Abstract:
An image forming system having an image information generation apparatus for generating image information, and an image forming apparatus capable of forming images on both sides of a printing medium on the basis of the image information generated by the image information generation apparatus, the image information generation apparatus comprises image information generation unit for generating image information, storage unit for storing the generated image information to allow identifying whether the image information is image information to be printed on a first surface or a second surface of the sheet.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an image forming system including a host computer, and an image forming method. 
   BACKGROUND OF THE INVENTION 
   An image forming apparatus which comprises an image read unit for reading an image and converting it into an image signal, a memory for holding the image signal, and an image output section for outputting the held image signal, and operates as a color copying machine has been proposed. 
   In addition, an image forming system has been proposed, which operates a color printer or operates both of a color copying machine and a color printer based on an instruction from the operator, in which the image forming apparatus and host computer are connected through a controller and which rasterizes, by the controller, color image information (PDL data) created by the host computer, holds the data in the internal memory of the controller, and then outputs the data from the image forming apparatus. 
   In such an image forming system, the controller has a large-capacity memory and compresses image information to store raster data corresponding an enormous number of pages in the memory. 
   In addition, while bitmapping PDL data, already bitmapped pages can be parallelly printed. The image forming apparatus is capable of double-side printing. The first surface pages are printed first and stored in an intermediate tray, and then, the second surface pages are printed from the intermediate tray. To print a document having pages in number larger than the stack count of the intermediate tray, paper sheets are repeatedly stacked/extracted in/from the intermediate tray in units of stack counts. 
   Communication control between the controller and the image forming apparatus in the prior art will be described. 
   The controller and image forming apparatus are connected by serial communication, and control commands are defined. As for the communication relationship, the controller serves as a master, and the image forming apparatus serves as a slave. Main control commands used for communication will be described below. 
   Status: A command for inquiring about the state of the image forming apparatus. The image forming apparatus returns “no paper”, “error”, “status (“operation in progress”, “door open”, and “prepare”)”, or “print report”. 
   Ready Page: The image forming apparatus is notified of information related to printable pages. The apparatus is notified of “the number of pages in the ready”, “paper feed/discharge positions”, and “color mode”. 
   Start: This command instructs the start of printing. 
   “Status” is transmitted every predetermined time. “Ready Page” is transmitted at least once before the start of printing. It may be transmitted during printing. 
     FIG. 6  shows the communication procedure between the controller and the image forming apparatus in executing double-side printing in the image forming system of the prior art. 
   In step S 601 , the controller sends “Status” to the image forming apparatus to inquire about its state. 
   In step S 602 , the image forming apparatus returns to the controller the paper state, the presence/absence of error, and the state of the image forming apparatus. 
   In step S 603 , pieces of information related to the page to be printed on the first surface are sent to the image forming apparatus. The controller sends to the image forming apparatus pieces of information including the number of pages, paper feed position, discharge position (intermediate tray), and color mode. 
   In step S 604 , the image forming apparatus stores the received information and notifies the controller of reception of data. If the image forming apparatus fails in storage, it transmits a resend request. 
   In step S 605 , pieces of information (the paper feed position is “intermediate tray”, the discharge position is “outside the machine”, and the like) related to the page to be printed on the second surface are sent to the image forming apparatus. 
   In step S 606 , the image forming apparatus notifies the controller that the pieces of received information are stored. 
   In step S 607 , the controller side instructs the start of printing. In step S 608 , the image forming apparatus side notifies the controller whether printing can be started. During printing, the controller periodically checks the state of the image forming apparatus by the “Status” command (S 609 ) and knows whether the image forming apparatus has stopped upon receiving the number of printed pages in a print report “Report” (S 610 ). 
   However, the conventional double-side printing has the following two problems. As the first problem, since the double-side printing uses the intermediate tray, paper feed to the intermediate tray and discharge from the machine are repeated. Hence, the productivity is low relative to through-path double-side printing using no intermediate tray. 
   As the second problem, since printing results of the first surface pages are temporarily stored in the intermediate tray, page processing is complicated, resulting in complexity in page control. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above problems. 
   It is another object of the present invention to provide an image forming system and method which allow double-side printing with high throughput, and a storage medium storing a program for causing a computer to execute the method. 
   In order to achieve the above objects, according to an aspect of the present invention, there is provided an image forming system comprising:
         image generation means for generating image information to be drawn on the basis of PDL data;   information management means for individually managing the generated image information as double-side printing information to be printed on upper and lower surfaces as first and second surfaces of a medium;   storage means for communicating page information which defines an attribute of the double-side printing information and individually storing the information in order to control double-side printing;   transmission request means for requesting start of transmission of the double-side printing information of the first surface or the second surface in accordance with the stored page information; and   image forming means for forming an image on the basis of the double-side printing information of the first surface or the second surface, which is received on the basis of the request of the start of transmission,   wherein the double-side printing is controlled on the basis of the transmission request from the transmission request means.       

