Patent Publication Number: US-10768566-B2

Title: Image forming apparatus for generating drive data by performing a magnification correction on image data

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The aspect of the embodiments relates to an image forming apparatus. 
     Description of the Related Art 
     As an image printing technology for use in image forming apparatuses (e.g., a copying machine), an electrophotographic technology has been developed. Electrophotographic image forming apparatuses form a latent image on a photoconductor by emitting a light beam to the photoconductor based on image data input from a document reader or an external device, such as a computer. The latent image is developed with a coloring material (toner). An example of a color image forming apparatus is an image forming apparatus including a plurality of photoconductors for developing yellow, magenta, cyan, and black toner images and a plurality of light sources each provided for one of the photoconductors and emitting a light beam.  FIG. 7A  illustrates the control blocks of the color image forming apparatus. 
     Image forming apparatuses perform correction in accordance with the characteristics of a laser scanner. An example of such correction is partial magnification correction described in Japanese Patent Laid-Open Nos. 2005-096351 and 2013-240994, which is magnification correction to be applied to each of a plurality of sub-areas obtained by dividing the image formation area in the main scanning direction. 
     In recent years, to meet the demands for higher image quality, the image formation area has been finely divided in the main scanning direction (for example, divided into 32). In addition, in many cases, a plurality of light beams are provided to improve the throughput of the image forming apparatus, and the output from a PWM output unit  5216  is transmitted to an optical scanning device  5104  for each of the light beams. Thus, the cost increases with increasing number of required signal lines. Accordingly, as illustrated in  FIG. 7B , the image processing unit is divided into a first image processing unit  6200  and a second image processing unit  6250  so as to reduce the number of signal lines. 
     In the image forming apparatus in a tandem configuration illustrated in  FIG. 8A , to form a color image, toner images of respective colors formed on photoconductive drums  7001  is transferred to  7004  so that the images overlap at the same position on a transfer belt  7009 . Therefore, as illustrated in  FIG. 8B , the color image forming apparatus in a tandem configuration forms the latent images of the respective colors by delaying the start time of formation from a reference timing signal  8000  by time periods Td 1 , Td 2 , and Td 3 , respectively. 
     Image forming apparatuses developed in recent years can insert a separator sheet between each printout during continuous printing. If the sizes of the print medium of the printout and the separator sheet differ from each other, the CPU is to send control data again and perform image formation in accordance with the size of the print medium after switching. The control data is transmitted in a period during which no image data is transmitted from the first image processing unit  6200  to the second image processing unit  6250 . As illustrated in  FIG. 7B , the control data is communicated (transmitted) via a common signal line  600  connected to a communication unit  6105  and a communication unit  6252 . 
     However, as illustrated in  FIG. 9B , when a single separator sheet having a size that differs from the size of the print media is inserted between the print media during continuous printing, the following situation arises. For example, even when the transmission start timing of M color control data is reached ( 9502   c ), Y control data is still being transmitted ( 9501   c ). For this reason, transmission of the M color control data is not performed in the communication period-β, and the transmission starts after a communication period α has elapsed. In contrast, since transmission of M color image data is started based on a reference timing signal, transmission of the M color image data is started before the transmission of the M control data is completed ( 9502   a ). As a result, an M color image cannot be properly formed and, thus, image defects may occur. The same applies to C and M colors. 
     To avoid such a situation, a method for increasing the communication speed of serial communication or a method for expanding the interval between the trailing edge of the previous image and the leading edge of the image can be employed. Alternatively, for example, a method for increasing a rotational speed v of the photoconductive drum while keeping the throughput constant or a method for communicating the Y color control data after transmission of the K color control data can be employed. However, these methods increase the cost or decrease the throughput. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the embodiments, an image forming apparatus includes a first toner image forming unit including a first photoconductor rotatingly driven, a first exposure unit configured to expose the first photoconductor, a first drive unit configured to drive the first exposure unit based on first drive data, and a first development unit configured to develop, with toner of a first color, a first electrostatic latent image formed on the first photoconductor through exposure in the first exposure unit, a second toner image forming unit including a second photoconductor rotatingly driven, a second exposure unit configured to expose the second photoconductor, a second drive unit configured to drive the second exposure unit based on second drive data, and a second development unit configured to develop, with toner of a second color, a second electrostatic latent image formed on the second photoconductor through exposure in the second exposure unit, and a transfer unit formed as an endless transfer belt rotatingly driven, where the transfer unit is configured to transfer the toner image on the first photoconductor and the toner image on the second photoconductor to a print medium via the transfer member. A transfer position of the toner image transferred from the first photoconductor to the transfer member is located upstream of a transfer position of the toner image transferred from the second photoconductor to the transfer member in a rotational direction of the transfer member, and a formation start timing of the second electrostatic latent image is delayed behind a formation start timing of the first electrostatic latent image on one print medium based on a delay amount in accordance with a distance between the transfer positions. The image forming apparatus further includes a data generation unit configured to generate first image data for the first color and second image data for the second color from input image data, a data processing unit configured to generate the first drive data obtained by performing a magnification correction process on the first image data and the second drive data obtained by performing a magnification correction process on the second image data based on set magnification correction data, and a controller configured to switch setting of the magnification correction data in accordance with a size of the print medium, where the controller switches the magnification correction data set in the data processing unit by outputting, to the data processing unit via a common signal line, the magnification correction data for the first image data and the magnification correction data for the second image data at different timings based on the delay amount corresponding to the distance between the transfer positions. If a timing of outputting the magnification correction data for the first image data to form the first electrostatic latent image for an (n+1)th print medium overlaps a timing of outputting the magnification correction data for the second image data to form an electrostatic latent image for an nth print medium having a size smaller than the (n+1)th print medium in a conveyance direction of the print medium, the controller outputs the magnification correction data for the second image data to form the second electrostatic latent image for the nth print medium before the magnification correction data for the first image data to form the first electrostatic latent image for the (n+1)th print medium is output, and the controller outputs the magnification correction data for the (n+1)th print medium after a magnification correction process performed by the data processing unit based on the magnification correction data for the nth print medium is completed. 
