Abstract:
There is disclosed an image forming apparatus comprising input means for inputting print data from the exterior, ID setting means for setting an ID for each page of the print data inputted by the input means, print condition designation means for designating the print condition, corresponding to the ID set by the ID setting means, print reservation means for reserving a print operation by storing the ID and the print condition in a print reservation memory, control means for causing the reserved print operation to be executed according to the content of the print reservation memory, and reservation cancellation means adapted, upon receiving an instruction to cancel a print reservation, to erase the corresponding ID and print condition from the print reservation memory, if a sheet corresponding to the designated ID in the print reservation memory is in a stand-by state for sheet feeding.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image forming apparatus capable of managing image formation status for each sheet.  
           [0003]    2. Related Background Art  
           [0004]    In the conventional image forming apparatus, there is only externally outputted the status information of the entire apparatus such as a state of preparing for printing, a state of image formation, a state of sheet carrying in progress, or the information on the number of sheet carried in the apparatus.  
           [0005]    As the conventional image forming apparatus does not externally output the information on the individual sheet managed in the apparatus, it has been associated with following drawbacks, since the external device or the user utilizing such image forming apparatus is incapable of recognizing the state of carrying or printing of the individual sheet.  
           [0006]    More specifically, in case the printing operation in the image forming apparatus is not completed in the normal state, the same image has to be printed after the image forming apparatus is restored to the normal state. As the completion of the image formation or of the sheet discharge on the individual sheet cannot be known from the outside, the external device sending the image information to the image forming apparatus has to retain the image formation with a certain marginal time after the output of the print instruction to the image formation apparatus, and such requirement limits the amount of the simultaneously storable image data.  
           [0007]    Also in case the image forming apparatus is connected to a network, the operator is unable to recognize the state of execution of the printing job from the position of the operator, and, in case of an abnormality in the image forming apparatus, the operator has to go to the location of the image forming apparatus in order to confirm whether the printing job of the operator has been properly completed. Also, if the printing job has not been completed in proper state, the operator has to return to the location of the output device and to repeat the image output.  
           [0008]    Also in case the image forming apparatus is connected to a sheet input/output device such as a large-capacity sheet feeding deck or a sheet sorter, the external control device for collectively controlling such input/output devices has to manage the sheet carrying state in the image forming apparatus by the timing based on a specified reference (for example the output of a print start command), and is therefore unable to exactly manage the transfer of the sheet among various devices. Particularly the sheet management becomes very difficult in case the order of printing in the image forming apparatus and the order of input/output of the sheets are mutually different, for example in case of two-side printing.  
           [0009]    Also, some printing apparatus have a print reserving function for designating the print conditions in the order of execution. However, in the conventional printing apparatus with such print reserving function, the reserved printing job can be deleted only by the owner of the printing job, but there is not provided a function of deleting the job, changing the print condition or changing the order of reservations after the execution is started.  
           [0010]    The printing operation by the printing apparatus is enabled after various units of the printing apparatus (printer engine) are activated in response to the print instruction from a host computer or the like. On the other hand, the print data forming apparatus (video controller) for supplying the printing apparatus with the print data forms the print data for supply to the printing apparatus based on the print information from an external device such as the host computer connected to the print data forming apparatus or from an operator. In general, in response to the reception of the print information from the exterior, the print data forming apparatus executes reservation of the printing operation in the printing apparatus, prior to the formation of the print data. Thus the starting-up of the printing apparatus to the printing state and the print data formation in the print data forming device are executed in parallel manner in order to shorten the time required to the start of the printing operation.  
           [0011]    In case the print data forming apparatus receives plural print information from the exterior, plural print jobs may be reserved in the printing apparatus prior to the completion of the formation of the plural print data.  
           [0012]    The printing apparatus having the print reserving function prepares for the execution of the printing in the order of the printing jobs reserved from the print data forming apparatus, and waits for the print instruction therefrom.  
           [0013]    In the printing apparatus, the reserved printing job may become not executable during the print instruction stand-by state, namely in the period from the reservation of the printing job to the completion of the print data formation in the print data forming apparatus.  
           [0014]    However, in the conventional printing apparatus not provided with the functions of changing the reserved print condition, changing the order of reservation or deleting the job after the start of execution, if plural print jobs are reserved and a print job with an earlier order of execution becomes not executable, all the print jobs of the subsequent order of execution become not executable.  
           [0015]    Otherwise, in the conventional printing apparatus provided only with the function of collectively deleting the print reservations, in case a reserved print job becomes not executable, it is necessary to collectively delete all the print reservations and to execute the print reservations anew.  
           [0016]    [0016]FIG. 36 is a block diagram of a control unit of a conventional printer.  
           [0017]    There are shown a printer controller  1101  for executing communication with and image data reception from a host computer, development of received image data into the information printable by the printer, and signal exchange and serial communication with a printer engine control unit to be explained later; an engine control unit  1102  for controlling various units of the printer engine through signal exchange and serial communication with the printer controller; a sheet size detection unit  1103  for detecting the sheet size in the cassette and transmitting the information to the engine control unit; a sheet feed unit sheet presence/absence detection unit  1104  for detecting the presence/absence of the sheet in the cassette, manual sheet insertion slot, optional cassette and envelope feeder and transmitting the information to the engine control unit; an option presence/absence detection unit  1105  for confirming the state of connection of the options such as the optional cassette and the envelope feeder; a sheet carrying control unit  1106  for controlling the sheet carrying; an optical system control unit  1107  for controlling an optical system including for example a scanner motor and a laser; a fixing temperature control unit  1108  for controlling the temperature of a fixing unit and detecting the abnormality thereof; an option control unit  1109  for instructing the operation of the options such as the optional cassette and the envelope feeder; and a sensor input unit  1110  for detecting the presence/absence of the sheet in various carrying paths such as a registration path, a discharge path, a two-side path and an inversion path.  
           [0018]    In the following there will be explained signals between the printer controller and the engine control unit. There are shown a /CPRDY signal  1111  indicating that the printer controller is in a state capable of communication with the engine; a /PPRDY  1112  indicating that the engine control unit is in a state capable communication with the printer controller; a /RDY signal  1113  indicating that the engine control unit is in a stand-by state capable of printing operation; a /PRNT signal  1114  for issuing a print demand from the printer controller to the engine control unit; a /TOP signal  1115  constituting a vertical synchronization signal outputted from the engine control unit to the printer controller; a /PRFD signal  1116  from the printer controller for causing the engine control unit to effect a sheet pre-feeding operation; a /BD signal  1117  constituting a horizontal synchronization signal outputted from the engine control unit to the printer controller; a /SCLK signal  1118  constituting a synchronization clock signal for serial communication; a /CMD signal  1119  constituting a command signal from the printer controller to the engine control unit; a /CBSY signal  1120  constituting a strobe signal for command output; a /STS signal  1121  indicating the status of the engine, to be outputted in response to a command from the printer controller; and a /SBSY signal  1122  for status output. FIG. 37 shows the timing of the serial communication mentioned above. In response to a command outputted from the printer controller, a corresponding status is outputted in 1-to-1 relationship. The synchronization clock signal is outputted from the printer controller. The serial communication is executed with  8  bits, in which the least significant bit LSB is used as a parity bit.  
           [0019]    [0019]FIG. 38 shows a conventional printing sequence, which is independent from the exchange of command and status by the serial communication shown in FIG. 37.  
           [0020]    Upon receiving the /PRNT signal from the printer controller  1101 , the engine control unit  1102  executes a sheet feeding operation and a pre-rotation process for starting up the electrophotographic process. Then the /TOP signal is outputted to the printer controller  1101  for vertical synchronization for the image writing. Subsequently the printer controller  1101  sends the /PRFD signal to the engine control unit  1102 , in order to start the feeding of a next sheet. In response, the engine control unit  1102  picks up a sheet. Then, in response to the /PRNT signal, the /TOP signal is outputted to execute the image writing. The above-explained sequence is repeated to execute the continuous printing operation.  
