming an image on a sheet, a plurality of trays each holding sheets on which the image is formed by the image forming unit, and a feed unit for feeding sheets held in any one of the trays to the image forming unit has been conventionally proposed. In such image forming apparatus, it is possible to form an image on a sheet by feeding sheets held in any one of the trays each holding sheets to the image forming unit by the feed unit. Further, in such image forming apparatus, there has been proposed that a unit for detecting the size and kind of sheet held in a tray is provided and, when the size and kind of sheet is designated by an upper apparatus, an image is formed by automatically selecting the tray holding sheets of the designated size and the kind and feeding the sheet (for example, see JP-A-2-182632). 
     SUMMARY 
     When both of the size and kind of sheet are designated and there is no sheet of the designated size and kind, the apparatus disclosed in JP-A-2-182632 enters into a sleep mode. Therefore, in order to form an image, there may be a case where the designation of size and kind of sheet has to be made repeatedly, resulting in degraded operability. The present invention provides an image forming apparatus or an image forming system capable of smoothly forming an image even when both the tray and the kind of sheet are designated and there is no sheet satisfying the designation of the tray and the kind. 
     According to an aspect of the invention, there is provided an image forming apparatus including: an image forming unit that forms an image on a sheet; a plurality of trays each for holding the sheet on which the image is formed by the image forming unit; a sheet feed unit that feeds the sheet held in one of the plurality of the trays; a correspondence storing unit that stores correspondence between trays and kinds of sheets held in the trays; a priority setting unit that sets priority to a designation of the tray or to a designation of the kind; a sheet feed control unit that causes the sheet feed unit to feed the sheet from one of the trays when the priority is given to the designation of the tray by the priority setting unit and the one of the trays is designated, and searches one of the trays that holds the sheet of the designated kind on the basis of the correspondence between trays and kinds of sheets stored in the correspondence storing unit to cause the sheet feed unit to feed the sheet from the searched tray when the priority is given to the designation of the kind by the priority setting unit and the kind of the sheet is designated; and a setting change unit that changes a setting of the image forming unit correspondingly to the kind of sheet fed by the sheet feed control unit. 
     In the aspect constructed as described above, the correspondence storing unit stores the trays and the kinds of sheets held in the respective trays by bringing them into connection. It is possible to set the tray priority or the sheet kind priority by the priority setting unit. The sheet feed control unit causes the sheet feed unit to feed sheets from one of the trays when the priority of the tray is set by the priority setting unit and the one tray is assigned. In this case, since the setting change unit changes the setting of the image forming unit according to the kind of sheet fed by the sheet feed control unit, it is possible to feed the sheets from the assigned tray and to form a good image on the sheet by the image forming unit. 
     Further, when the sheet kind priority is set by the priority setting unit and the sheet kind is assigned, the sheet feed control unit searches one of the trays, which holds sheets of the assigned kind of sheet, on the basis of the correspondence between trays and sheet kinds stored in the correspondence storing unit and, when the one tray is searched, feeds the sheets from the searched tray. In this case, since the setting change unit changes the setting of the image forming unit according to the kind of sheet fed by the sheet feed control unit, it is also possible to form a good image on the assigned sheet. 