     According to another aspect of the embodiments, an image forming apparatus includes a first toner image forming unit including a first photoconductor rotatingly driven, a first exposure unit configured to expose the first photoconductor, a first drive unit configured to drive the first exposure unit based on first drive data, and a first development unit configured to develop, with toner of a first color, a first electrostatic latent image formed on the first photoconductor through exposure in the first exposure unit, a second toner image forming unit including a second photoconductor rotatingly driven, a second exposure unit configured to expose the second photoconductor, a second drive unit configured to drive the second exposure unit based on second drive data, and a second development unit configured to develop, with toner of a second color, a second electrostatic latent image formed on the second photoconductor through exposure in the second exposure unit, and a transfer unit formed as a endless transfer belt rotatingly driven, where the transfer unit is configured to transfer the toner image on the first photoconductor and the toner image on the second photoconductor to a print medium via the transfer member. A transfer position of the toner image transferred from the first photoconductor to the transfer member is located upstream of a transfer position of the toner image transferred from the second photoconductor to the transfer member in a rotational direction of the transfer member, and a formation start timing of the second electrostatic latent image is delayed behind a formation start timing of the first electrostatic latent image on one print medium based on a delay amount in accordance with a distance between the transfer positions. The image forming apparatus further includes a data generation unit configured to generate first image data for the first color and second image data for the second color from input image data, a data processing unit configured to generate the first drive data obtained by performing a position correction process on the first image data to correct a position of a toner image relative to the print medium and the second drive data obtained by performing a position correction process on the second image data to correct a position of a toner image relative to the print medium based on set position correction data, and a controller configured to switch setting of the position correction data in accordance with a size of the print medium, where the controller switches the position correction data set in the data processing unit by outputting, to the data processing unit via a common signal line, the position correction data for the first image data and the position correction data for the second image data at different timings based on the delay amount corresponding to the distance between the transfer positions. If a timing of outputting the position correction data for the first image data to form the first electrostatic latent image for an (n+1)th print medium overlaps a timing of outputting the position correction data for the second image data to form an electrostatic latent image for an nth print medium having a size smaller than the (n+1)th print medium in a conveyance direction of the print medium, the controller outputs the position correction data for the second image data to form the second electrostatic latent image for the nth print medium before the position correction data for the first image data to form the first electrostatic latent image for the (n+1)th print medium is output, and the controller outputs the position correction data for the (n+1)th print medium after a position correction process performed by the data processing unit based on the position correction data for the nth print medium is completed. 
     Further features of the disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates the overall configuration of an image forming apparatus according to an exemplary embodiment, and  FIG. 1B  illustrates a main part of an optical scanning device. 
         FIG. 2  is a block diagram of an image forming apparatus according to an exemplary embodiment. 
         FIG. 3  illustrates a transmission period of image data according to the exemplary embodiment. 
         FIG. 4  illustrates control data according to the exemplary embodiment. 
         FIG. 5  is a flowchart illustrating control processing of transmission timing of the control data according to the exemplary embodiment. 
         FIG. 6  is a flowchart for determining whether transmission timings of control data overlap according to the exemplary embodiment. 
         FIGS. 7A and 7B  are block diagrams of an image forming apparatus according to an existing example. 
         FIG. 8A  illustrates the arrangement of photoconductive drums of an existing example, and  FIG. 8B  illustrates transmission start timings of image data. 
         FIG. 9A  illustrates a time period during which control data for each color is transmitted, and  FIG. 9B  illustrates transmission timings of image data of each color and control data according to the existing example. 
         FIG. 10  illustrates a conversion table according to the existing example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the disclosure are described in detail below with reference to the accompanying drawings. As used herein, the direction in which the laser beam is scanned, namely, the direction of the rotation axis of a photoconductive drum is referred to as a “main scanning direction” or a “second direction”, and a direction substantially perpendicular to the main scanning direction, namely, the direction of rotation of the photoconductive drum is referred to as a “sub-scanning direction” or a “first direction”. 
     Configuration of Image Forming Apparatus 
     The transmission timing of each of the image data and the control data in the above-described existing image forming apparatus is described in detail below.  FIG. 7A  illustrates an example of control blocks for controlling a light beam based on image data input to the electrophotographic image forming apparatus. The image forming apparatus includes a CPU  5100  that performs overall control of the operation of the image forming apparatus, an image processing unit  5200  that performs a variety of image processing tasks on the input image data, and an optical scanning device  5104 . For example, the image processing unit  5200  is a single integrated circuit (IC) chip. Because a circuit board and an optical scanning device  5104  (a laser scanner) that emits a light beam are disposed inside the image forming apparatus at different locations, the optical scanning device  5104  is disposed away from the CPU  5100  and the image processing unit  5200 . During formation of an image, the CPU  5100  stores, in a register (not illustrated), control data used to control the image processing unit  5200 , and the image processing unit  5200  operates based on the information stored in the register. In addition to data on the size of the image, the control data includes correction data for the laser scanner (described in more detail below) and time data used to transmit image data. 
     Arrows pointing from left to right in  FIG. 7A  indicate the sequence of the processes to be applied to image data input from an external device, such as a document reader or a computer. The image data input from an external device is configured for each of the colors, that is, red (R), green (G), and blue (B). The image data is input to an image input unit  5210 . The image processing unit  5200  converts the image data of each of the colors (R, G, and B) input from the external device into image data corresponding to the colors of toner of the image forming apparatus by using the color conversion unit  5211 . In this case, the colors of the toner of the image forming apparatus are, for example, yellow (Y), magenta (M), cyan (C) and black (K). The color conversion unit  5211  converts the image data of each of R, G, and B colors into image data of each of Y, M, C, and K colors. Image data for each of Y, M, C, and K colors is 8-bit density data. The image processing unit  5200  performs density correction processing on the image data of each of Y, M, C, and K colors. That is, the first data processing unit  5212  of the image processing unit  5200  performs image data processing, such as gamma correction, on the image data of each of Y, M, C, and K colors. A halftone generation unit  5213  generates halftone data by performing screen processing and error diffusion processing on the image data subjected to the gamma correction performed by the first data processing unit  5212 . The halftone generation unit  5213  stores the generated halftone data in the storage unit  5214 . The halftone data is 4-bit image data. 
     In addition, the image processing unit  5200  performs correction on the image data (halftone data) stored in the storage unit  5214  in accordance with the characteristics of the laser scanner by using the second data processing unit  5215 .  FIG. 10  illustrates a conversion table for converting halftone data into drive data for generating a PWM signal. The conversion table is stored in a ROM  5101 . A first column of the conversion table illustrated in  FIG. 10  represents 4-bit image data, which corresponds to one pixel. Each row of the conversion table illustrated in  FIG. 10  represents 16-bit drive data, each row corresponding to one of the 4-bit density values. For example, when the image data input from the storage unit  5214  to the second data processing unit  5215  is a bit pattern of “0110”, the conversion is as follows. The second data processing unit  5215  converts the image data “0110” into drive data having a bit pattern “0000000000111111” by using the conversion table. 
     The second data processing unit  5215  performs magnification correction (magnification correction processing) on the bit pattern obtained through conversion using the conversion table. In the magnification correction processing, bit data is inserted into or removed from the bit pattern. The second data processing unit  5215  sets magnification correction data sent from the CPU  300  in an internal register and performs magnification correction processing based on the set magnification correction data. The accuracy of the magnification correction processing performed by the second data processing unit  5215  is not guaranteed unless the setting of the magnification correction data in the internal register is completed. By inserting bit data into the bit pattern, the image width in the main scanning direction can be increased. By removing bit data from the bit pattern, the image width in the main scanning direction can be reduced. 
     In addition, the second data processing unit  5215  inserts bit data into a bit pattern of the margin on the upstream side in the scanning direction of the laser beam or deletes the bit data from the bit pattern of the margin. In this manner, the second data processing unit  5215  can correct the position of the image relative to the print medium in the main scanning direction (position correction processing). The second data processing unit  5215  sets, in an internal register, the position correction data sent from the CPU  300  and performs position correction processing based on the set position correction data. The accuracy of the position correction processing performed by the second data processing unit  5215  is not guaranteed until setting of the position correction data in the internal register is completed. 