   According to another aspect of the present invention, there is also provided an image forming method comprising:
         the image generation step of generating image information to be drawn on the basis of PDL data;   the information management step of individually managing the generated image information as double-side printing information to be printed on upper and lower surfaces as first and second surfaces of a medium;   the storage step of communicating page information which defines an attribute of the double-side printing information and individually storing the information in a memory in order to control double-side printing;   the transmission request step of requesting start of transmission of the double-side printing information of the first surface or the second surface in accordance with the stored page information; and   the image forming step of forming an image on the basis of the double-side printing information of the first surface or the second surface, which is received on the basis of the request of the start of transmission,   wherein the double-side printing is controlled on the basis of the transmission request in the transmission request step.       

   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing the flow of an image signal; 
       FIG. 2  is a view showing the arrangement of an image forming apparatus; 
       FIG. 3  is a schematic view showing the system arrangement; 
       FIG. 4  is a timing chart showing operation of the system as a standalone copying machine; 
       FIG. 5  is a timing chart showing operation as a system including a host computer; 
       FIG. 6  is a view showing communication between a controller and an image forming apparatus in a prior art; 
       FIG. 7  is a view showing communication between a controller and a printer according to an embodiment; 
       FIG. 8  is a timing chart showing the operation of an image control signal according to the embodiment; and 
       FIGS. 9A to 9D  are views showing the hard circuit arrangement for controlling double-side printing according to the embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   As a preferred embodiment, an image forming system related to a full-color copying machine will be described below in detail. However, the present invention is not limited to this embodiment. 
   [Outline of System Arrangement] 
     FIG. 3  is a view showing the system arrangement according to the first embodiment of the present invention. Reference numeral  101  denotes a host computer; and  102 , a controller. An image forming apparatus  103  copies an original placed on the original table in colors and outputs a color image sent from the computer  101  through the controller  102 . On the host computer  101 , so-called DTP (Desk Top Publishing) application software runs to create or edit various kinds of documents and graphics. 
   The host computer  101  converts a created document or graphic into PDL (Page Description Language; e.g., Post Script available from Adobe) and sends it to the controller  102  through a connection cable  243 . 
   The controller  102  translates and rasterizes the PDL sent from the host computer  101 . The rasterized image signal is sent to the image forming apparatus  103  through a connection cable  242  and output. The controller  102  may be incorporated in the image forming apparatus  103 . 
   [Outline of Image Forming Apparatus] 
     FIG. 2  is a view showing the arrangement of the image forming apparatus  103  shown in  FIG. 3 . In copying an original as a copying machine, an original  202  to be read is placed on an original glass table  201 . The original  202  is irradiated with an illumination lamp  203 , so an image is formed on a CCD  208  by an optical system  207  through mirrors  204 ,  205 , and  206 . By a motor  209 , a first mirror unit  210  including the mirror  204  and illumination lamp  203  is mechanically driven at a velocity V, and a second mirror unit  211  including the mirrors  205  and  206  is driven at a velocity ½V to scan the entire surface of the original  202 . 
   An image processing circuit section  212  processes the read image information as an electrical signal, temporarily holds the signal on an image memory  108 , and outputs it as a print signal. 
   The print signal output from the image processing circuit section  212  is sent to a laser driver (not shown) to drive four semiconductor lasers (not shown). 
   A polygon mirror  213  receives four laser beams emitted by the four semiconductor lasers (not shown). The first beam scans a photosensitive drum  217  through mirrors  214 ,  215 , and  216 , the second beam scans a photosensitive drum  221  through mirrors  218 ,  219 , and  220 , the third beam scans a photosensitive drum  225  through mirrors  222 ,  223 , and  224 , and the fourth beam scans a photosensitive drum  229  through mirrors  226 ,  227 , and  228 . 