           [0021]    In the above-described conventional printer, however, the engine control unit  1102  cannot execute the sheet feeding operation for the next page until the establishment of the vertical synchronization for the image data of the preceding page, namely until the reception of the /PRFD signal responding to the output of the /TOP signal. Consequently, the high-speed printing can only be achieved by selecting a sheet feeding speed higher than in the ordinary printing operation, thereby reducing the time required for sheet feeding.  
           [0022]    Such high-speed sheet feeding is achieved for example by selecting the high or low sheet feeding speed with an exclusive motor in the sheet feeding system or providing a mechanism for switching the gear ratio, thus involving an increased cost. This drawback is particularly conspicuous in an image forming apparatus with a long sheet feeding path.  
         SUMMARY OF THE INVENTION  
         [0023]    An object of the present invention is to provide an image forming apparats not associated with the above-mentioned drawbacks, and a control method therefor.  
           [0024]    Another object of the present invention is to provide an image forming apparatus capable of managing the reserved print job for each sheet, and easily achieving interchange of jobs, alteration of job, cancellation thereof etc., and a control method therefor.  
           [0025]    Still another object of the present invention is to provide an image forming apparatus capable of managing the reserved print job for each sheet and informing the status of image formation for each sheet to the exterior of the printer engine, and a control method therefor.  
           [0026]    Still another object of the present invention is to provide an image forming apparatus capable of managing the reserved print job by an ID for each sheet, thereby facilitating the control from the exterior of the printer engine, and a control method therefor.  
           [0027]    Still another object of the present invention is to provide an image forming apparatus capable of increasing the throughput of image formation by a process outside the printer engine, and a control method therefor.  
           [0028]    Still another object of the present invention is to provide an image forming apparatus capable of increasing the throughput of image formation by executing the pre-feeding of the recording sheet as quickly as possible. 
       
    
    
       [0029]    Other objects and features of the present invention will become fully apparent from the following description to be taken in conjunction with the attached drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a lateral cross-sectional view showing the configuration of a printer in which the present invention is applicable;  
         [0031]    [0031]FIG. 2 is a block diagram showing the functional configuration of the printer;  
         [0032]    [0032]FIG. 3 is a view showing the data structure of ID management information;  
         [0033]    [0033]FIG. 4 is a block diagram showing the functional configuration in case an optional input/output device is connected to the printer;  
         [0034]    [0034]FIG. 5 is a view showing the data structure of ID management information;  
         [0035]    [0035]FIG. 6 is a view showing the configuration of status of two-face sheet feed requesting ID;  
         [0036]    [0036]FIG. 7 is a view showing the content of each bit of the ID status information;  
         [0037]    [0037]FIG. 8 is a flow chart showing the printing process in the engine control unit;  
         [0038]    [0038]FIG. 9 is a flow chart showing the printing process in the printer controller;  
         [0039]    [0039]FIG. 10 is a view showing the content of each bit of the ID status information;  
         [0040]    [0040]FIG. 11 is a cross-sectional view showing the sheet carrying state in the printer;  
         [0041]    [0041]FIG. 12 is a view showing the configuration of an ID table for each sheet, indicating the sheet position;  
         [0042]    [0042]FIG. 13 is a view showing the configuration of a table for each carrying unit, indicating the sheet position;  
         [0043]    [0043]FIG. 14 is a cross-sectional view of a printing apparatus in which the present invention is applicable;  
         [0044]    [0044]FIG. 15 is a block diagram showing the functional configuration of the printing apparatus;  
         [0045]    [0045]FIG. 16 is a view showing a printing reservation table;  
         [0046]    [0046]FIG. 17 which is composed of FIGS. 17A and 17B is a flow chart showing the control of the printing operation in a print control unit;  
         [0047]    [0047]FIG. 18 is a flow chart showing the printer control of the printing reservation table in the print control unit;  
         [0048]    [0048]FIG. 19 is a flow chart showing the cancellation control for the print reservation of the designation ID in a fifth embodiment;  
         [0049]    [0049]FIG. 20 is a view showing the print reservation table at the cancellation of print reservation in the fifth embodiment;  
         [0050]    [0050]FIG. 21 is a view chart showing the print information changing control of the designation ID in a sixth embodiment;  
         [0051]    [0051]FIG. 22 is a flow chart showing the exchanging control of the print reservations of the designation ID in a seventh embodiment;  
         [0052]    [0052]FIG. 23 is a view showing a print reservation table at the exchange of the print reservations in an embodiment;  
         [0053]    [0053]FIG. 24 is a block diagram of a control unit of the printer in an eighth embodiment;  
         [0054]    [0054]FIG. 25 is a timing chart of the serial communication in the eighth embodiment;  
         [0055]    [0055]FIG. 26 is a view showing the command configuration in the eighth embodiment;  
         [0056]    [0056]FIG. 27 is a block diagram showing an example of the configuration of the printer control unit in the eighth embodiment;  
         [0057]    [0057]FIG. 28 is a flow chart showing the function of the engine control unit in the eighth embodiment;  
         [0058]    [0058]FIG. 29 is a flow chart showing the function of the engine control unit in the eighth embodiment;  
         [0059]    [0059]FIG. 30 is a view showing the command structure in a ninth embodiment;  
         [0060]    [0060]FIG. 31 is a flow chart showing the function of the engine control unit in the ninth embodiment;  
         [0061]    [0061]FIG. 32 is a flow chart showing the function of the engine control unit in the ninth embodiment;  
         [0062]    [0062]FIG. 33 is a view showing the command structure in a tenth embodiment;  
         [0063]    [0063]FIG. 34 is a flow chart showing the function of the engine control unit in the tenth embodiment;  
         [0064]    [0064]FIG. 35 is a flow chart showing the function of the engine control unit in the tenth embodiment;  
         [0065]    [0065]FIG. 36 is a block diagram of a conventional engine control unit;  
         [0066]    [0066]FIG. 37 is a timing chart of the serial communication in the conventional configuration; and  
         [0067]    [0067]FIG. 38 is a timing chart showing a conventional printing sequence. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0068]    [0068]FIG. 1 is a view showing the configuration of a printer in which the present invention is applicable, wherein shown are a photosensitive drum  101  constituting an image bearing member; a semiconductor laser  102  serving as a light source; a rotary polygon mirror  103  rotated by a scanner motor  104 ; and a laser beam  105  emitted from the semiconductor laser  102  and scanning the photosensitive drum  101 .  
         [0069]    There are also shown a charging roller  106  for uniformly charging the photosensitive drum  101 ; a developing unit  107  for developing an electrostatic latent image, formed on the photosensitive drum  101 , with toner; a transfer roller  108  for transferring the toner image, developed by the developing unit  107 , onto a predetermined recording sheet; and a fixing unit  109  for thermally fusing the toner transferred onto the recording sheet.  
         [0070]    There are also shown a cassette sheet feeding roller  110  capable, by a rotation, of feeding a sheet from a cassette, provided with the function of identifying the size of the recording sheet, and advancing the sheet to a carrying path; a manual insertion sheet feeding roller  111  for advancing a sheet, from a manual insertion sheet feeding slot, not provided with the function of identifying the size of the recording sheet, to the carrying path; an optional cassette sheet feeding roller  112  for advancing a sheet from an optional cassette, not provided with the function of identifying the size of the recording sheet, to the carrying path; an envelope feeder sheet feeding roller  113  for advancing, one by one, the recording sheet from an envelope feeder capable of stacking envelopes only, to the carrying path; and carrying rollers  114 ,  115  for carrying the recording sheet fed from the cassette or the optional cassette.  