     The second data processing unit  5215  transmits, to the PWM output unit  357 , the bit pattern obtained after performing the magnification correction processing and the position correction processing. In response to a clock signal (not illustrated), the PWM output unit  357  serially outputs the bit data included in the bit pattern bit by bit to a laser driver (hereinafter referred to as an “LD”) for the color of the bit pattern (i.e., LD  5301 Y,  5301 M,  5301 C, or  5301 K). The signal generated when the PWM output unit  357  serially outputs bit data is a PWM signal. When the PWM output unit  357  outputs “1”, a laser beam is emitted from the light source. In contrast, when the PWM output unit  357  outputs “0”, a laser beam is not emitted from the light source. The BD  5207 Y, BD  5207 M, BD  5207 C and BD  5207 K are described later in exemplary embodiments (BD  207 Y, BD  207 M, BD  207 C, and BD  207 K). In addition, the CPU  5100 , the ROM  5101 , the RAM  5102 , and the I/O  5103  are described later in the exemplary embodiment (a CPU  300 , a ROM  301 , a RAM  302 , and an I/O  303 ). 
     An example of correction in accordance with the characteristics of a laser scanner is partial magnification correction, which is correction of magnification to be applied to each of sub-areas obtained by dividing an image area in the main scanning direction. The partial magnification correction is performed, for example, in order to correct the difference in magnification caused by the difference in scanning speed between the end portion and the middle portion in the main scanning direction. The difference in scanning speed occurs in laser scanners not including a lens having the f-O characteristic. In addition, even in laser scanners including a lens having the f-O characteristic, the magnification difference occurs due to product-to-product variation in fabricating lens and fluctuation of the lens characteristic due to the environmental changes (a temperature change). Accordingly, partial magnification correction is required for optical scanning devices including an f-O lens. In recent years, to meet the demand for high image quality, the image area has been finely divided into a plurality of sub-areas in the main scanning direction (for example, 32 sub-areas). 
     In recent years, in order to increase the image forming speed of the image forming apparatus (for example, the number of output sheets per minute), many image forming apparatuses have scanned the photoconductive drum with a plurality of light beams (about 2 to 8). The number of outputs from the PWM output unit  5216  to the optical scanning device  5104  is the same as the number of these light beams. Note that  FIG. 7A  illustrates the control blocks of a color image forming apparatus using four light beams. 
     In this case, due to the available space inside the image forming apparatus, the following situation arises in an apparatus in which a circuit board having the image processing unit  5200  thereon and the optical scanning device  5104  are disposed apart from each other. That is, cost related to the number of signal lines (for example, 16) between the PWM output unit  5216  and the LDs  5301 Y,  5301 M,  5301 C, and  5301 K is to be incurred. In addition, since the configuration of such an image forming apparatus is complicated, it is difficult to assemble the image forming apparatus at the time of production and it is difficult to maintain the image forming apparatus on site (at the place where the image forming apparatus is installed). 
     Furthermore, if the PWM output unit  5216  is connected to the LDs  5301 Y,  5301 M,  5301 C, and  5301 K by using LVDS (Low voltage differential signaling), the number of required signal lines is doubled. 
     Accordingly, such an image forming apparatus sometimes adopts a configuration illustrated in  FIG. 7B  as an example. In  FIG. 7B , the image processing unit is divided into a first image processing unit  6200  and a second image processing unit  6250 , and the image processing unit  6200  fabricated on a single IC chip and the second image processing unit  6250  also fabricated on a single IC chip are mounted on different circuit boards. The circuit board having the second image processing unit  6250  thereon is disposed closer to the optical scanning device  6104  than the circuit board having the first image processing unit  6200  thereon. The second image processing unit  6250  disposed in the vicinity of the optical scanning device  6104  includes a second data processing unit  6255  and a PWM output unit  6256  that perform correction in accordance with the characteristics of the optical scanning device  6104 . The control data is received from the CPU  6100  via a communication unit  6105  and a communication unit  6252  which serve as serial communication interfaces (hereinafter referred to as “IFs”). The data are transmitted to the communication unit  6252 , the second data processing unit  6255 , and the PWM output unit  6256  via a bus  6251 . Note that only difference between the configurations of the other units in  FIG. 7B  and those in  FIG. 7A  is the reference numerals (5000s for those in  FIG. 7A  and 6000s for those in  FIG. 7B ). Thus, descriptions of the units are not repeated. 
     As illustrated in  FIG. 7B , by mounting the first image processing unit  6200  and the second image processing unit  6250  on different circuit boards, the following effects are provided. The first image processing unit  6200  is a general-purpose IC that performs processing that can be widely used for image forming apparatuses with different specifications, such as the image forming speed or the image quality. In contrast, the second image processing unit  6250  is an IC for increasing the performance of the laser scanner, and in one embodiment, the second image processing unit  6250  is designed and fabricated for each of the laser scanners having different specifications. 
     As illustrated in  FIG. 7A , when like the image processing unit  5200 , an IC is designed to perform various image processing tasks, the IC is individually designed and fabricated for each of laser scanners having different specifications, which leads to an increase in the cost of a product. In contrast, the first image processing unit  6200  is designed and fabricated so as to be adopted as a general-purpose IC for a plurality of image forming apparatuses having different specifications, and the second image processing unit  6250  is designed and fabricated as an IC having a specification for a laser scanner. As a result, the overall cost of designing and fabricating ICs that perform image processing operations can be reduced. 
       FIG. 8A  illustrates an example of the arrangement of photoconductive drums  7001  to  7004  of a color image forming apparatus in a tandem configuration. For example, the photoconductive drum  7001  is used for a yellow image, the photoconductive drum  7002  is used for a magenta image, the photoconductive drum  7003  is used for a cyan image, and the photoconductive drum  7004  is used for a black image. Arrows illustrated in the photoconductive drums  7001  to  7004  indicate the rotation direction (the counterclockwise direction) of the photoconductive drums  7001  to  7004 , and “v” indicates the rotational speed. Reference numerals  7005  to  7008  denote irradiation positions of the light beams for forming latent images on the photoconductive drums  7001  to  7004 , respectively. After latent images formed on the photoconductive drums  7001  to  7004  are developed by developers (not illustrated) to form toner images, the toner images are transferred to the transfer belt  7009 , which is an endless belt for transferring the toner images formed thereon.  FIG. 8A  illustrates part of the transfer belt  7009 . 
     When a color image is formed by an image forming apparatus in a tandem configuration, the different color toner images formed on the photoconductive drums  7001  to  7004  are to be stacked one on top of the other at the same position on the transfer belt  7009 . The photoconductive drums  7001  to  7004  are arranged apart from each other. Let ld be the distance between neighboring ones of the photoconductive drums. Then, if the latent images of respective colors are formed on the photoconductive drums at the same timing, the latent images are transferred to the positions on the transfer belt  7009  which are offset from each other by a distance of ld. Therefore, as illustrated in  FIG. 8B , in the color image forming apparatus in a tandem configuration, the latent images of the respective colors are formed by shifting the time of formation. The reference numeral  8000  in  FIG. 8B  denotes a timing signal which is used as a reference signal (also referred to as a “reference timing signal”), and a latent image  8001  corresponding to the photoconductive drum  7001  is formed at the same timing as the reference timing signal. A latent image  8002  corresponding to the photoconductive drum  7002  is formed by delaying the formation time behind the time of the reference timing signal  8000  by a time period Td 1 . The latent image  8003  corresponding to the photoconductive drum  7003  is formed by delaying the formation time behind the time of the reference timing signal  8000  by a time period Td 2 . The latent image  8004  corresponding to the photoconductive drum  7004  is formed by delaying the formation time behind the time of the reference timing signal  8000  by a time period Td 3 . Here, the time periods Td 1 , Td 2 , and Td 3  are calculated as follows:
 