   A developing unit  230  for supplying yellow (Y) toner forms a yellow toner image on the photosensitive drum  217  in accordance with the laser beam. A developing unit  231  for supplying magenta (M) toner forms a magenta toner image on the photosensitive drum  221  in accordance with the laser beam. A developing unit  232  for supplying cyan (C) toner forms a cyan toner image on the photosensitive drum  225  in accordance with the laser beam. A developing unit  233  for supplying black (Bk) toner forms a black toner image on the photosensitive drum  229  in accordance with the laser beam. 
   The four-color (Y, M, C, and Bk) toner images are transferred to a paper sheet, and a full-color output image can be obtained. 
   A paper sheet fed from one of paper cassettes  234  and  235  and manual feed tray  236  is passed through registration rollers  237 , chucked on a transfer belt  238 , and conveyed. In synchronism with the paper feed timing, the respective color toners are developed on the photosensitive drums  217 ,  221 ,  225 , and  227  in advance. As the paper sheet is conveyed, the toners are transferred to the paper sheet. 
   The paper sheet having the respective color toners transferred thereon is separated, conveyed by a conveyor belt  239 , and after the toners are fixed on the paper sheet by a fixing unit  240 , discharged to a discharge tray  241 . In double-side operation, a paper sheet fed from one of the paper cassettes  234  and  235  and manual feed tray  236  is passed through the registration rollers  237 , chucked on the transfer belt  238 , and conveyed. In synchronism with the paper feed timing, the respective color toners are developed on the photosensitive drums  217 ,  221 ,  225 , and  227  in advance. As the paper sheet is conveyed, an image is formed on the first surface, and the toners are transferred to the paper sheet. 
   The paper sheet having the respective color toners transferred thereon is separated, conveyed by the conveyor belt  239 , after the toners are fixed on the paper sheet by the fixing unit  240 , passed through a vertical discharge path  246  by a discharge deflecting plate, and conveyed to a double-side inverting section  245 . After a predetermined time from the passage of the paper sheet, double-side inverting section inlet rollers are reversed. The paper sheet is inverted and conveyed to a double-side path preconvey section  247  and then to a double-side path  244 . At this time, the paper sheet on the double-side path  244  has its first-page image facing upward. The paper sheet is conveyed to the double-side path, aligned, and immediately after this, refed for image formation on the second surface, passed through the fixing unit  240 , and discharged to the discharge tray  241 . When the double-side operation is to be continuously performed for a plurality of paper sheets, refeed from the double-side path and feed from the paper tray alternate. 
   In outputting an image sent from the host computer  101  through the controller  102 , the image is directly transferred to a PWM circuit (not shown) through the interface cable  242  and formed, as in the copying operation. 
   The four photosensitive drums  217 ,  221 ,  225 , and  229  are arranged at an equidistant interval d. The paper sheet is conveyed by the conveyor belt  239  at a predetermined velocity v. The four semiconductor lasers are driven in synchronism with that timing. 
   [Flow of Image Signal] 
     FIG. 1  shows the flow of an image signal. The CCD sensor  208  outputs the three, i.e., red (R), green (G), and blue (B) color components of a read image as digital signals. A masking circuit  112  converts the input (R 0 , G 0 , B 0 ) signals into standard (R, G, B) signals by calculation using 
               (         R           G           B         )     =       (           C   11           C   12           C   13               C   21           C   22           C   23               C   31           C   32           C   33           )     =     (         R0           G0           B0         )               (   1   )               
where c ij (i=1, 2, 3; j=1, 2, 3) is a constant unique to the apparatus, which considers various characteristics including the sensitivity characteristic of the CCD sensor and the spectral characteristic of the illumination lamp.
 
   A luminance/density conversion section  104  formed from a lookup table in a RAM or ROM executes conversion using 
                     C1   =       -   K     ×       LOG   10     ⁡     (     R   /   255     )                     M1   =       -   K     ×       LOG   10     ⁡     (     G   /   255     )                     Y1   =       -   K     ×       LOG   10     ⁡     (     B   /   255     )                 }           (   2   )             
 
where K is a constant.
 