     As mentioned, it is possible in the present invention to select sheets based on the assignment set by the priority setting unit and to form an image on the sheets by the setting according to the sheet kind. Therefore, according to the present invention, it is possible to prevent the image forming apparatus from being fallen in the sleep state unnecessarily. Further, since the setting of the image forming unit is changed according to the sheet kind, it is also possible to form a good image according to the kind of the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects of the invention may be more readily described with reference to the accompanying drawings: 
         FIG. 1  is a schematic cross sectional view showing a construction of a laser printer according to an aspect of the present invention; 
         FIG. 2  is a perspective view showing an outer configuration of the laser printer; 
         FIG. 3  is a perspective view showing a construction of an optional tray of the laser printer; 
         FIG. 4  is a perspective view showing a construction of a light emitting element and a light receiving element, etc., of the optional tray; 
         FIG. 5  is a block diagram showing a control system of the laser printer and a control system of a personal computer as an upper apparatus; 
         FIG. 6  shows a correspondence setting screen in the laser printer; 
         FIG. 7  shows a priority order setting screen in the laser printer; 
         FIG. 8  shows a setting screen in the personal computer; 
         FIG. 9  shows a priority order setting screen in the personal computer; 
         FIG. 10  is a flowchart showing a processing executed by the control system of the laser printer; and 
         FIG. 11  is a flowchart showing a processing executed by the personal computer in another aspect. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described with reference to aspects of the present invention. Incidentally, the present invention is not limited to the aspects to be described and may be modified in various manners within the technical scope of the present invention.  FIG. 1  is a schematic cross sectional view of a laser printer  1  as an image forming apparatus to which the present invention is applied and  FIG. 2  is a perspective view showing an outer configuration of the laser printer  1 . 
     [Construction of Laser Printer  1 ] 
     As shown in  FIG. 1 , the laser printer  1  of this aspect includes a main casing  2 , in which a feeder  4  for feeding sheets  3  on which images are recorded, a multi-purpose tray  14 , a process unit  18  for forming an image on a fed sheet  3  and a fixing unit  19 , etc. The laser printer  1  has an optional tray  41  provided below the feeder  4 . The optional tray  41  can be added optionally. In the following description, a side (left side in  FIG. 1 ) of the main casing  2  of the laser printer  1 , on which the multi-purpose tray  14  is mounted, is referred to as a “front side” and the other side of the main casing  2  is referred to as a “rear side”. 
     [Construction of Feeder  4 ] 
     As shown in  FIG. 1 , the feeder  4  includes a sheet feed tray  6  detachably mounted in the bottom of the main casing  2 , a sheet urging plate  8  provided within the sheet feed tray  6 , a feed roller  12  and a separating pad  13 , which are provided above one end portion of the sheet feed tray  6 . Further, a curved transporting path  7  extends from the feed roller  12  to an image forming position P (a contact portion between a photosensitive drum  23  and a transfer roller  25 , that is, a transferring position on the photosensitive drum  23  at which a toner image on the drum is transferred to the sheet  3 ). 
     A rear end portion of the sheet urging plate  8 , which can hold a stack of sheets  3 , is swingably supported and a front end thereof in the vicinity of the sheet feed roller  12  is vertically movable. The sheet urging plate  8  is urged upward by a spring  8   a . The separating pad  13  is provided below and against the sheet feed roller  12  and has a pad  13   a  formed by a member having large coefficient of friction. The pad  13   a  is urged against the sheet feed roller  12  by a spring  13   b.    
     Incidentally, the pad  13   a  and the sheet feed roller  12  have widths in a direction perpendicular to the transporting direction of the sheet  3  smaller than the width of the sheet  3  and are arranged such that the pad  13   a  and the sheet feed roller  12  are in contact with only substantially a center portion in the width direction of the sheet  3  during a sheet feed operation (see  FIG. 4 ). 
     The transporting path  7  is formed by a pair of guide plates  7   a  and  7   b  for guiding the surfaces of the sheet  3 . Along the transporting path  7 , the sheet feed roller  12 , a pair of transporting rollers  11  including a drive roller and a driven roller, a pair of transporting rollers  10  including a drive roller and a driven roller and a pair of registration rollers  9  including a drive roller and a driven roller and provided immediately before the image forming position P are arranged from an upstream side of the transporting path to a downstream side with appropriate intervals. 