 Td 1= ld/v,  
 
 Td 2=1 d/v× 2, and
 
 Td 3= ld/v× 3  (1).
 
     Referring to the control block diagram in  FIG. 7A  (or  FIG. 7B ), after the time periods Td 1 , Td 2 , and Td 3  are calculated by the CPU  5100  ( 6100 ), the calculated time periods are stored in a register (not illustrated) in the image processing unit  5200  ( 6200 ) as control data. 
     The image data of each of Y, M, C, and K colors that is input from the image input unit  5210  ( 6210 ) and that is subjected to several image processing operations is temporarily stored in the storage unit  5214  ( 6214 ). At the stage of forming images on the photoconductive drums  7001  to  7004 , the CPU  5100  ( 6100 ) instructs the image processing unit  5200  ( 6200 ) to generate a reference timing signal. Note that the reference timing signal is generated to be used for starting image-writing for one page. The image processing unit  5200  ( 6200 ) sequentially transmits the image data of respective colors to the second data processing unit  5215  ( 6255 ) and the PWM output unit  5216  ( 6256 ) in accordance with the time periods Td 1 , Td 2 , and Td 3  stored in the above-described register. Note that in the case of the configuration illustrated in  FIG. 7B , the image data of respective colors are transmitted from the first image processing unit  6200  to the second data processing unit  6255  via the signal lines  601 Y to  601 K. Finally, the image data is converted into an on/off operation of the laser beam by the LD  5301  ( 6301 ), and the laser beam is emitted onto the surface of each of the photoconductive drums  7001  to  7004 . In this manner, latent images are formed. The image processing unit  5200  ( 6200 ) transmits the image data corresponding to each of the photoconductive drums  7001  to  7004  in the following manner. That is, by using the reference timing signal, the image processing unit  5200  ( 6200 ) transmits the image data at the timings based on the distances from the photoconductive drum  7001 , which is disposed most upstream in the movement direction of the transfer belt  7009 , to each of the other photoconductive drums  7002 ,  7003 , and  7004 . In this manner, when forming an image on a print medium, the image forming apparatus delays the start timing of formation of the electrostatic latent image on, for example, the photoconductive drum  7002  from the start timing of formation of the electrostatic latent image on the photoconductive drum  7001  based on the delay amount corresponding to the distance (ld) between the transfer positions. 
     Note that image forming apparatuses widely used in recent years can not only print pages consecutively but also insert a separator sheet between printouts, for example, between chapters each composed of a plurality of print media or between the print media when a plurality of pages are printed. While consecutively printing sheets having a predetermined length in the conveyance direction of the sheets, the image forming apparatuses can form an image on a sheet having a length that differs from the predetermined length. In particular, when the sizes of the printout and the separator sheet differ from each other, that is, when the size of the print medium to be printed is switched during continuous printing, the following control is required. That is, the CPU ( 5100 ,  6100 ) is to newly set the control data in the image processing units ( 5200 ,  6200 , and  6250 ). Thereafter, the CPU is to perform image formation in accordance with the switched print medium. Transmission of the control data for the print medium after the sheet size is switched is performed in a period during which transmission of the image data is not performed, as indicated by reference numerals  9001   b  to  9004   b  in  FIG. 9A . In  FIG. 9A , a horizontally long hexagon indicates a period during which the image data is being transmitted (hereinafter referred to as a “transmission period”). The same applies to the following drawings. Reference numeral  9000  denotes a reference timing signal. Reference numeral  9001   a  denotes a period during which image data for forming a Y latent image is being transmitted, and reference numeral  9002   a  denotes a period during which image data for forming an M latent image is being transmitted. In addition, reference numeral  9003   a  denotes a period during which image data for forming a C latent image is being transmitted, and reference numeral  9004   a  denotes a period during which image data for forming a K latent image is being transmitted. 
     As illustrated in  FIG. 7B , in order to avoid an increase in the cost of the signal lines and a decrease in the maintainability, some image forming apparatuses have a configuration in which the CPU  6100  transmits the control data to the second image processing unit  6250  via the signal line  600  which is a serial communication line. In the image forming apparatuses having such a configuration, the time period from the start to the end of transmitting control data is determined by the baud rate of communication and the number of communication data. In the image forming apparatuses, in order to increase the image quality of the image forming apparatus, control data, that is, the number of communication data is increased, while a low baud rate is employed. Thus, the cost for reducing noise is reduced and, at the same time, the total cost is reduced. Particularly, in the case of the image forming apparatus having such a configuration, the ratio of the time period required to transmit the control data to the periods  9001   b  to  9004   b  during which no image data is transmitted via the signal lines  601 Y to  601 K, respectively, has a predetermined value. 
     In this case, when a separator sheet having a different size is inserted between print media during continuous printing as described above, the sizes of the latent images formed before and after the separator sheet is inserted are different. Accordingly, the CPU  6100  is to continuously transmit the control data to the second image processing unit  6250  via the signal line  600  before and after transmission of the image data of the separator sheet from the first image processing unit  6200  to the second image processing unit  6250 .  FIG. 9B  illustrates signal and data transmission when a separator sheet having a different size is inserted between the print media during continuous printing. In addition,  FIG. 9B  illustrates the case where the user intends to perform setting of the image forming apparatus so that a print medium is inserted as the nth sheet between the (n−1)th print medium and the (n+1)th print medium. Furthermore,  FIG. 9B  illustrates the timings of transmission of the image data from the first image processing unit  6200  to the second image processing unit  6250  and the control data from the CPU  6100  to the second image processing unit  6250  in this case. Reference numeral  9500  denotes the reference timing signal, and reference numerals  9501   a  to  9504   a  denote transmission periods of the image data of respective colors via the signal lines  601 Y to  601 K. Reference numerals  9501   b  to  9504   b  denote the signals that trigger the start of communication of the control data via the signal line  600  which is a serial communication line (hereinafter, the signals are referred to as “communication start triggers”). Reference numerals  9501   c  to  9504   c  denote actual communication periods of control data from the first image processing unit  6200  to the second image processing unit  6250  via the common signal line  600 . More specifically, reference numeral  9501   a  denotes data transmitted via the signal line  601 Y. Reference numeral  9502   a  denotes data transmitted via the signal line  601 M. Reference numeral  9503   a  denotes data transmitted via the signal line  601 C. Reference numeral  9504   a  denotes data transmitted via the signal line  601 K. Reference numerals  9501   c ,  9502   c ,  9503   c , and  9504   c  are data transmitted via the common signal line  600 . For ease of description, reference numerals  9501   c ,  9502   c ,  9503   c , and  9504   c  separately appear in  FIG. 9B . In addition, reference numerals  9500 ,  9501   b ,  9502   b ,  9503   b , and  9504   b  are trigger signals generated inside the CPU  6100 . These signals may be the same signal or a signal generated separately, as illustrated in  FIG. 9B . 
     The Y color, which is a first color, is described with reference to  FIG. 9B . The image data for the (n−1)th print medium is transmitted via the signal line  601 Y ( 9501   a _ n −1). Immediately after the transmission of the image data for the (n−1)th print medium is completed, a communication start trigger ( 9501   b _ n ) of the control data for the nth print medium is generated. The communication period ( 9501   c _ n ) of the control data for the nth print medium transmitted via the signal line  600  is terminated before transmission ( 9501   a _ n ) of the nth image data via the signal line  601 Y is started. Similarly, the image data for the nth print medium is transmitted ( 9501   a _ n ) via the signal line  601 Y. Immediately after the transmission of the image data for the nth print medium is completed, the communication start trigger ( 9501   b _ n +1) of the control data for the (n+1)th print medium is generated. The communication period ( 9501   c _ n +1) of the control data transmitted for the (n+1)th print medium via the signal line  600  is terminated before the transmission ( 9501   a _ n +1) of the image data for the (n+1)th print medium via the signal line  601 Y is started. 