         [0071]    There are further shown a pre-feed sensor  116  for detecting the leading end and trailing end of the fed sheet; a pre-transfer roller  117  for advancing the carried sheet to the photosensitive drum  101 ; a top sensor  118  for synchronizing the carrying of the fed sheet with the image writing (recording/printing) onto the photosensitive drum  101  and measuring the length of the fed sheet in the carrying direction; a sheet discharge sensor  1109  for detecting presence/absence of the sheet after fixation; and discharge rollers  120  for discharging the sheet after fixation from the apparatus.  
         [0072]    There are further shown a flapper  121  for switching the destination (discharge tray or detachable two-face unit) of the printed sheet; carrying rollers  122  for carrying the sheet, carried to the detachable two-face unit, to an inversion unit; an inversion sensor  123  for detecting the leading/trailing end of the sheet carried to the inversion unit; and re-feeding rollers  124  for sequentially effecting forward/reverse rotation to invert the sheet, thereby advancing the sheet to a re-feeding unit.  
         [0073]    [0073]FIG. 2 is a block diagram showing the circuit configuration of a control system for controlling the above-described mechanism, wherein show are a printer controller  201  for developing image code data, transferred from an external device such as an unrepresented host computer, into bit data reburied for the printing operation in the printer, and reading and displaying the internal information of the printer.  
         [0074]    A printer engine control unit  202  controls the various units of the printer engine according to the instruction of the printer controller  201  and informs the internal information of the printer to the printer controller  201 . A sheet carrying control unit  203  drives and stops the motors and rollers for carrying the recording sheet, according to the instruction of the printer engine control unit  202 . A high voltage control unit  204  controls the high voltage output in the steps of charging, development and transfer according to the instruction of the printer engine control unit  202 . An optical system control unit  205  drives and stops the scanner motor  104  and turns on the laser beam according to the instruction of the engine control unit  202 . A fixing unit control unit  207  executes and stops the power supply to a fixing heater according to the instruction of the printer engine control unit  202 .  
         [0075]    A control unit  208  for the detachable optional cassette drives and stops the driving system according to the instruction of the printer engine control unit, and informs the printer engine control unit  202  of the presence/absence of the sheet and the sheet size information.  
         [0076]    A control unit  209  for the detachable two-face unit executes inversion and re-feeding of the sheet according to the instruction of the printer engine control unit  202  and informs the engine control unit  202  of the status of such operations.  
         [0077]    A control unit  210  for a detachable envelope feeder drives and stops the driving system according to the instruction of the printer engine control unit  202 , and informs the printer engine control unit  202  of the presence/absence of sheet.  
         [0078]    The printer controller  201  and the engine control unit  202  mutually execute 16-bit serial communication, and, in response to 16-bit command data transmitted from the printer controller  201 , the engine control unit  202  returns 16-bit status data indicating the internal information of the engine, corresponding to the content of the received command. Also there is executed an operation corresponding to the content of the command.  
         [0079]    Also the printer controller  201  and the engine control unit  202  manage the printing operation by an ID which is different for each sheet. The ID is designated from the printer controller  201  to the engine control unit  202 , for each print instruction.  
         [0080]    [0080]FIG. 3 shows the status structure for managing the ID. In FIG. 3, a status of registered ID is composed of 16 bits in which the first bit constitutes the MSB and the 16th bit constitutes the LSB. Among these 16 bits, the first bit is defined as “0” while the 16th bit is used as the parity bit, and the remaining 14 bits are assigned for ID  1  to ID  14 .  
         [0081]    When an ID is designated by the printer controller  201 , the engine control unit  202  sets “1” in a bit corresponding to the ID number, and resets the bit corresponding to the ID number to “0” when the printing operation corresponding to the ID number is completed and the discharge of the printed sheet to the discharge tray is completed. Consequently, a bit “ 1 ” corresponding to each ID indicates an ID registered in the engine control unit  202  for a printing operation in progress or to be executed, while a bit “ 0 ” indicates an ID that can be designated by the printer controller  201 . The printer controller  201  can thus easily recognize and manage the print status of the printer, by managing the status of registered ID.  
         [0082]    In the present embodiment, there has been explained the sheet managing method by ID in the single printer alone, but such method becomes more effective in case the printer is connected to a sheet feeding unit and a sheet discharging unit as shown in FIG. 4 and the printer controller  201  manages the status of sheet in these units. More specifically it is possible to recognize and manage the sheet status in the entire printer system by giving sheet status not only to the printer itself but also the sheet feeding unit and the sheet discharging unit, also giving an ID to each status and calculating, by the printer controller  202 , the logical sum of the bits of the status shown for each unit as shown in FIG. 5.  
       Second Embodiment  
       [0083]    In the image forming apparatus of the present embodiment, there will be explained a method of managing first and second faces in the two-face printing operation with a same ID. In the present embodiment, there is provided a status indicating the ID that can be designated in the second face in the two-face printing operation (hereinafter called status of two-face sheet feed demand ID. The structure of the status is shown in FIG. 6.  
         [0084]    When the sheet after the first-face printing is designated to be discharged to the two-face unit, the engine control unit  202  sets a corresponding ID bit of the two-face sheet feed demand ID status at “1”, and clears this bit to “0” when the sheet feeding from the two-face units is designated again with the same ID. Therefore the printer controller  201  can recognize and manage the sheet carrying state in the two-face printing operation, by calculating the logical product or logical sum of the registered ID status explained in the first embodiment the two-face sheet feed demand ID status.  
       Third Embodiment  
       [0085]    In the third embodiment, there will be explained an error recovery process to be executed between the printer controller  201  and the engine control unit  202 .  
         [0086]    [0086]FIG. 7 shows the content of status information transmitted from the engine control unit  202  to the printer control  201 , in the serial communication executed therebetween.  
         [0087]    Each of 16-bit information indicates ID information by “1” or “0”. In FIG. 7, a mark “*1” indicates that a print instruction is received after the print reservation and the printer engine has either completed the vertical synchronization or has started the image printing after the vertical synchronization, when the ID bit of this status information is “1”. Also the ID bit “ 0 ” indicates a state where the print reservation is not made, the print instruction is not made though the print reservation has been made, the vertical synchronization is not made though the print instruction has been given, or the vertical synchronization has been made and the corresponding sheet has been discharged from the apparatus.  
         [0088]    [0088]FIG. 8 is a flow chart showing the engine control sequence of the engine control unit  202  relating to the ID status in case of a jam.  
         [0089]    Upon receiving a print reservation (S 801 ), the engine control unit  202  searches an empty ID among the ID status (S 802 ) and determines the ID (S 803 ). Then it starts the sheet carrying process at a predetermined timing (S 804 , S 805 ). When the sheet is carried to a predetermined sheet feed position (S 806 ), the sheet carrying is terminated (S 807 ).  
         [0090]    It then waits until a print request is received (S 808 ), and, upon receiving a print instruction from the printer controller  201 , there is executed an ordinary printing operation (S 809 ). Subsequently, when a timing is reached for starting the vertical synchronization (S 810 ), it executes the vertical synchronization with the printer controller  201  (S 811 ), and sets a corresponding ID bit of the above-mentioned status information at “1” (S 812 ).  
         [0091]    In case a jam is subsequently generated by some reason and the engine is stopped (S 813 ), the engine control unit  202  retains the above-mentioned status information on ID at the state of jam generation (S 815 ). This status is retained until a status clearing instruction is given from the printer controller  201  (S 816 ).  
         [0092]    [0092]FIG. 9 shows the sequence executed at the printer controller. At first, upon receiving data from a host computer (S 901 ), the printer controller analyzes the content of the received data and executes development into image data (S 902 ). Then it executes print reservation in the engine control unit  202  (S 903 ). Then, upon completion of development of the data of the first page (in case of processing the initial page), it instructs the printing operation to the engine control unit (S 904 , S 905  and S 906 ).  