     In the feeder  4  constructed as described above, an uppermost sheet  3  of the sheets  3  stuck on the sheet urging plate  8  is pressed toward the feed roller  12 , pinched between the rotating feed roller  12  and the separating pad  13  so that the sheets  3  are fed one by one. The sheet  3  thus fed is transported sequentially by the transporting rollers  11  to the transporting rollers  10  and the registration rollers  9  and sent to the image forming position P after the sheet  3  is subjected to the certain registration. 
     [Construction of Multi-Purpose Tray  14 ] 
     On the front side of main casing  2  above the feeder  4 , the multi-purpose tray  14  for feeding the sheet  3  by hand or automatically, and a sheet feed mechanism  15  for feeding a stack of sheets  3  on the multi-purpose tray  14  are provided. The sheet feed mechanism  15  includes a feed roller  15   a  and a feed pad  15   b . The feed pad  15   b  is urged against the multi-purpose feed roller  15   a  by a spring  15   c  provided on the back of the feed pad  15   b . The multi-purpose tray  14  includes a pair of transporting rollers  16  including a drive roller and a driven roller. 
     In the multi-purpose tray  14  constructed as described above, the sheets  3  stuck on the tray  14  are pinched between the rotating feed roller  15   a  and the feed pad  15   b  one by one and sent to the registration rollers  9  through the transporting rollers  16 . 
     [Construction of Scanner Unit  17 ] 
     A scanner unit  17  is provided below a discharge tray  36  provided in an upper portion of the main casing  2 . The scanner unit  17  includes a laser light emitter (not shown), a polygon mirror  20 , which is rotated, lenses  21   a  and  21   b  and a reflection mirror  22 . Laser beam containing an image data and emitted by the laser emitter is reflected and transmitted through the polygon mirror  20 , the lens  21   a , the reflection mirror  22  and the lens  21   b  and scans the surface of the photosensitive drum  23  in the process unit  18  at high speed. 
     [Construction of Process Unit  18 ] 
     The process unit  18  is constructed with a drum cartridge, which includes the photosensitive drum  23  as an electrostatic latent image carrier, a scorotron type electrostatic charger  37  and a transfer roller  25  as a transfer unit and a developer cartridge  24 . The developer cartridge  24  includes a toner holder  26 , a developing roller  27  as a developing unit, a layer thickness control blade  28  and a toner supply roller  29 , etc. 
     The toner holder  26  is filled with non-magnetic, positively chargeable mono-component polymer toner as the developer. The toner is supplied to the developing roller  27  by the toner supply roller  29  while being charged positively by friction between the rollers  27  and  29 . Toner supplied on the developing roller  27  forms a thin layer, thickness of which is regulated to a constant value by the thickness control blade  28 . The photosensitive drum  23  is arranged oppositely to the developing roller  27  and electrically grounded. On the photosensitive drum  23 , a positively chargeable photosensitive layer of an organic photosensitive material such as polycarbonate is formed. 
     In this laser printer  1 , residual toner on the surface of the photosensitive drum  23  after a toner image is transferred to the sheet  3  by the transfer roller  25  is recovered by the developing roller  27 . Since this recovering system, which is so-called “cleaner-less system”, recovers the residual toner on the surface of the photosensitive drum  23 , it does not require a cleaner such as a cleaning blade and a reservoir of used toner, and therefore it is possible to make the device simple and reduce the size and cost of the device. 
     The scorotron type electrostatic charger  37  is arranged above the photosensitive drum  23  with a predetermined gap with respect to the drum  23  such that it is not in contact with the drum. The scorotron type electrostatic charger  37 , which generates corona discharge by charging wires of such as tungsten, is constructed such that the surface of the photosensitive drum  23  is positively charged uniformly. 
     With rotation of the photosensitive drum  23 , the surface thereof is positively charged uniformly by the scorotron type charger  37  and exposed by high speed scanning of laser beam from the scanner unit  17 , resulting in formation of the electrostatic latent image based on the image data. 