     The processing for M color, which is the next color, is described below. The image data for the (n−1)th print medium is transmitted via the signal line  601 M ( 9502   a _ n −1). Immediately after the transmission of the image data for the (n−1)th print medium is completed, the communication start trigger of the control data for the nth print medium is generated ( 9502   b _ n ). However, the processing for the next M color is performed during the transmission of the control data for Y color via the signal line  600  (during a period α of  9501   c ). Therefore, communication is not started in the expected communication period β, and the communication is started with a delay. Thus, the communication of the control data does not end before transmission of the image data of the n-th print medium via the signal line  600 M ( 9502   a _ n ) starts. Image formation for M color is to be performed based on the time interval indicated by the above-described expression (1), since image formation of the nth print medium for Y color has already started. However, if, as described above, communication of the control data via the common signal line  600  is too late for transmission of the image data, there is a possibility that the control data is not transmitted from the CPU  6100  to the second image processing unit  6250  before the image is formed. In this case, the image cannot be formed correctly. The same also applies to the C and K colors. Note that such a situation does not always occur, and the situation may occur depending on the interval ld between neighboring ones of the photoconductive drums  7001  to  7004 , the distance between the neighboring print media, the length of the print medium for forming an image in the sub-scanning direction, the baud rate, and the amount of the control data. 
     In the existing technology, to avoid the occurrence of such a situation, a method for increasing the communication speed of serial communication can be applied first. However, to increase the communication speed, the clock speed for serial communication is increased and, thus, parts for blocking noise, such as a shield, are required, which leads to an increase in the cost. As another method for avoiding such a situation, a method for expanding the interval between the trailing edge of a first image and the leading edge of a second (next) image can be employed. This can be accomplished simply by lowering the throughput. However, in this case, the performance achieved by the original specification of the product is degraded. Alternatively, to keep the throughput of the image forming apparatus constant, if the rotational speed v of the photoconductive drum is increased, the distance between the leading edge of an image and the trailing edge of the next image is increased and, thus, the increased time is available for communication of the control data. However, in this case, a higher-power motor for driving the photoconductive drum or the intermediate transfer belt may be needed, which also leads to an increase in the cost. Still alternatively, the following method can be employed. Only when the control data is switched, the next control data for Y color is communicated after completion of communication of the control data for K color. Thus, overlapping of the communication periods of the control data can be reliably prevented. However, according to the method, the sheet-to-sheet interval increases more than necessary. Accordingly, for example, in a mode of inserting a separator sheet between printouts, the throughput decreases with increasing number of separator sheets inserted between printouts. 
     EXEMPLARY EMBODIMENT 
     Image Forming Apparatus 
       FIG. 1A  is a schematic sectional view of a color image forming apparatus having toner of a plurality of colors. The image forming apparatus  100  includes four image forming units  101 Y,  101 M,  101 C, and  101 K that form images for respective colors. The image forming unit  101 Y functions as a first toner image forming unit, and the image forming unit  101 M functions as a second toner image forming unit. As used herein, Y, M, C, and K represent yellow, magenta, cyan, and black, respectively. The image forming units  101 Y,  101 M,  101 C, and  101 K perform image formation using toner of yellow, magenta, cyan, and black, respectively. Hereinafter, suffixes Y, M, C, and K of reference numerals are removed except when necessary. The image forming unit  101  is provided with a photoconductive drum  102  which is a photoconductor. The photoconductive drum  102 Y for yellow functions as a first photoconductor, and the photoconductive drum  102 M for magenta functions as a second photoconductor. A charging device  103 , an optical scanning device  104 , and a developing device  105  are provided around the photoconductive drum  102 . Note that an optical scanning device  104 Y for yellow functions as a first exposure unit, and an optical scanning device  104 M for magenta functions as a second exposure unit. A developing device  105 Y functions as a first development unit for developing, with the toner of the first color, the first electrostatic latent image formed on the photoconductive drum  102 Y by the optical scanning device  104 Y that performs exposure. The developing device  105 M functions as a second development unit that develops, with the toner of the second color, the second electrostatic latent image formed on the photoconductive drum  102 M by the optical scanning device  104 M that performs exposure. Note that a cleaning device  106  is further disposed around the photoconductive drum  102 . 
     Below the photoconductive drum  102 , an intermediate transfer belt  107 , which is an endless belt, is disposed. The intermediate transfer belt  107  is entrained about a driving roller  108  and the driven rollers  109  and  110 . The intermediate transfer belt  107  rotates in the direction of an arrow B (the clockwise direction) in  FIG. 1A  during image formation. In addition, a primary transfer device  111  is provided at a position facing the photoconductive drum  102  with the intermediate transfer belt  107  therebetween. The transfer position of the toner image from the photoconductive drum  102 Y to the intermediate transfer belt  107  in the rotational direction of the intermediate transfer belt  107  is located upstream of the transfer position of the toner image from the photoconductive drum  102 M to the intermediate transfer belt  107 . In addition, the image forming apparatus  100  further includes a secondary transfer roller  112  and a fixing device  113 . The secondary transfer roller  112  transfers a toner image on the intermediate transfer belt  107  (on the belt) to a sheet P, which is a print medium. The fixing device  113  fixes an unfixed toner image on the sheet P. The primary transfer device  111 Y, the primary transfer device  111 M, the intermediate transfer belt  107 , the driving roller  108 , the driven rollers  109  and  110 , and the secondary transfer roller  112  function as a transfer unit. 
     During the printing operation, the photoconductive drum  102  and the intermediate transfer belt  107  are driven to rotate in the direction of the arrow in  FIG. 1A  by a drive mechanism (not illustrated), and a printed image is formed through a series of steps for image formation. The surface of the photoconductive drum  102 Y is uniformly charged to have a predetermined potential by a voltage applied by the charging device  103 Y in a charging step. Thereafter, the surface of the photoconductive drum  102 Y is exposed to a laser beam emitted from the optical scanning device  104 Y in an exposure step. Normally, the laser beam is turned on and off in accordance with the data of the document image and, thus, a potential difference corresponding to the data of the document image is generated on the surface of the photoconductive drum  102 Y. In this manner, an electrostatic latent image is formed. Thereafter, by applying a voltage to the developing device  105 Y to keep the toner in the developing device  105 Y at a predetermined potential, the electrostatic latent image is developed to form a yellow toner image on the surface of the photoconductive drum  102 Y in the next development step. For the magenta, cyan, and black colors, toner images are formed on the surfaces of the photoconductive drums  102 M,  102 C, and  102 K, respectively, through the same process as described above. In the next primary transfer step, the toner images of respective colors formed on the photoconductive drums  102  are transferred from the surfaces of the photoconductive drums  102  to the surface of the intermediate transfer belt  107  by applying a primary transfer voltage to the primary transfer device  111 . At this time, the toner images of respective colors are stacked one on top of the other. 