   An output masking/UCR circuit section  106  for converting the signals M 1 , C 1 , and Y 1  into signals Y, M, C, and Bk corresponding to the toner colors of the image forming apparatus executes calculation by 
               (         C           M           Y           Bk         )     =       (           a   11           a   21           a   31           a   41               a   12           a   22           a   32           a   42               a   13           a   23           a   33           a   43               a   14           a   24           a   34           a   44           )     ⁢     (         C1           M1           Y1           Bk1         )               (   3   )             
 
where a ij  (i=1, 2, 3, 4; j=1, 2, 3, 4) is a constant unique to the apparatus, which considers the tint characteristic of toner. The signals C 1 , M 1 , Y 1 , and Bk 1  have the relationship represented by
 
 Bk   1 =min( C   1 ,  M   1 ,  Y   1 )  (4)
 
   On the basis of equations (2), (3), and (4), the signals C 1 , M 1 , Y 1 , and Bk 1  based on the signals R, G, and B read by the CCD sensor are corrected to the signals C, M, Y, and Bk based on the spectral distribution characteristics of toners and output. 
   A character/line drawing detection circuit  105  determines, for each pixel of the original image, whether it is part of a character or line drawing and generates a determination signal TEXT. 
   A compression/expansion circuit  107  compresses the image signals (R, G, B) and character/line drawing determination signal TEXT to reduce the information amount and then stores them in the memory  108 . Alternatively, the compression/expansion circuit  107  expands the image signals (R, G, B) and character/line drawing determination signal TEXT on the basis of data read out from the memory  108 . 
   The controller  102  is controlled by a CPU  110  such that the signals Y, M, C, and Bk matching the spectral sensitivity characteristics of the toners are stored and read out in synchronism with the image formation timing on the copying machine side. A memory  109  holds an image signal read by the CCD  208  through the cable  242  or a computer image sent from the host computer  101  through the cable  243 . 
   [Operation of System as Standalone Copying Machine] 
   The system of this embodiment operates as a standalone copying machine or as a system including a controller. 
   The operation of the system as the standalone copying machine will be described first. In copying machine operation, an image signal read by the CCD  208  passes through the masking circuit  112  and luminance/density conversion section  104 , is compressed by the compression/expansion circuit  107 , and written in the memory  108 . The character/line drawing determination signal TEXT detected by the character/line drawing detection circuit  105  is also compressed by the compression/expansion circuit  107  and then written in the memory  108 . 
   Data read out from the memory  108  is expanded by the compression/expansion circuit  107 , sent in accordance with the image formation timing of the copying machine, and sent to the laser driver through the PWM circuit (not shown).  FIG. 4  is the timing chart of this operation. 
   Referring to  FIG. 4 , an image read by the CCD  208  is written in the memory  108  at a timing  401 . The pieces of image information written in the memory  108  are read out at timings  402 ,  403 ,  404 , and  405 . The timings  402 ,  403 ,  404 , and  405  have the relationship shown in  FIG. 4 , so the pieces of information are read at a time interval d/v. As described above, d is the equidistant interval between the four drums, and v is the velocity of a paper sheet conveyed by the conveyor belt. 
   [System Operation Including Controller] 
   System operation including the controller will be described next. The system operation including the controller includes scanning operation, PDL bitmapping operation, and printing operation. 
   The scanning operation will be described first. In this operation, an image read by the CCD  208  is received by the controller. The image information is converted into RGB data or YMCK data and held in the memory  109 . 
   In the PDL bitmapping operation, PDL data sent from the host computer  101  is interpreted by the CPU  110  of the controller, bitmapped to a full-color image, and written in the image memory  109 . This full-color image is bitmapped as image information color-separated into four colors, i.e., yellow (Y), magenta (M), cyan (C), and black (Bk) in accordance with the output characteristics of the image forming apparatus  103 . 
   In the printing operation, the full-color image bitmapped on the image memory  109  and the signal TEXT which identifies a character or line drawing are read out in synchronism with the four drums  217 ,  221 ,  225 , and  229 , sent to the laser driver through the PWM circuit (not shown), and output by printing. 
     FIG. 5  is a timing chart showing the operation of the system including the controller. 
   Referring to  FIG. 5 , PDL bitmapping operation is performed in a section  501 , the determination signal TEXT is generated in a section  502 , and simultaneously, write operation in the memory  109  is performed. Pieces of image information written in the memory  109  are read out at timings  503 ,  504 ,  505 , and  506 . The timings  503 ,  504 ,  505 , and  506  have the relationship shown in  FIG. 5 , so the pieces of information are read at the time interval d/v. As described above, d is the equidistant interval between the four drums, and v is the velocity of a paper sheet conveyed by the conveyor belt. 
   As a characteristic feature, PDL bitmapping operation and generation of the signal TEXT are simultaneously executed. Hence, high-speed processing can be realized as compared to a case wherein these operations are sequentially performed. The simultaneous processing of these operations is controlled by the CPU  110 . 
   [Double-Side Printing] 
   A case wherein processing is executed by designating double-side printing from the host computer  101  will be described. On the host computer, a printer driver generates PDL data for double-side printing. The PDL data is transferred from the host computer  101  to the controller  102  through a parallel cable or a network such as Ethernet. The controller  102  stores the transferred PDL data in an internal hard disk  111 . The PDL data is bitmapped on the memory  109  by the CPU  110  of the controller  102  as a raster image. The raster data bitmapped on the memory  109  is sent to the laser driver through the interface cable  242 , and printing starts. 
   When printing starts, a paper sheet is fed from one of the cassettes  234  and  235  and manual feed tray  236  in accordance with an instruction from the controller  102  and chucked on the transfer belt. As the paper sheet is conveyed, the image is formed on the first surface of the paper sheet. At this time, the controller  102  must send the image information of the first surface to the printer first. While the paper sheet whose first surface has the formed image is being conveyed to the double-side path  244 , the second, third, and fourth paper sheets are sequentially fed, and an image is formed on the first surfaces of the respective paper sheets (third, fifth, and seventh pages). After this, after the first paper sheet is passed through the double-side path  244  and refed for image formation on the second surface (second page), paper feed from the cassette  235  or manual feed tray  236  and refeed from the double-side path  244  alternate. For example, to send a document having 16 pages in ascending order, the controller sends to the printer the pieces of information in the order of
         1, 3, 5, 7, 2, 9, 4, 11, 6, 13, 8, 15, 10, 12, 14, and 16
 