     Thereafter, with the rotation of the developing roller  27 , positively charged toner on the developing roller  27  is supplied onto the electrostatic latent image formed on the photosensitive drum  23 , which is in contact with the developing roller  27 , that is, onto a portion of the positively charged surface of the photosensitive drum  23 , potential of which is lowered by the exposure with the laser beam, resulting in formation of a visible toner image. 
     The transfer roller  25  is arranged below the photosensitive drum  23  and opposes thereto. The transfer roller is supported by the drum cartridge rotatably in clockwise direction in  FIG. 1 . The transfer roller  25  is constructed with a metal roller shaft and a roller, which is formed of an ion-conductive rubber and fitted on the roller shaft, such that it is biased by a transfer biasing power source during the transfer operation. Therefore, the toner image on the surface of the photosensitive drum  23  is transferred onto the sheet  3  in the image forming position P during the sheet  3  passes between the photosensitive drum  23  and the transfer roller  25 . 
     [Construction of Fixing Unit  19 ] 
     The fixing unit  19  is arranged on a downstream side of the transporting direction with respect to the process unit  18 , as shown in  FIG. 1 , and includes a heating roller  31 , a pressing roller  32  provided to urge the heating roller  31  and a pair of transporting rollers  33  provided on a downstream side of the pressing roller  32 . The heating roller  31  is formed of metal such as aluminum and includes a heater such as a halogen lamp. The heating roller  31  functions to thermally fix toner transferred onto the sheet  3  in the process unit  18  during the sheet  3  passes between the heating roller  31  and the pressing roller  32 . Thereafter, the sheet  3  is transported by the transporting rollers  33 , transporting rollers  34  and discharge rollers  35  along a discharge path formed in the rear portion of the main casing  2  and discharged onto the discharge tray  36 . Incidentally, the scanner unit  17 , the process unit  18  and the fixing unit  19  function as the image forming unit. 
     [Construction of Optional Tray  41 ] 
     An internal construction of the optional tray  41 , which is optionally provided below the feeder  4 , is substantially the same as that of the feeder  4 . That is, the optional tray  41  includes an optional sheet feed tray  46  detachably provided in a bottom portion of the optional tray  41 , a sheet urging plate  48  having one end swingably supported by the optional sheet feed tray  46  and urged by a spring  48   a , a sheet feed roller  52  provided above the other end of the sheet urging plate  48  and a separating pad  53 . The separating pad  53  is arranged oppositely to the sheet feed roller  52  and has a pad  53   a  urged to the sheet feed roller  52  by a spring  53   b.    
     A passage  6   b , which is capable of passing a sheet  3  vertically between a swinging range of the sheet urging plate  8  and a handle  6   a  on the front side of the sheet feed tray  6 , is provided in the sheet feed tray  6 , which is attached to the feeder  4 . Further, a pair of transporting rollers  54  including a drive roller and a driven roller are provided below the passage  6   b . Therefore, when the sheet feed tray  6  of the feeder  4  is put in a normal position (position at which the sheet  3  can be fed to the transporting path  7  by the sheet feed roller  12 ), the sheets  3 , which are supplied one by one by the sheet feed roller  52  of the optional tray  41 , are transported by the transporting rollers  54  and, thereafter, sent to the transporting passage  7  through the passage  6   b  by the transporting rollers  11 . 
     As shown in  FIG. 3 , a connector  55  and pins  56  are provided in an upper surface of the optional tray  41 . When the optional tray  41  is mounted below the feeder  4 , the connector  55  is electrically connected to a CPU  61  ( FIG. 5 ) of the laser printer  1 , which is provided in the main casing  2 . The pins  56  are provided for positioning the optional tray  41  with respect to the main casing  2 . Incidentally,  FIG. 3  is a perspective view showing the construction of the optional tray  41 , with the sheet urging plate  48 , the sheet feed roller  52  and the separating pad  53  being removed. 