         [0093]    Upon receiving error information such as a jam from the engine control unit  202  after the print instruction (S 907 ), it analyzes the error information (S 908 ) and reads the ID status information at this point (S 909 ). Based on this operation, it determines the job of an ID bit at “1” as the page to be resent (S 910 ), and executes the printing operation again after the jam processing (S 911 ). On the other hand, if the printer engine has no abnormality and the vertical synchronization signal is requested from the engine control unit  201  (S 912 ), the vertical synchronization signal and the image data are outputted to the engine control unit (S 913 ). These operations are repeated for all the pages.  
         [0094]    In this manner an appropriate recovery process can be provided to the user.  
         [0095]    In addition to the structure shown in FIG. 7, the status information may assume the structure shown in FIG. 10 in which each ID has 16-bit information to indicate more precise positional information.  
       Fourth Embodiment  
       [0096]    [0096]FIG. 11 shows the position of the sheet in the course of carrying in the printer. The feed paths are divided into blocks of a lower sheet feed path A-C, an upper sheet feed path B-C, a common sheet feed path C-D, an image forming path D-E, an FD sheet discharge path E-F, a two-face sheet discharge path E-G, a two-face inversion path G-H, a two-face sheet feed path G-I and a two-face sheet feed path I-J.  
         [0097]    There is illustrated a state where four sheet of ID=2, 3, 4 and 5 are fed in the sheet feed paths as indicated in an ID table shown in FIG. 12. Sheet position table information is provided for each sheet, and the sheet position table indicates the sheet feed path in which the sheet is currently present. FIG. 12 shows the sheet position table for the sheets of ID=2, 3, 4 and 5. More specifically, for the sheet with ID=2, as the leading end has reached the FD discharge path after image formation, a bit  5  (FD discharge path) of the sheet position table is set at “1”. Similarly, for the sheet with ID=3, as the leading end is in the two-face inversion path, a bit  7  is set at “1”. For the sheet with ID=4, a bit  3  is set at “1” as the leading end is in the common sheet feed path. For the sheet with ID=5, a bit  1  is set at “1” since the leading end is in the lower sheet feed path. When a sheet position status is requested from the printer controller  201 , the engine control unit  202  returns the sheet position status corresponding to the sheet ID. The printer controller  201  can recognize the positions of all the sheets, by acquiring the sheet position statuses corresponding to all the sheet ID&#39;s and calculating the logical sum.  
         [0098]    In the present embodiment, the sheet position is represented by the position of the leading end of the sheet, but more complete sheet position status can be obtained by preparing the sheet position table including also the trailing end of the sheet.  
         [0099]    [0099]FIG. 13 shows the sheet position regrouped in the respective sheet feeding blocks, indicating the sheet ID present in each sheet feed path. The example shown in FIG. 13 indicates that the lower sheet feed path contains the sheet of ID=5, the upper sheet feed path contains no sheet, the common sheet feed path contains the sheet of ID=4, the FD discharge path contains the sheets of ID=2, and the two-face inversion path contains the sheet of ID=3. The printer controller  201  can recognize the positions of all the sheet present in the sheet feed paths also by this method.  
         [0100]    In the present embodiment, there is employed information indicating the sheet feeding block in which the leading end of the sheet is present, but more complete sheet position status can be obtained by preparing the sheet feed path information including also the trailing end of the sheet.  
       Fifth Embodiment  
       [0101]    In the following there will be explained the deletion of the job for which the print reservation is made.  
         [0102]    [0102]FIG. 14 is a cross-sectional view of a laser beam printer  401  having the function of managing the print information ID of the present invention.  
         [0103]    The laser beam printer  401  is provided with a cassette  402  for containing recording sheets S, a pickup roller  403  for picking up a recording sheet from the cassette  402 , feed rollers  404  for advancing the picked-up recording sheet, and, at the downstream side of the feed rollers  404 , a carrying roller  406  for carrying the fed recording sheet to registration rollers  411  to be explained later.  
         [0104]    The laser beam printer  401  is also provided with a multi purpose tray (MPT)  407  for enabling manual insertion of a sheet of predetermined size or unfixed size, an MPT sheet feed roller  409  for advancing the recording sheet from the MPT  407 , and an MPT lifter  408  for pressing the recording sheets, set on the MPT  407 , to the MPT sheet feed roller  409 .  
         [0105]    The sheet feed path  405  from the cassette  402  and that  410  from the MPT  407  are united at a junction point C and reach the registration rollers  411  to be explained later.  
         [0106]    At the downstream side of the above-mentioned junction point C, there are provided registration rollers  411  for synchronized carrying of the recording sheet S, and an image forming unit  304  for forming an image by a laser beam from a laser scanner unit  303  is provided at the downstream side of the registration rollers  411 . Also a fixing unit  306  is provided at the downstream side of the image forming unit  304 , and sheet discharging rollers  412 ,  413 ,  414 ,  415  are provided at the downstream side of the fixing unit  306  to carry the recording sheet to a discharge tray  416 .  
         [0107]    The laser beam printer  401  is also provided with an automatic two-face printing function by inverting the recording sheet, bearing the image on a face, and feeding the recording sheet into the image forming unit  412  again to form images on both faces of the recording sheet.  
         [0108]    A flapper  417  for changing the destination of the recording sheet S is provided at the downstream side of the discharge rollers  415 , and, at a further downstream position there are provided an inversion roller  418  for changing the direction of the recording sheet S thereby effecting sheet inversion, and an inverted sheet container  419  for containing the recording sheet S at the inverting operation. At the downstream side of the carrying path after inversion of the recording sheet S by the inversion roller  418 , there are provided a D-cut roller  421 , a movable lateral registration plate  420 , two-face carrying rollers  422 . At the downstream side thereof, the sheet carrying path joins the sheet feed rollers  406 .  
         [0109]    For detecting the position of the recording sheet S, there are provided, on the sheet carrying paths, a pre-registration sensor  423  in front of the registration rollers  411 , a fixation-discharge sensor  424  after the fixing unit  306 , a discharge sensor  425  in front of the discharge tray  416 , an inversion sensor  426  in front of the inversion roller  418  and a two-face sensor  427  in front of the two-face carrying rollers  422 . Each of the sensors is so constructed as to detect the presence of the recording sheet S at the position of such sensor, that a light shielding plate upon contact with the recording sheet intercepts a photointerruptor from the light.  
         [0110]    [0110]FIG. 15 is a block diagram showing the configuration of the printing apparatus shown in FIG. 14.  
         [0111]    The laser beam printer  401  is composed of an engine unit  200  for forming an image by an electrophotographic process, and a video controller unit  302  for generating an image signal for effecting image formation in the engine unit, based on drawing data transmitted from a host computer. The engine unit is provided with an engine control unit  301  for controlling the entire engine unit. The engine control unit  301  is composed of a CPU, a ROM, a RAM, gate elements etc., and the principal part of the control is realized by a software recorded in the ROM.  
         [0112]    The engine control unit  301  is connected with the video controller  302  through signal lines HSYNC, VDO, PRNT, VSYNC, VSREQ to be explained later and a serial I/F for serial communication.  
         [0113]    The engine control unit  301  is composed of an engine test print pattern generation unit  307 , an image signal control unit  308 , a print control unit  309 , a sheet carrying control unit  310 , an image print information memory unit  311 , a serial communication unit  312 , a print reservation information control unit  313  etc., and controls the laser scanner unit  303 , image forming unit  304 , fixing unit  306  described in the foregoing and a sheet carrying mechanism  305  consisting of driving mechanisms for the above-mentioned carrying rollers and the above-mentioned sheet sensors.  