     The toner images stacked on the surface of the intermediate transfer belt  107  are transferred onto the surface of the sheet P conveyed from the first paper feed cassette  120   a  by applying a secondary transfer voltage to the secondary transfer roller  112  in the next secondary transfer step. Note that the sheet P is conveyed from the paper feed cassette  120   a  to the secondary transfer unit by conveyance rollers  121   a ,  122   a ,  123   a , and  124  that are rotationally driven by a driving mechanism (not illustrated). Furthermore, the image forming apparatus includes a second paper feed cassette  120   b  and a manual paper feed tray  120   c . The sheet P fed from the second paper feed cassette  120   b  is conveyed to the secondary transfer unit by conveyance rollers  121   b ,  122   b ,  123   b , and  124  that are rotationally driven by a drive mechanism (not illustrated). The sheet P fed from the manual paper feed tray  120   c  is conveyed to the secondary transfer unit by conveyance rollers  121   c ,  122   c , and  124  that are rotationally driven by a drive mechanism (not illustrated). The first paper feed cassette  120   a  and the second paper feed cassette  120   b  allow the sheets P having a plurality of sizes to be set therein. The size of the sheets P set in each of the first paper feed cassette  120   a  and the second paper feed cassette  120   b  is detected by a size detection device (not illustrated), and the result of detection is output to the CPU  300 . Thus, the CPU  300  can detect the size of the sheets P set in each of the first paper feed cassette  120   a  and the second paper feed cassette  120   b . In addition, the manual paper feed tray  120   c  allows the sheets P having a plurality of sizes to be set therein. The manual paper feed tray  120   c  has a size sensor  117  disposed therein. The size sensor  117  detects the size of sheets set in the manual paper feed tray  120   c . The CPU  300  can identify the size of the sheet P conveyed from the manual paper feed tray  120   c  to the secondary transfer unit based on the result of detection output from the size sensor  117 . Note that the CPU  300  may identify the size of the sheet P set in the manual paper feed tray  120   c  based on the information input from the operation panel (not illustrated) by the user. The above-mentioned separator sheet (a print medium inserted between printouts) is fed from the second paper feed cassette  120   b  or the manual paper feed tray  120   c.    
     The toner that is not transferred to the sheet P and is remaining on the intermediate transfer belt  107  is collected by a cleaner  114  disposed downstream of the secondary transfer unit in the conveyance direction so as to face the intermediate transfer belt  107 . Note that the secondary transfer roller  112  can apply a voltage having a polarity opposite to the secondary transfer voltage for transferring the toner on the surface of the intermediate transfer belt  107  to the sheet P. As a result, the toner adhering to the secondary transfer roller  112  can be moved toward the surface of the intermediate transfer belt  107  and can be corrected by the cleaner  114 . Furthermore, the toner on the surface of each of the photoconductive drums  102  that have completed the transfer process is removed by the cleaning device  106 . The photoconductive drum  102  from which the toner remaining on the surface has been removed returns to the charging step again as the photoconductive drum  102  rotates. The sheet P having the toner image transferred in the secondary transfer unit is conveyed to the fixing device  113  by the conveyance belt  115 . The toner image transferred onto the sheet P is heated and fixed on the sheet P by the fixing device  113 . Finally, the sheet P having the full color image formed thereon in this manner is output to a discharge unit  140  via conveyance rollers  141  and  142  that are rotatingly driven. 
     The sensor  116  serving as a detection unit is a sensor for detecting an image formed on the intermediate transfer belt  107 . In some cases, to control the image quality, the image forming apparatus  100  forms one of detection toner images called “patches” having a variety of sizes and patterns between a toner image to be transferred onto the sheet P and a toner image to be transferred to the succeeding sheet P during continuous printing. Hereinafter, the detection toner image called a patch of a variety of sizes and patterns is referred to as a “patch image”. The sensor  116  detects a patch image formed on the intermediate transfer belt  107  and outputs the result of detection to the CPU  300  (described in more detail below). The CPU  300  corrects the image data based on the result of detection performed by the sensor  116 . When a patch image, which is a predetermined toner image, is formed during continuous printing, a situation that is the same as the above-described situation occurring when a separator sheet is inserted arises, since the size of the sheet P differs from the size of the patch image (refer to  FIG. 9B ). 
     Optical Scanning Device 
       FIG. 1B  illustrates the internal configuration of the optical scanning device  104  that emits a light beam. The optical scanning device  104  includes a semiconductor laser  201  serving as a light source, a collimator lens  202 , a cylindrical lens  203 , and a rotary polygon mirror  204 . The semiconductor laser  201  generates, for example, four laser beams as the light beam. The collimator lens  202  shapes the laser beams emitted from the semiconductor laser  201  into a parallel light beam. The cylindrical lens  203  condenses the laser beam that has passed through the collimator lens  202  in the sub-scanning direction. Furthermore, the optical scanning device  104  includes a first scanning lens  205  on which the laser beam (the scanning beam) deflected by the rotary polygon mirror  204  is incident and a second scanning lens  206 . The rotary polygon mirror  204  is rotated by a drive motor (not illustrated) which drives the rotary polygon mirror  204  during the printing operation. The angle of the laser beam emitted from the semiconductor laser  201  is continuously changed by the reflecting surfaces of the rotary polygon mirror  204  that is rotating. Thus, the laser beam is deflected. The laser beam deflected by the rotary polygon mirror  204  passes through the first scanning lens  205  and the second scanning lens  206  and scans the photoconductive drum  102  in the main scanning direction which is the scanning direction. In this manner, the surface of the photoconductive drum  102  is exposed to form an electrostatic latent image. An area where an electrostatic latent image is formed in the main scanning direction is defined as an image formation area. 
     A mirror  208  is disposed between the first scanning lens  205  and the second scanning lens  206  at an end portion of the scanning range of laser beam (outside the image formation area on the photoconductive drum  102 ). The mirror  208  reflects the laser beam incident through the first scanning lens  205  and folds back the optical path of the laser beam. The laser beam whose optical path is folded is detected by a beam detector (BD)  207  through a lens  209 . Upon detecting the laser beam emitted from the semiconductor laser  201 , the BD  207  outputs a signal to the CPU  300  (described in more detail below). By using the signal input from the BD  207  (hereinafter referred to as a “synchronization signal”) as a reference, the CPU  300  emits a laser beam corresponding to the image data from the semiconductor laser  201  to the image formation area. Thus, the CPU  300  aligns the image forming start positions of the electrostatic latent image (the image) in the main scanning direction for all of the scanning operations. As described above, the synchronization signal is a signal for synchronizing the writing start timings in the main scanning direction. Note that the image forming unit  101  does not necessarily have to be of a type that exposes the photoconductive drum  102  by deflecting and scanning a laser beam with the rotary polygon mirror  204  as described above. For example, another technique in which the photoconductive drum  102  is directly irradiated with LED light and is exposed may be used. 
     Control Block Diagram 