     Further, as shown in  FIGS. 3 and 4 , a light emitting element  57  and a light receiving element  58  are provided in the upper surface of the optional tray  41  provided above the handle  46   a  of the optional tray  46 . That is, a shutter  6   c  for blocking an optical path from the light emitting element  57  to the light receiving element  58  when the sheet feed tray  6  is positioned in the normal position shown in  FIG. 4  is provided in a rear surface (on the rear portion side) of the handle  6   a  of the sheet feed tray  6 . Therefore, when the optional tray  41  is added, it is possible to know that the tray  6  is positioned in the normal position be determining whether light emitted by the light emitting element  57  is received by the light receiving element  58 . 
     [Control System of Laser Printer  1 ] 
     Now, a control system of the laser printer  1  will be described with reference to  FIG. 5 . As shown in  FIG. 5 , this control system includes a CPU (Central Processing Unit)  61  for executing various operations, a ROM (Read Only Memory)  62  for storing control programs and a RAM (Random Access memory)  63 , which backs up in order to prevent content thereof from being erased when a power switch is OFF and stores various data temporarily. The CPU  61  can receive a printing job and an image forming instruction from a personal computer  70  as an upper apparatus through a communication circuit and an input/output interface (input output I/F)  64 . 
     The personal computer  70  has a general construction including a main body  75 , which includes a CPU  71 , a ROM  72 , a RAM  73  and a hard desk drive (HDD)  74 , and a display  76 , a keyboard  77  and a mouse  78 , which are connected to the main body  75 . 
     An input button  90  ( FIG. 2 ) including a plurality of arrow keys, execution keys and canceling keys, etc., and a display  91 , which are provided on a surface of the main casing  2 , are further connected to the CPU  61  of the laser printer  1 . Further, a main motor  94  for driving the photosensitive drum  23  and the various rollers is connected to the CPU  61  through a drive circuit  93  and a first clutch  95 , a second clutch  96  and a third clutch  97  for transmitting driving force of the main motor  94  to the sheet feed roller  12 , the multi-purpose side sheet feed roller  15   a  or the optional sheet feed roller  52  are also connected to the CPU  61 . As previously mentioned, the third clutch  97  is connected to the CPU  61  when the connectors  55  are connected thereto. 
     [Printing Processing] 
     Now, the printing processing to be executed by the control system of the laser printer  1  will be described. In order to start the printing processing, a user displays a correspondence setting screen shown in  FIG. 6  on the display  91  by operating the input button  90  and sets the kinds of sheets  3  held in the respective trays  6 ,  14  and  46  by operating the input button  90 . It is possible to arbitrarily change the kind of sheet corresponding to one of the trays  6 ,  14  and  46  by operating such an arrow key after a cursor is pointed to one of the trays displayed on the display  91 . Further, this correspondence displayed on the display  91  is stored in a predetermined memory area of the RAM  63  as needed. 
     Incidentally, though various kinds of sheets such as plain sheet, thin sheet, thick sheet, envelope and OHP sheet can be set to each of the trays  6 ,  14  and  46  in the correspondence setting screen, it is possible to set the thick sheet in only the multi-purpose tray  14 . 
     Further, the user displays a priority order setting screen shown in  FIG. 7  on the display  91  by operating the input button  90  and sets one of the trays or the kind of sheet to be printed (image-formed) first by operating the input button  90 . In this priority order setting screen, one of three ways, that is, the printing in the sheet kind priority, in the tray priority and without priority setting, can be arbitrarily selected. The priority order displayed on the display  91  is stored in a predetermined memory area of the RAM  63  as needed. 
     In transmitting printing data from the personal computer  70  by the user, the user displays a setting screen shown in  FIG. 8  on the display  76  and sets the tray holding the sheets  3  on which the printing data is to be printed and the kind of sheet on which the printing data is to be printed. The tray and the sheet kind set in the screen are stored in a predetermined memory area of the RAM  73  by operating the keyboard  77  or the mouse  78 . The setting of the tray and the sheet kind is transmitted to the laser printer  1  and stored in the predetermined memory area of the RAM  63 . 