         [0114]    The video controller  302 , upon receiving the drawing data from the host computer or the like, develops such drawing data into video data for output to the engine unit  300 . It also assigns an ID for each image to be printed, designates information required for printing (source of sheet feeding, destination of sheet discharge, sheet size etc.) for each ID by serial communication to the engine control unit  301 , and makes the print reservation by ID. For example 10 ID&#39;s are set for a job consisting of images of 10 pages. The engine control unit  301  stores the information of the source of sheet feeding, destination of sheet discharge, sheet size etc., designated for each ID, in an area assigned for each ID in the image print information memory unit  311 , and also stores the order of printing in a print reservation table therein.  
         [0115]    [0115]FIG. 16 is a view showing the conceptual structure of the print reservation table, in which the reserved ID codes and the pointers to the print information memory areas for the respective ID&#39;s, arranged in the order of printing. In the image print information memory unit  311 , there is provided an area for collectively storing, for each ID, the above-mentioned print information such as the source of sheet feeding, destination of sheet discharge, sheet size etc. The pointer to the print information memory area includes a print reservation pointer and a print execution pointer, and designates the print information memory area of the corresponding ID. In an empty area, a special value indicating the empty state (“0” in the present embodiment) is set in the ID code area.  
         [0116]    In the following there will be explained, with reference to a flow chart shown in FIGS. 17A and 17B, a printing sequence by the engine control unit  301 .  
         [0117]    Upon receiving the drawing data from the host computer or the like, the video controller  302  executes print reservation by setting an ID for each image in the engine control unit  301 . The print reservation information management unit  113  stores the print reservation from the video controller  302  in the above-mentioned print reservation table. If at least a print reservation is present (step S 301 ), the print control unit  109  waits until the sheet feed enabling condition from the source of sheet feeding designates by the reserved ID is satisfied (S 303 ), and instructs the sheet carrying control unit  310  to execute the sheet pre-feeding operation (S 304 , S 305 , S 306 , S 307 , S 308 , S 309  and S 310 ). The discrimination of the presence of at least a print reservation will be explained later. The sheet feed enabling condition means, for example, that a predetermined distance to the preceding sheet can be secured and that the driving source for the sheet feeding mechanism has been started up. Also, the pre-feeding operation is to advance the recording sheet from the designated source to a position in front of the junction point C (cf. FIG. 14) with the carrying path from another sheet feeding source. In response to the sheet pre-feeding instruction from the print control unit  309 , the sheet carrying control unit  310  executes the pre-feeding of sheet from the designated source.  
         [0118]    If any of the laser scanner unit  330 , the image forming unit  304  and the fixing unit  306  is not in a state enabling the printing operation at the reception of the print reservation from the video controller  302 , the print control unit  309  causes such unit to initiate the starting-up to the state enabling the printing operation. Then the print control unit  309  discriminates whether the pre-feeding of the sheet corresponding to the ID of the order to be printed next is completed (S 311 ), and, if completed, instructs the sheet carrying control unit  310  to advance such sheet to the registration rollers  411  (S 313 ).  
         [0119]    When the video data become ready for transmission, the video controller  302  sends a print instruction signal (hereinafter simply represented as PRNT signal) to the engine control unit  301  through a PRNT signal line.  
         [0120]    The print control unit  309  awaits the starting-up of the laser scanner unit  330 , image forming unit  304  and fixing unit  306 , the completion of feeding of the recording sheet S to the registration rollers  411 , and the output of the PRNT signal from the video controller  302  (S 314 ). In response to the output of the PRNT signal, it starts the output of a horizontal (main scanning) synchronization signal (hereinafter simply represented as HSYNC signal) to the video controller  302 . Also the print control unit  309  sends a vertical (sub scanning in the sheet carrying direction) synchronization request signal (hereinafter simply represented as VSREQ signal) to the video controller  302  through a VSREQ signal line (S 315 ). In response thereto, the video controller  302  outputs a vertical synchronization signal VSYNC to the engine control unit  301  through a VSYNC signal line, and sends the video data as an image signal VDO to the engine control unit  301  through a VDO signal line. In this operation, the video controller  302  synchronizes the VDO signal with the HSYNC signal in the main scanning direction and with the VSYNC signal in the sub scanning direction. When the print control unit  309  receives the VSYNC signal (S 316 ), the engine control unit  301  instructs the image signal control unit  308  to transmit the received VDO signal to the laser scanner unit  303 , the print control unit  309  to control the image forming unit  304  and the sheet carrying control unit  310  to advance the recording sheet S, stopped at the registration rollers  411 , in synchronization with the VSYNC signal (S 317 ).  
         [0121]    Through the above-described procedure, a predetermined image is formed on the recording sheet S. After the image formed on the sheet S is then fixed by the fixing unit  306 , there is discriminated the destination of discharge designated for the ID of such image (S 319 ). If the discharge destination designated by ID is the two-face carrying unit, the print control unit  309  shifts the flapper  417  toward the inversion roller  418  upon detection of the leading end of the recording sheet S, thereby controlling the carrying direction thereof (S 326 , S 327 ), thus achieving sheet inversion for two-face process. When the trailing end of the recording sheet is detected by the fixation discharge sheet sensor  424  (S 329 ), the flapper  417  is returned to the side of the sheet discharge path (S 330 ), whereupon the two-face sheet carrying operation is terminated at a predetermined timing (S 332 ). On the other hand, if the discharge destination designated by the ID is the discharge tray, the print control unit  309  maintains the flapper  417  in the direction toward the discharge tray, and executes the sheet discharging operation when the fixation sheet discharge sensor detects the trailing end of the recording sheet (S 320 , S 321 ).  
         [0122]    Steps S 302 , S 308 , S 309 , S 310 , S 318 , S 322  and S 331  set the information of the sheet carrying state for each ID in the image print information memory unit  311 . The sheet carrying state is a stand-by state for sheet feeding, a state in sheet feeding, a state in sheet carrying for printing or a state in discharge. In addition there is provided an unused state as a special case, indicating the absence of the image related to the corresponding ID. Also in case the leading and trailing ends of the sheet in discharge are detected by the discharge sensor  425  (S 323 , S 324 ) to terminate the sheet discharging state, there is also assumed the unused state (S 325 ).  
         [0123]    Also the image print information memory unit  311  has a print reservation pointer and a print execution printer as information for controlling the print reservation table mentioned above. The print reservation pointer indicates an area for writing the reservation information on the print reservation table, in case a print reservation is received next from the video controller  302 . The print execution pointer indicates an area in the table where the pointer is set for the print information of an ID to be printed next when a PRNT signal is received from the video controller  302 . Upon receiving a print reservation from the video controller  302 , the print reservation information management unit  313  writes a pointer to the print information of the reserved ID, in an area indicated by the above-mentioned print reservation pointer, and advances the print reservation pointer to a next position. In case the area indicated by the print reservation pointer is the last area of the print reservation table, the print reservation pointer is advanced to the top of thereof. Also at the start of the printing operation (S 317  in FIG. 17A), the print reservation information management unit  313  shifts the print execution pointer to the pointer of a next reserved ID information. This process is shown in a flow chart in FIG. 18.  