       FIG. 2  is a block diagram of the configuration of a control circuit for controlling driving of the optical scanning device  104 . The image forming apparatus  100  includes the CPU  300  serving as a control unit, a ROM  301  that stores a control program of the CPU  300 , and a RAM  302  that provides a work area. The image forming apparatus  100  further includes an I/O  303  used to receive input signals from a variety of sensors and output signals to the actuators, such as motors, a communication unit  305  for performing serial communication, and an image processing unit  320  (a first image processing unit). The image processing unit  320  is a data generation circuit (a data generation unit) that generates first image data for a first color and second image data for a second color from input image data. These units communicate data via a bus. Furthermore, the image forming apparatus  100  according to the present exemplary embodiment includes an image processing unit  350  (a second image processing unit). The image processing unit  320  and the image processing unit  350  are different ICs. The image processing unit  350  is disposed at a position closer to the optical scanning device  104  than the image processing unit  320 . The image processing unit  320  and the image processing unit  350  are different ICs mounted on different circuit boards. The CPU  300  is mounted on the circuit board having the image processing unit  320  mounted thereon. Transmission of the control data from the CPU  300  to the image processing unit  320  is performed electrically by printed wiring formed on the circuit board. The image processing unit  350  includes a second data processing unit  356  and a PWM output unit  357 . The second data processing unit  356  performs correction in accordance with the characteristics of the optical scanning device. Reception of the control data from the CPU  300  is performed by the communication unit  305  and the communication unit  355  which are serial communication interfaces (IFs). The second data processing unit  356  is a data processing circuit (a data processing unit) that generates first drive data obtained by performing the magnification correction processing on the first image data and generates second drive data obtained by performing the magnification correction processing on the second image data based on the magnification correction data that has been set. In addition, the second data processing unit  356  generates first drive data obtained by performing, for the first image data, position correction processing for correcting the position of the toner image relative to the print medium based on the set position correction data. The second data processing unit  356  generates second drive data obtained by performing, for the second image data, position correction processing for correcting the position of the toner image relative to the print medium based on the set position correction data. The communication unit  305  and the communication unit  355  are connected by a second signal line  380 . That is, the common signal line  380  is connected to the circuit board having the image processing unit  320  mounted thereon and the circuit board having the second data processing unit  356  mounted thereon. The CPU  300  serially transmits, to the second data processing unit  356 , the magnification correction data or the position correction data for the first image data and the magnification correction data or the position correction data for the second image data by using the common signal line  380 . In addition, the CPU  300  transmits control data other than the magnification correction data or control data other than the position correction data to the second data processing unit  356  via the common signal line  380 . By employing such a configuration, an increase in the cost of the signal lines between the PWM output unit  357  having a large number of signal lines and the LD  371  ( 371 Y,  371 M,  371 C, and  371 K) can be prevented. In addition, a decrease in the maintainability can be prevented. Note that the LD  371 Y for yellow functions as a first drive unit that drives the optical scanning device  104 Y based on the first drive data. The LD  371 M for magenta functions as a second drive unit that drives the optical scanning device  104 M based on the second drive data. 
     Arrows pointing from the left to the right in the image processing unit  320  indicate the processes to be applied to image data input from an external device, such as a document reader or a computer. The image data input from the external device is composed of data for each of colors red (R), green (G) and blue (B) and is input to the image input unit  321 . The image processing unit  320  converts the image data of each of R, G, and B colors input from the external device into an image for each of the colors (Y, M, C, and K) of the toner of the image forming apparatus  100  by the color conversion unit  322 . The image processing unit  320  performs image processing, such as gamma correction, on the image data of each of the colors Y, M, C, and K by using the first data processing unit  323 . By using the halftone generation unit  324 , the image processing unit  320  performs screen processing or error diffusion processing on the image data subjected to image processing. Thus, the image processing unit  320  generates halftone data and supplies the generated halftone data to the storage unit  325 , which stores the halftone data. 
     In addition, the image data for each color stored in the storage unit  325  is transmitted from the image processing unit  320  to the image processing unit  350 . For example, the Y color image data is transmitted via the signal line  381 Y, the M color image data is transmitted via the signal line  381 M, the C color image data is transmitted via the signal line  381 C, and the K image data is transmitted via the signal line  381 K. The image processing unit  350  corrects the image data of each color transmitted from the image processing unit  320  via the signal lines  381  (the plurality of first signal lines) by using the second data processing unit  356  in accordance with the characteristics of the optical scanning device  104 . Thereafter, by using the PWM output unit  357 , the image processing unit  350  converts the image data corrected in accordance with the characteristics of the optical scanning device  104  into the PWM analog signal representing the laser on/off pattern. The image processing unit  350  outputs the PWM analog signal converted by the PWM output unit  357  to the LD  371  in the optical scanning device  104  for each color to form a latent image on the surface of each of the photoconductive drums  102 . 
     The CPU  300  stores, in a register (not illustrated) of the image processing unit  320 , the time periods Td 1 , Td 2 , and Td 3  calculated based on Expression (1) described above. Thereafter, at the stage of forming an image, the CPU  300  instructs the image processing unit  320  to generate a reference timing signal. Upon receiving the instruction, the image processing unit  320  sequentially transmits the image data for each color from the storage unit  325  to the image processing unit  350  in accordance with the time periods Td 1 , Td 2 , and Td 3  stored in the register. 
     Image Formation Timing 
       FIG. 3  illustrates a method for use in the CPU  300  to calculate the image formation timing for each color. In  FIG. 3 , images to be printed on three print media n−1, n, and n+1 are generated. Reference numeral  400  in  FIG. 3  denotes a reference timing signal, and a reference timing signal is generated for each of the print media. Reference numerals  401 ,  402 ,  403 , and  404  denote transmission periods for Y, M, C, and K, respectively. Image data  401   a  is transmitted via the signal line  381 Y. Image data  402   a  is transmitted via the signal line  381 M. Image data  403   a  is transmitted via the signal line  381 C. Image data  404   a  is transmitted via the signal line  381 K. Control data  401   c ,  402   c ,  403   c , and  404   c  are transmitted via the common signal line  380 . For ease of description, in  FIG. 3 , the control data  401   c ,  402   c ,  403   c , and  404   c  are separately illustrated. In addition, trigger signals  400 ,  401   b ,  402   b ,  403   b , and  404   b  are generated inside the CPU  300 . These signals may be the same signal or may be generated separately as illustrated in  FIG. 3 . In the following description, the timing control for image formation performed by the CPU  300  is described by focusing on the nth sheet. Note that Y image data for the nth sheet is referred to as “image data  401   a _ n ”, M image data for the nth sheet is referred to as “image data  402   a _ n ”, C image data for the nth sheet is referred to as “image data  403   a _ n ”, and K image data for the nth sheet is referred to as “image data  404   a _ n”.    
     In addition to the above-described time periods Td 1 , Td 2 , and Td 3 , the CPU  300  calculates a time period tp required for transmission of the image data  401   a _ n ,  402   a _ n ,  403   a _ n , and  404   a _ n . Let  1   p  be the length of the image to be formed on the sheet P in the sub-scanning direction, and let v be the driving speed (i.e., the rotational speed) of the photoconductive drum  102  and the intermediate transfer belt  107 . Then, the time period tp is given as follows:
 