Double-side printing of image information from the controller has been described above. Printing can be executed according to the same operation as described above even when originals of a plurality of pages are read by the CCD  208 , the pieces of image information are stored in the memory  108 , and the images from the memory  108  are subjected to double-side printing. The basic operation of this processing is the same as in double-side printing of image information from the controller, and a description will be made using the former case as an example.
 
[Communication Between Controller and Printer in Double-Side Printing]
       

   Communication control between the controller  102  and the printer  103  will be described with reference to  FIG. 7 . Upon receiving PDL data for the host computer  101 , the controller  102  bitmaps the data on the frame buffer memory  109 . In step S 71 , the controller  102  sends “Status” to the printer to inquire about its state before the start of printing. 
   In step S 72 , the printer returns to the controller the paper state, the presence/absence of error, and the state of the printer (preparation state or printable state). 
   In step S 73 , the controller  102  sends to the printer  103  pieces of information related to the pages for double-side printing. “Ready Page” shown in  FIG. 7  has a portion for sending the number of pages, a portion for sending pieces of information such as the feed position of the first surface, discharge position (intermediate tray), and color mode, and a portion for sending pieces of information such as the feed position of the second surface (double-side path), discharge position (outside the machine), and color mode. Specifically, when the bitmap data of the first and the second are made ready, the data as a pair are sent to the printer  103  by the controller  102 . 
   In step S 74 , the printer  103  stores the received information and notifies the controller of reception. If the printer fails in storage, it transmits a resend request. The printer separately manages the information for the first surface and that for the second surface. If the information related to the first or second surface is different from that for the previous paper sheet, the information is sent to the printer by “Ready Page” every time. The second or subsequent “Ready Page” may be transmitted during printing. 
   In step S 75 , the start of printing is instructed. In step S 76 - 1 , the printer  103  notifies the controller whether printing can be started and requests the controller to transmit predetermined image information to the printer. 
   In step S 77 - 1 , the predetermined image information (e.g., image information related to the first surface of the medium) is transmitted to the printer on the basis of the image information transmission request. The printer forms an image on the basis of the data (S 78 - 1 ). 
   In step S 79 - 1 , it is determined whether image information to be transmitted next is image information for the first surface or image information for the second surface. In the above-described printer operation or operation shown in  FIG. 8  to be described later, after images are formed on the first surfaces of the first to fourth paper sheets, image formation on the second surface and image formation on the first surface are alternately performed, and then, image formation on the second surface is performed four times. However, this order changes depending on the paper size or the like. In this embodiment, when it is detected that a paper sheet has reached the refeed standby position of the double-side path  244 , it is determined that image information to be transmitted next is image information for the second surface, and a refeed signal is output (S 79 - 2 ). With this processing, it is specified that the image information transmission request to the controller in step S 76 - 2  is a transmission request of the image information for the second surface. The controller transmits to the printer predetermined image information (e.g., image information related to the second surface of the medium) on the basis of the image information transmission request (S 77 - 2 ). The printer forms an image on the basis of the data (S 78 - 2 ). 
   During printing, the controller periodically transmits the “Status” command, as in step S 80 , to check the state of the printer. The controller  102  knows whether the printer has stopped upon receiving the number of printed pages in a printer state report in step S 81 . 
   [Management of Images for Double-Side Printing] 