     Further, the user displays the priority order setting screen shown in  FIG. 9  on the display  76  and sets one of the tray and the sheet kind as to be printed first. In this priority order setting screen, one of three ways, that is, the printing in the sheet kind priority, in the tray priority and without priority setting, can be arbitrarily selected. The priority order displayed on the screen is stored in a predetermined memory area of the RAM  73  by operating the keyboard  77  or the mouse  78 . Incidentally, the priority order set on the personal computer  70  by the user may be transmitted to the laser printer  1  through an input/output I/F  64  and stored in a predetermined memory area of the RAM  63 . 
       FIG. 10  is a flowchart showing a processing to be executed by the CPU  61  of the laser printer  1  on the basis of the program stored in the ROM  62 . When the processing is started, a preset tray, for example, the main tray, is set in a predetermined memory area of the RAM  63  (S 1 ) as a variable A and then a preset sheet kind, for example, the plain sheet, is set in a predetermined memory area of the RAM  63  as a variable B (S 2 ). In the step S 3 , it is decided whether or not there is data received through the input/output I/F  64  and, when there is no data received (NO in S 3 ), the processing becomes standby state in the step S 3 . 
     When there is the data received (YES in S 3 ), the processing is shifted to the step S 4  in which it is decided whether or not the data indicates a selection of the tray. When the received data indicates the tray selection (YES in S 4 ), the processing is shifted to the step S 5  in which the tray set by the data, that is, the content set in the setting screen shown in  FIG. 8  in the personal computer  70  by the user, is set to the variable A and the processing is shifted to the step S 3 . 
     When the received data does not indicate the tray selection (NO in S 4 ), the processing is shifted to the step S 6  in which it is decided whether or not the received data indicates the sheet kind selection. When the received data indicates the sheet kind selection (YES in S 6 ), the processing is shifted to the step S 7  in which the sheet kind specified by the data, that is, the content set in the setting screen shown in  FIG. 8  in the personal computer by the user, is set to the variant B and the processing is shifted to the step S 3 . 
     When the received data does not indicate the tray selection or the sheet kind (NO in S 6 ), the processing shifts to S 8  in which it is decided whether or not the received data is the main portion of the printing data indicating the printing content. When the received data is the main portion of the printing data (YES in S 8 ), the processing is shifted to the step S 9  in which the printing image processing such as expansion of the printing data into bit map data is executed and then the processing is shifted to the step S 3 . 
     When the received data does not belong to any of these data (NO in S 8 ), it is decided in the step S 10  whether or not the received data is a page printing request. When the received data is the page printing request (YES in S 10 ), the printing is executed by the processing from the step S 15  and, when the received data is not the page printing request (NO in S 10 ), the processing is shifted to the step S 3 . 
     In the step S 15 , it is decided whether or not the priority order set in the screen shown in  FIG. 7  and stored in the RAM  63  is the tray priority. Incidentally, when the priority order set on the side of the personal computer  70  is stored in the RAM  63 , the decision may be made on the basis of the stored priority order. When it is tray priority (YES in S 15 ), it is decided in the step S 16  whether or not the sheet kind is designated in the screen shown in  FIG. 6  to the tray set as the variable A. When the sheet kind is designated (YES in S 16 ), the kind of sheet is set as the variable B in the step S 17 . When the kind is not designated (NO in S 16 ), the processing is shifted to the step S 18 . 