         [0124]    When the print control unit  309  initiates the printing operation in response to the PRNT signal from the video controller  302 , the print reservation information management unit  311  writes “0” in the ID code of the area indicated by the print execution pointer, and shifts the print execution pointer to a next area in a step S 502 . Then a step S 503  discriminates whether the next area indicated by the print execution pointer coincides with the area indicated by the print reservation pointer and, if so, the print execution pointer is stopped at such area and the process is terminated. If the step S 503  identifies that the position of the print execution pointer is different from that of the print reservation pointer, a step S 504  discriminates whether the ID code indicated by the print execution pointer is “0”. If it is “0”, a step S 505  shifts the print execution pointer to the top of the print reservation table, and the sequence proceeds to a step S 506 . If the ID code of the area indicated by the print execution pointer is not “0”, the pointer is stopped at such position and the sequence is terminated. A step S 506  discriminates whether the next are indicated by the print execution pointer is same as that indicated by the print reservation pointer, and, if same, the print execution pointer is stopped at such position and the sequence is terminated. If the step S 506  identifies that the position of the print execution pointer is different from that of the print reservation pointer, a step S 507  discriminates whether the area indicated by the print execution pointer is “0”, and, if “0”, a step S 508  advances the print execution pointer to a next area and the sequence returns to the step S 506 . If not “0”, the pointer is stopped at such position and the sequence is terminated.  
         [0125]    At the start of the printing operation, the print information reservation management unit  313  executes the above-described process ont he print execution pointer, whereby the print execution pointer always indicates the ID for the print job to be executed next. Also, in case absence of the print reservation, the print execution pointer indicates an area same as the area indicated by the print reservation pointer having a value “0”, whereby the print control unit  309  can judge presence or absence of the print reservation.  
         [0126]    As explained in the foregoing, the engine control unit  301  executes preparations for the printing operation such as the sheet pre-feeding and the starting up of various units of the engine, in response to the print reservation from the video controller  302 . Therefore, by the execution of the print reservation to the engine prior to the start of development of the video data, there can be executed the preparation for the transmission of the video data in the video controller  302  and the preparation for the printing operation in the engine in parallel manner. Consequently, in case plural drawing data are received from the host computer or the like, the video controller  302  executes the print reservation as early as possible in the engine control unit  301 . However, in case the video controller  302  makes print reservations on plural images and if a part of the reserved print jobs becomes not executable because of the situation in the engine prior to the completion of development of the video data for a certain page, all the executable print jobs reserved after such print job that has become not executable also become not executable.  
         [0127]    In the engine control unit  301  of the present embodiment, the print reservation information management unit  313  is provided with a function of deleting the reservation information, prior to the start of the sheet feeding operation, according to an instruction from the video controller  302 .  
         [0128]    [0128]FIG. 19 shows a flow chart of the print reservation deleting process of the print reservation information management unit  313 . At first when an instruction is given by serial communication from the video controller  302  for canceling the print reservation for an ID, the print reservation information management unit  313  sets, in a step S 601 , the object of ID check at the start of the print reservation table, and discriminates, in a step S 602 , whether the ID code of the checked area is the designated ID. If not, the sequence proceeds to a step S 605  to discriminate whether the checked area is the end of the print reservation tables. If so, the corresponding reservation is judged absent, and a step S 606  returns information indicating that the execution is impossible to the video controller  302  and the sequence is terminated. If the checked area is not the end of the table, a step S 607  advances the area to be checked to a next area and the sequence returns to the step S 602 . If the step S 602  identifies that the ID of the checked area is the designated ID, a step S 603  discriminates, by the pointer to the print information indicated by such ID, whether the recording sheet for such ID is in the stand-by state for sheet feeding, and, if not, the reservation for such designated ID is identified as not cancelable. Then a step S 606  returns information indicating that the execution is impossible to the video controller  302  and the sequence is terminated. If the step S 603  identifies that the recording sheet for such designated ID is in the stand-by state, a step S 604  deletes the information for such ID from the print reservation table.  
         [0129]    [0129]FIG. 20 is a schematic view showing the mode of deletion of information from the print reservation table.  
         [0130]    Thus, even in case the video controller  302  makes print reservations on plural images and in case a part of the reserved print jobs becomes not executable because of the situation in the engine prior to the completion of development of the video data for a certain page, all the executable print jobs reserved after such print job that has become not executable can be executed in continuation by deleting such print job that has become not executable through the above-described process of the print reservation information management unit  313  in response to the instruction from the video controller  302 .  
       Sixth Embodiment  
       [0131]    The foregoing embodiment is only given a function of deleting the ID of the print reservation, but the present embodiment allows to change the designated print conditions (source of sheet feeding, destination of sheet discharge, sheet size etc.) prior to the start of the recording sheet feeding operation.  
         [0132]    The hardware configuration is same as that shown in FIGS. 14 and 15.  
         [0133]    The print reservation information management unit  313  of the present embodiment, upon receiving an instruction from the video controller  302  for changing the print information for the reserved ID, changes the print information for the designated ID in the image print information memory unit  311 .  
         [0134]    [0134]FIG. 21 shows a flow chart of the reservation information changing process in the print reservation information management unit  313  of the present embodiment. In FIG. 21, steps S 801 , S 801  and S 803  and S 805 , S 806  and S 807  are same as those S 601 , S 602  and S 603  and S 605 , S 606  and S 607  in FIG. 19 and will not, therefore, be explained further. If there exists a print reservation of the ID designated by the video controller  302  and the recording sheet designated by such ID is in the stand-by state for feeding, a step S 804  makes the access to the print information corresponding to the ID designated by the pointer of the print reservation table and changes it to the print condition designated from the video controller  302 .  
         [0135]    Thus, even in case the video controller  302  makes print reservations on plural images and in case a part of the reserved print jobs becomes not executable because of the situation in the engine prior to the completion of development of the video data for a certain page, the printing operation can be continued by changing the print condition of the print job that has become not executable through the above-described process of the print reservation information management unit  313  in response to the instruction from the video controller  302 . For example, in case the recording sheets in the original designated source of sheet feeding have been exhausted, the printing operation can be continued by re-designating another source having the sheets of the same kind.  
       Seventh Embodiment  
       [0136]    This embodiment enables interchange of the print jobs by the reservation ID&#39;s before the start of the recording sheet feeding operation.  
         [0137]    The hardware configuration is same as that shown in FIGS. 14 and 15.  
         [0138]    The print reservation information management unit  313  of the present embodiment, upon receiving an instruction from the video controller  302  for interchanging two reservation ID&#39;s, interchanges the print reservation information for the designated ID&#39;s in the image print information memory unit  311 .  
         [0139]    [0139]FIG. 22 shows a flow chart of the reservation information changing process in the print reservation information management unit  313  of the present embodiment. In FIG. 22, steps S 901  and S 907  discriminates the possibility of change for each of the ID&#39;s designated for interchange. The discrimination for the possibility of change for each ID (S 902 , S 903 , S 904 , S 905 , S 906 , S 907 , S 908 , S 909 , S 911 , S 912  and S 913 ) is same as the steps S 601 , S 602 , S 603 , S 605 , S 606  and S 607  in FIG. 19 and will not, therefore, be explained further. If there exist print reservations for the ID&#39;s designated by the video controller  302  and the recording sheet designated by each designated ID is in the stand-by state for feeding, a step S 910  interchanges the information corresponding to the two designated ID&#39;s in the print reservation table.  
         [0140]    [0140]FIG. 23 is a schematic view showing the mode of interchange of the two information of the print reservation table.  
         [0141]    Thus, even in case the video controller  302  makes print reservations on plural images and in case a part of the reserved print jobs becomes not executable because of the situation in the engine prior to the completion of development of the video data for a certain page, the above-described process of the print reservation information management unit  313  allows to continue the printing operation, in response to the instruction from the video controller  302 , by interchanging the print job that has become not executable with another executable reservation, thereby executing such another print job at first and postpone execution of such not executable print job until it becomes executable.  
         [0142]    The job cancellation, change of the print condition or job interchange explained in the foregoing is executed according to the instruction from the video controller  302 , and the video controller  302  may either independently determine the execution of such processes based on various signals from the engine control unit  301 , or may determine according an instruction from the host computer or from the operation unit of the printer.  
       Eighth Embodiment  
       [0143]    [0143]FIG. 24 is a block diagram of the control unit of a printer of the eighth embodiment of the present invention. The configuration of the printer is same as that of the first embodiment shown in FIG. 1.  