   Image signal transfer from the controller  102  to the printer  103  after the start of printing will be described with reference to  FIG. 8 . 
   The controller  102  transfers image signals in accordance with page enable signals (cyan CPE  82 , magenta MPE  83 , yellow YPE  84 , and black KPE  85 ) sent from the printer in units of colors.  FIG. 8  shows an example of a print job having 10 pages in ascending order. At C 1  shown in  FIG. 8 , a cyan image signal of the first page is transferred. A refeed signal  81  (to be described later) is connected to the CPU in the controller  102 . The leading or trailing edge of the refeed signal can be detected as an interrupt signal. 
   When printing starts, the controller  102  is set to output image information for the first surface, and transfers image signals in accordance with the page enable signals. When the paper sheet for the first page is to be refed, the refeed signal  81  goes high. When this leading edge is detected as an interrupt signal, the controller  102  is set to output image information for the second surface from the subsequent first station (cyan in this case), and transfers the image signals in accordance with the page enable signals. When the trailing edge of the refeed signal  81  is detected, the controller is set to output the image information for the first surface again from the subsequent first station, and transfers the image signals in accordance with the page enable signals. As described above, at the start of printing, the image of the first surface is set. When the leading or trailing edge of the refeed signal output from the printer is detected, the image information to be transmitted is switched between the image information for the first surface and that for the second surface, thereby managing the images for double-side printing. 
   Control signals are exchanged between the controller and the printer through the interface cable  242 , like the video signal. 
   Other Embodiment 
   In the above embodiment, the refeed signal from the printer is transmitted through a dedicated signal line. However, it may be transmitted as one status of the printer. In the above embodiment, refeed signal detection and image information switching in the controller are done by software. However, they may be done by hardware.  FIGS. 9A to 9D  are views showing the circuit arrangement in this case. A memory  1   901  is a page memory for the first surface for C (cyan), and a memory  2   902  is a page memory for the second surface for C. In accordance with a signal obtained by latching a refeed signal with CPE by a latch circuit  904 , a selector  903  selects data for the first surface or second surface. By switching the data in accordance with the CPE latch signal, even when the refeed signal is switched during output from the memory, the data bus from the memory is not switched. For the remaining colors as well, the data bus from each memory is switched in accordance with a signal obtained by latching the refeed signal with MPE ( FIG. 9B ), YPE ( FIG. 9C ), or KPE ( FIG. 9D ), thereby managing images for double-side printing. 
   When the number of paper sheets feedable on the double-side path can be variably controlled, the following flexible control becomes possible. 
   More specifically, when the paper sheet on the double-side path has reached the refeed standby position at a timing delayed from the normal timing, in the conventional method, a convey error occurs, or image formation on the second surface of the paper sheet is performed after waiting arrival of the paper sheet at the refeed standby position. In this embodiment, it can be determined by the printer whether the image of the first surface or the image of the second surface is to be formed next. Hence, in the above case, without waiting arrival of the paper sheet at the refeed standby position, a paper sheet can be fed from the cassette  235  or manual feed tray  236  for image formation on the first surface. The order of pages for image information can be flexibly controlled in accordance with the convey state of paper sheets. This can also improve the throughput. 
   As has been described above, in double-side printing by an image forming system including a host computer, the printer is notified of pieces of information for both surfaces of a paper sheet before the start of printing, and of the paper feed position before image signal transfer, thereby allowing double-side printing with high productivity by through-path double-side printing. 
   As many apparently widely different embodiments of the present invention can be, made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.