     In the step S 18 , the first clutch  95 , the second clutch  96  or the third clutch  97  is connected on the basis of the variable A and the sheet feeding processing is executed from the tray set as the variable A. Thereafter, in the step S 19 , the setting processing of the respective portions corresponding to the sheet kind of sheet set as the variable B is performed. For example, the fixing temperatures of the fixing unit  19  for OHP sheet, plain sheet and envelope are set higher in the order and an absolute value of transfer current to be supplied to the transfer roller  25  is larger for the envelope than that for the OHP sheet or the plain sheet. Incidentally, when the kind of sheet is not defined as in the case of the optional tray  46  shown in  FIG. 6 , the setting is performed according to a command indicated by the transmitted data or according to a preset value. In the step S 20 , the printing processing is executed according to the printing data and the processing is ended. 
     On the other hand, when the priority order set in the screen shown in  FIG. 7  is not the tray priority (NO in S 15 ), it is decided in the step S 21  whether or not the priority order stored in the RAM  63  is the sheet kind priority. Incidentally, when the priority order set on the side of the personal computer  70  is stored in the RAM  63 , the decision may be made on the basis of the priority order in the RAM  63 . When it is not the sheet kind priority (NO in S 21 ), the processing is shifted to the step S 18  and, when it is the sheet king priority (YES in S 21 ), the processing is shifted to the step S 22 . 
     In the step S 22 , it is decided on the basis of the correspondence set in the screen shown in  FIG. 6  whether or not there is a tray corresponding to the sheet kind set as the variable B at that time. When there is such sheet feed tray (YES in S 22 ), the processing is shifted to the step S 18  after the tray is set as the variable A in the step S 23 . When there is no such tray (NO in S 22 ), the processing is shifted to the step S 18 . Incidentally, there may be a case where a plurality of trays for one and the same sheets. In such case, one of these trays is selected according to the priority order of tray in the step S 23 . 
     In this aspect, when both the tray priority and the sheet kind priority are designated (S 5 , S 7 ), it is possible to feed sheets as follow. When the tray priority is set (YES in S 15 ), it is possible to feed sheets from the set tray and, when the sheet kind priority is set (YES in S 21 ), a tray holding the sheets  3  is searched and, when there is the tray holding the sheets  3  (YES in S 22 ), the sheets  3  are fed from the tray (S 18 ). The setting of the fixing temperature and the transfer current, etc., is changed correspondingly to the kind of sheets fed (S 19 ). According to this aspect, it is possible to properly select the sheets  3  on the basis of the priority order even when there is no designated tray and there is no designated kind of sheet. Therefore, it is possible to prevent the laser printer  1  from becoming the sleep state and to thereby improve the operability. In addition, since the image forming condition corresponding to the kind of selected sheet  3  is set, it is possible to obtain a high printing performance without errors such as fixing error. 
     Further, in this aspect, when only one of the tray and the sheet kind can be designated on the personal computer  70 , that is, for example, when an application software run on the personal computer  70  can designate only one of the tray and the sheet kind or when only one of the tray and the kind of sheet is designated, it is always possible to form the image on the basis of the tray or the sheet kind, which can be designated by the user, by setting the priority order on the side, in which the designation is possible or the designation is set, and to thereby improve the operability. 
     Incidentally, the main sheet feed tray  6 , the multi-purpose tray  14  and the optional sheet feed tray  46  function as the tray. The main feed roller  12 , the multi-purpose side feed roller  15   a  and the optional feed roller  52  function as the sheet feed unit. The processing related to the display on the screen shown in  FIG. 6  and the memory area of the ROM  62  storing the program for executing the processing function as the correspondence storing processing and the correspondence storing unit, respectively. The processing related to the display on the screen shown in  FIG. 7  and the memory area of the ROM  62  storing the program for executing the processing function as the priority setting processing and the priority setting unit, respectively. The processing in the steps S 15  to S 23  shown in  FIG. 10  and the memory area of the ROM  62  storing the program for executing the processing function as the sheet feed control processing and the sheet feed control unit, respectively. The processing in the step S 19  shown in  FIG. 10  and the memory area of the ROM  62  storing the program for executing the processing function as the setting change processing and the setting change unit, respectively. 