         [0144]    In FIG. 24, components equivalent to those in the conventional configuration shown in FIG. 36 are represented by corresponding numbers and will not be explained further.  
         [0145]    As shown in FIG. 24, the printer control unit of the present invention is additionally provided with a pre-feed control unit  1203 , while the /PRFD signal and the /PRNT signal are not used between the printer controller  1201  and the engine control unit  1202 .  
         [0146]    In the serial communication, there are employed signals/SCLK,/CMD,/STS,/SBSY and/CBSY as in the conventional configuration, but the data transmitted or received in a communication are composed of 16 bits.  
         [0147]    [0147]FIG. 25 is a timing chart of the serial communication of the present embodiment.  
         [0148]    The serial communication is executed in a mode where the engine control unit  1202  always returns a status in response to a command from the printer controller  1201 . The engine control unit  1202  returns the status immediate after the reception of the command.  
         [0149]    The printer controller  1201 , after setting the /CBSY signal at the low level state, transmits the command by the /CMD signal to the engine control unit  1202  in synchronization with the /SCLK signal. In response to the command, the engine control unit  1202  shifts the /SBSY signal to the low level state to inform the printer controller  1201  of the preparation of status, and transmits the /STS signal in synchronization with the /SCLK signal outputted from the printer controller  1201 .  
         [0150]    [0150]FIG. 26 shows an example of the command from the printer controller  1201 .  
         [0151]    Within 16 bits, the MSB (first bit) is used to transmit a command error to the printer controller  1201  in case an error is detected in the command, and is always maintained at “0” in the serial communication in the normal state.  
         [0152]    Also the LSB (16th bit) is used as an odd number parity bit. Therefore, 14 bits are used for the command codes. Within such  14  bits, the upper 6 bits indicate the command code, while the succeeding 4 bits are used for designating the source of sheet feeding, the succeeding 3 bits are used for designating the designation of sheet discharge, and the remaining 1 bit is used for designating the pre-feed reservation for a sheet next to the current recording sheet. Upon receiving a print command with the pre-feed reservation bit “ 1 ”, the engine control unit  1202  executes pre-feeding with the source of sheet feeding and destination of sheet discharge same as those in the printing operation.  
         [0153]    Upon receiving a print job from the host computer, the printer controller  1201  issues the print command to the engine control unit  1202  at the completion of image development of a predetermined page. If the image information for a next page is received from the host computer at this point, the print command is issued with the pre-feed reservation bit set at “1”. The printing operation can be started immediately by advancing the recording sheet to a predetermined position in advance by the pre-feed reservation and by issuing the print command after the image development for the next page is completed. Also, if the image information for the next page is not yet received at the completion of the image development for the current page, the printing command is issued with the pre-feed reservation bit “ 0 ” to execute the printing operation without the reservation for the pre-feeding.  
         [0154]    Thus, in the conventional configuration, the printing operation is started by the /PRNT signal from the printer controller while the sheet feeding operation is started by the /PRFD signal. In contrast, according to the present invention, the command transmitted from the controller  1201  to the engine control unit  1202  by the serial communication is given the functions of the /PRNT signal and the /PRFD signal, and the engine control unit is to improved as to be capable of starting the sheet feeding operation, independently from the establishment of the vertical synchronization by the /TOP signal.  
         [0155]    [0155]FIG. 27 shows an example of the configuration of the engine control unit.  
         [0156]    The engine control unit  1202  is provided with a CPU  2021  for control, a register  2026  connected to a bus  2023 , a ROM  2027  storing advance a process for the received command, and a RAM  2028  serving as a work memory area. It is connected with the control units  1103 ,  1104 ,  1105 ,  1106 ,  1107 ,  1108 ,  1109  and  1110 ,  1203  through an I/O controller  2022  and a bus  2024 , and is also provided with a communication shift register  2025  for communication with the printer controller  1201 . The communication with the printer controller  1201  is achieved by fetching the 16-bit command, received by the shift register  2025  when the /CBSY signal is in the true state as explained in the foregoing, into the register  2026  for example at the leading edge of the /CBSY signal. The status information of the engine control unit can be transmitted to the controller  1201  by shifting the content of the register  2026  to the shift register  2025  and also shifting the /SBSY signal to the true state.  
         [0157]    [0157]FIGS. 28 and 29 are flow charts showing the process of the engine control unit  1202  in the present embodiment. At first the command transmission strobe (/CBSY) signal from the printer controller  1201  is read (S 401 ), and, if it is not in the true state, the signal reading is repeated until it becomes true (S 402 ). When the strobe signal is shifted to the true state, the transmitted command is received (S 403 ), and the command code contained therein is analyzed (S 404 ). If the read command code is a print command, the sequence proceeds to a next step (S 405 ).  
         [0158]    If the read command is a print end command, there is executed an ending process (S 406 , S 407  and S 408 ), but, if it is another command, there is executed a corresponding process (S 406  and S 407 ) and the sequence returns to the step S 401  to awaits the transmission of a next command.  
         [0159]    In case the received command code is a print command, there is discriminated presence or absence of the pre-feed reservation (S 409 ), and, if present, information indicating such pre-feed reservation is stored for example in a memory area of the engine control unit  1202  (S 410 ).  
         [0160]    After the storage of the pre-feed reservation (or without such storage in the absence of the pre-feed reservation), there is confirmed the state of the pre-feed sensor  116  (FIG. 1) (S 411 ), and, if the pre-feeding operation is already executed, the electrophotographic process is started (S 415 ) and the sequence proceeds to a step S 416 . If the pre-feeding operation is not yet executed, the sheet feeding operation is started by driving the necessary ones among the rollers  9 ,  13 ,  31 ,  32 ,  33  and the starting-up of the electrophotographic process is initiated (S 412 ). When the pre-fed sheet is detected by the pre-feed sensor  116  (S 413 ), the feeding operation is terminated (S 414 ) and the completion of starting-up of the electrophotographic process is awaited (S 416 ).  
         [0161]    When the starting-up of the electrophotographic process is completed, the rollers  117 ,  115  are driven to re-start the sheet feeding, and, upon detection of the sheet by the registration sensor  118  (S 418 ), the /TOP signal is outputted (S 419 ) to execute the printing operation (S 420 ).  
         [0162]    On the other hand, simultaneous with the starting-up of the electrophotographic process, the pre-feed control unit  1203  discriminates the presence or absence of the pre-feed reservation (S 450 ), and, if present, awaits the sheet feed timing of the next page (S 451 ). The sheet feeding timing for the next page can be calculated for example from the sheet feed timing for the preceding page. It may also be suitably determined, depending on the conditions such as the sheet carrying path length, sheet size or printing speed, for example when the sheet of the preceding page ceases to be detected by the pre-feed sensor  116  or by the registration sensor  118  or when a predetermined time elapses after the start of the printing operation. Otherwise there may be provided a sensor for detecting such timing.  
         [0163]    The pre-feeding operation is started when the sheet feed timing for the next page is reached (S 452 ), and the operation is terminated after the sheet is carried until it is detected by the pre-feed sensor (S 453  and S 454 ).  
         [0164]    Thus, in contrast to the conventional configuration in which the sheet feeding cannot be started until the vertical synchronization is established by the transmission of the /TOP signal and the /PRFD signal is outputted from the printer controller, the configuration of the present invention allows to discriminate presence of absence of the pre-feed reservation independently from the reception of the /PRFD signal thereby enabling to start the pre-feeding operation, and is therefore effective in case the printer has a long sheet carrying path or in case of improving the throughput.  
         [0165]    Though not illustrated in FIGS. 28 and 29, it is possible to cancel the designated pre-feeding by outputting a command with the lowermost bit “ 0 ”, and to instruct the pre-feeding operation by thereafter sending a command with the lowermost bit “ 1 ”.  