     The present invention is not limited to the above described aspect and can be modified in various manners within the scope of the present invention. For example, though the designation of the tray and the sheet kind is performed from the personal computer  70  in the described aspect, it can be made on the display  91  of the laser printer  1 . In such case, the input button  90  and the display  91  function as the tray designating unit and the sheet kind designating unit, respectively. Further, the table storing the correspondence shown in  FIG. 6  may be provided on the side of the personal computer  70 . In the step S 15 , the decision may be made on the basis of the priority order set in the screen on the personal computer  70  side shown in  FIG. 9 . Further, when the tray and the sheet kind are designated by the personal computer  70 , the processing for selecting the tray according to the designation may be executed on the side of the personal computer  70 . 
     [Other Embodiments of Printing Processing] 
       FIG. 11  shows a flowchart showing a processing of the personal computer  70  in another aspect of the present invention. When the execution of printing is instructed by the keyboard  77  or the mouse  78 , the CPU  71  executes this processing on the basis of the program stored in the ROM  72 . In this processing, the tray selected by the user in the screen shown in  FIG. 8  is acquired from the user setting and is set as the variable A (S 31 ) In the next step S 32 , the sheet kind selected by the user in the screen shown in  FIG. 8  is acquired from the user setting and is set as the variable B. 
     In the step S 33 , it is decided whether or not the priority order set in the screen shown in  FIG. 9  is the tray priority. When it is the tray priority (YES in S 33 ), it is decided in the step S 34  whether or not the sheet kind is designated from the screen similar to that shown in  FIG. 6  on the side of the personal computer  70  to the tray set as the variable A at that time. When the kind of sheet is designated (YES in S 34 ), the processing is shifted to the step S 36  after the sheet kind is set as the variable B in the step S 35 . When the sheet kind is not designated (NO in S 34 ), the processing is shifted to the step S 36  as it is. 
     In the step S 36 , the tray set as the variable A is transmitted as data to the laser printer  1  and, in the step S 37 , the sheet kind set as the variable B at that time is transmitted to the laser printer  1  as data. Further, in the step S 38 , the printing data is produced and transmitted to the laser printer  1  and the processing is ended. 
     On the other hand, when the priority order set in the screen shown in  FIG. 9  is not the tray priority (NO in S 33 ), it is decided in the step S 41  whether or not the priority order is the sheet kind. When it is not the sheet kind priority (NO in S 41 ), the processing is shifted to the step S 36 . In the case of the sheet kind priority (YES in S 41 ), the processing is shifted to the step S 42 . 
     In the step S 42 , it is decided whether or not there is a tray corresponding to the sheet kind set as the variable B at that time on the basis of the correspondence set in the screen similar to that shown in  FIG. 6  on the side of the personal computer  70 . When there is the tray (YES in S 42 ), the processing is shifted to the step S 36  after the tray is set as the variable A in the step S 43 . When there is no such tray (NO in S 42 ), the processing is shifted to the step S 36 . 
     In this aspect, the tray is selected on the side of the personal computer  70  on the basis of the priority order, etc. Therefore, the effect similar to that obtained by the above described aspect is obtained even when the laser printer  1  does not have the data table shown in  FIG. 6 . 
     Incidentally, in the described aspect, the processing related to the screen display similar to that shown in  FIG. 6  on the side of the personal computer  70  and the memory area of the ROM  72  storing the program for executing the processing function as the correspondence storing processing and the correspondence storing unit, respectively. The processing related to the screen shown in  FIG. 9  and the memory area of the ROM  72  storing the program for executing the processing function as the priority setting processing and the priority setting unit, respectively. The processing in the steps S 33  to S 43  shown in  FIG. 11  and the memory area of the ROM  72  storing the program for executing the processing function as the data transmission processing and the data transmission unit, respectively. Further, the present invention is not limited to the described image forming apparatus of the electronic photography system and the present invention can be applied to various image forming apparatus such as ink jet printer, etc.