       Ninth Embodiment  
       [0166]    The present embodiment is featured by a print command configuration as shown in FIG. 30, modified from that in the eighth embodiment.  
         [0167]    In the present embodiment, the command contains the print command of 4 bits, the source of sheet feeding of 3 bits, the destination of sheet discharge of 2 bits, the pre-feeding reservation of remaining 5 bits.  
         [0168]    In the pre-feed reservation area, the lowermost bit indicates presence or absence of pre-feed reservation as in the eighth embodiment, and the remaining 4 bits indicate the number of pre-feed reservations after the execution of the print command, namely a number equal to the total print number minus one.  
         [0169]    Because of the presence of such field for designating the number of pre-feed reservations in the command, the printer controller  1201 , upon receiving the job of a predetermined sheet number from the host computer, can instruct the pre-feeding operation to the engine control unit  1202  by a single command until the end of such job instead of executing the pre-feed reservation for each sheet, by designating the number of pre-feed reservations to the engine control unit  1202 .  
         [0170]    Thus, the printer controller  1201 , after designating the number of pre-feed reservations in the initial command, is only required in the subsequent print commands to set only the lowest bit in the pre-feed reservation field. If the pre-feed reservation has to be canceled by the situation of the printer controller  1201 , there can be issued a print command with the lowest bit “ 0 ”.  
         [0171]    [0171]FIGS. 31 and 32 are flow charts showing the function of the engine control unit in the present embodiment, wherein operations same as those in the eighth embodiment (FIGS. 28 and 29) are represented by same numbers as therein and will not be explained further.  
         [0172]    The process of the present embodiment is featured by a fact that the process branches depending on the sequential page number of printing after the presence/absence of the pre-feed reservation is discriminated (S 409 ), and that the process for the pre-feed numbers is added.  
         [0173]    More specifically, in case the pre-feeding reservation is present and in case of the print instruction for the first page, the pre-feed number field in the command is referred to and the content of the field is stored in a predetermined memory area in the pre-feed control unit  1203  (S 601  and S 602 ). For the print instruction for the second or subsequent page, the pre-feed number is not stored. This is because the pre-feed number is already designated in a preceding command and the pre-feed number field in the command received at this point may not contain any meaningful value. After the reservation information is stored in the predetermined memory area of the pre-feed control unit  1203  (S 411 ), the operations are same as those in the eighth embodiment.  
         [0174]    On the other hand, even if the pre-feed reservation is absent, there is discriminated whether the command is the print instruction for the first page (S 604 ), and, if for the first page, the sequence shifts to a process similar to that in the eighth embodiment. If for the second or subsequent page, there is erased the predetermined area storing the pre-feed reservation (S 605 ), whereby the pre-feed reservation can be canceled in the course of the printing operation.  
         [0175]    Simultaneous with the initiation of the starting-up of the electrophotographic process (S 412  or S 415 ), the pre-feed control unit  1203  discriminates presence or absence of the pre-feed reservation (S 450 ), and, if present and if the remaining pre-feed number is not zero (S 450 →S 606 ), there is awaited the sheet feed timing for the next page (S 451 ) and the pre-feeding operation is executed (S 453  and S 454 ). Upon completion of the pre-feed operation, the remaining pre-feed number stored in the memory area of the pre-feed control unit  1203  is decreased by one.  
         [0176]    The above-described process allows to designate the number of pre-feed operations in the beginning in a job for which the number of prints is known in advance, thereby alleviating the load of the printer controller and enabling the output operation fully exploiting the maximum ability of the print engine.  
       Tenth Embodiment  
       [0177]    [0177]FIG. 33 shows another example of the command configuration employable in the present invention.  
         [0178]    In the present embodiment, the command contains the print command of 5 bits, the source of sheet feeding of 3 bits, the destination of sheet discharge of 2 bits and the condition of pre-feed reservation of remaining 4 bits. As in the eighth and ninth embodiments, the lowermost bit (15th bit) indicates the presence or absence of pre-feed reservation as in the above-mentioned foregoing embodiments, designating the pre-feed reservation for the next page by “1”, while the remaining 3 bits designate the source of sheet for pre-feeding. Such source of sheet pre-feeding may be same as or different from that for the printing operation. In this manner the source of sheet pre-feeding can be set independently from the source of sheet feeding for the preceding page.  
         [0179]    The destination of discharge of the pre-fed sheet is designated by the field for designating the destination of sheet discharge in the print command issued subsequently by the printer controller  1201 .  
         [0180]    In case the source of sheet feeding designated for the pre-fed sheet is different from the source of sheet feeding in the succeeding print command, the engine control unit  1202  returns an error command status to the printer controller  1201 , indicating the error in the designation of the source of sheet feeding.  
         [0181]    [0181]FIGS. 34 and 35 are flow charts showing the process of the engine control unit in the present embodiment, wherein steps same as those in FIGS. 28 and 29 are represented by same numbers and will not be explained further.  
         [0182]    In case the pre-feed reservation is already made and the pre-feeding operation is already completed at the reception of the print instruction (S 411 ), there is discriminated whether the source of sheet feeding designated in the print instruction coincides with the source of sheet feeding designated in the pre-feeding (S 801 ), and, if not, an error process is executed (S 802 ). Such error process may be to discharge the pre-fed sheet and to start the sheet feeding operation again from the source designated at the print instruction, or to continue the printing operation while outputting an error message if the sheet size is same. It is naturally possible also be interrupt the printing operation.  
         [0183]    Other processes will not be explained further as they are same as those in the eighth embodiment, except that the pre-feeding is executed from the designated source (S 803 ).  
         [0184]    In the eighth to tenth embodiments, there may be applied the ID management for each sheet according to any of the first to seventh embodiments.  
         [0185]    In the preceding embodiments, the recording sheets waits n the pre-feed position until the electrophotographic process is started up, but, as long as the recording sheet does not overlap with the preceding sheet, the recording sheet may be advanced from the pre-feed position to the position of the registration sensor in the course of printing of the preceding page or of the starting-up of the electrophotographic process and stopped in such position to awaits the completion of starting-up of the process. Such configuration allows to further shorten the carrying distance from the starting-up of the process to the actual printing position.  
         [0186]    The present invention may be applied to a configuration in which the printer controller and the engine control unit are present different equipment, or to an apparatus consisting of a single equipment (such as a copying apparatus or a facsimile apparatus). It is also applicable to a recording apparatus of non-electrophotographic process.  
         [0187]    Further, the objects of the present invention can naturally be attained in a case of supplying a system or an apparatus with a memory medium storing program codes of a software realizing the functions of the foregoing embodiments and a computer (or CPU or MPU) in such system or apparatus reads and executes the program codes stored in the memory medium.  
         [0188]    In such case the program codes themselves of the software realize the functions of the foregoing embodiments, and the memory medium storing the program codes constitutes the present invention.  
         [0189]    The memory medium storing such program codes can be, for example, a floppy disk, a hard disk, an optical disk, a magnetooptical disk, a CD-ROM, a CR-R, a magnetic tape, a non-volatile memory card, or a ROM.  
         [0190]    The present invention also includes not only a case where the functions of the above-mentioned embodiments are realized by the execution of the program codes read by the computer but also a case where an operating system or the like functioning on the computer executes all or a part of the actual processes under the control of such program codes thereby realizing the functions of the foregoing embodiments.  
         [0191]    The present invention further includes a case wherein the program codes read from the memory medium are once stored in a function expansion board inserted into the computer or a function expansion unit connected to the computer, and a CPU provided in the function expansion board or the function expansion unit executes all the process or a part thereof under the control of such program codes, thereby realizing the functions of the above-mentioned embodiments.