 Tp=lp/v   (2)
 
From Expressions (1) and (2), the timing (hereinafter, referred to as “transmission end timing”) at which the transmission of the image data of each of colors Y, M, C, and K with respect to the nth reference timing signal  400  (hereinafter referred to as “400_ n ”) is completed is given as follows:
 
 Y: tp  
 
 M: Td 1+ tp  
 
 C: Td 2+ tp  
 
 K: Td 3+ tp   (3)
 
     Accordingly, the CPU  300  instructs the image processing unit  320  to generate the reference timing signal  400 _ n  at a timing to. In addition, to determine the transmission end timing for the image data of each color given by Equation (3), the CPU  300  starts an internal timer. Upon receiving the instruction to generate the reference timing signal  400 _ n  from the CPU  300 , the image processing unit  320  starts transmitting the image data at the timings based on the time periods Td 1 , Td 2 , and Td 3  stored in the register (not illustrated). 
     If the CPU  300  determines that the time tp has elapsed since the time of the reference timing signal  400 _ n  by referring to the timer, that is, the transmission end timing of the Y color image data has been reached, the CPU  300  operates as follows. That is, the CPU  300  starts communication of control data for Y color for the (n+1)th sheet via the common signal line  380  at a timing Ty indicated by a broken line as necessary. Note that Ty is the timing to start communication of the control data for Y color based on the timing t 0  at which the reference timing signal for the nth sheet is generated. When communication of the control data for the Y color for the (n+1)th sheet is started, the CPU  300  refers to the timer and, in addition, stores the current time in the RAM  302 . The details of the process are described below with reference to  FIG. 5 . 
     Similarly, if, by referring to the timer, the CPU  300  determines that each of the predetermined time periods has elapsed since the time of the reference timing signal  400   n , that is, if the CPU  300  determines that the transmission end timing of each of the M, C, and K image data has been reached, the CPU  300  operates as follows. In this case, the predetermined time periods are Td 1 +tp, Td 2 +tp, and Td 3 +tp. The CPU  300  starts communication of control data for each of the colors M, C, and K for the (n+1)th sheet at timings Tm, Tc, and Tk indicated by broken lines, respectively, via a common signal line  380  as needed. Note that at the timing Tm, communication of control data for M color based on the timing t 0  at which the nth reference timing signal is generated starts. At the timing Tc, communication of the control data for the C color based on the timing t 0  at which the reference timing signal for the nth sheet is generated starts. At the timing Tk, communication of the control data for the K color based on the timing t 0  at which the reference timing signal for the nth sheet is generated starts. Communication of control data for each of the colors Y, M, C, and K for the succeeding print medium may be performed every time an image is formed on one print medium or when control data (e.g., the size of the print medium and the correction data) is switched. According to the present exemplary embodiment, description is given on the assumption that control data is transmitted to the second image processing unit  350  every time an image is formed on one print medium. 
     At the transmission end timing of the Y image data (Ty), the CPU  300  calculates a time period tb used for an instruction to generate a reference timing signal  400 _ n +1 for the (n+1)th sheet is to be sent as follows:
 
 tb=Tcyc−tp   (4)
 
At the same time, the CPU  300  starts the timer (timer setting).
 
     Note that the time Tcyc is determined based on the specification of the product. For example, in the case of an image forming apparatus capable of printing A3-size sheets at 30 sheets per minute,
 
 Tcyc =60 seconds/30 sheets=2 seconds
 
where Tcyc is the time period from the leading edge of the print medium to the trailing edge of the succeeding print medium during continuous printing. Alternatively, in the case where the same image forming apparatus can print A4-size sheets at 60 sheets per minute,
 
 Tcyc =60 seconds/60 sheets=1 second.
 
     The correspondence between the sheet size that can be output by the image forming apparatus and the throughput (the number of printable sheets per minute (ppm)) is stored in the ROM  301  in the form of a table in advance, as illustrated in Table. For example, the throughput for A3-size sheet is 30 sheets per minute (30 ppm) and the throughput for A4 size paper is 60 sheets per minute (60 ppm). By referring to Table, the CPU  300  calculates the time period Tcyc. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Sheet Size 
                 Throughput 
               
               
                   
                   
               
             
            
               
                   
                 A3 
                 30 ppm 
               
               
                   
                 A4 
                 60 ppm 
               
               
                   
                   
               
            
           
         
       
     
     If the CPU  300  refers to the timer and determines that the time period tb has elapsed since the transmission end timing of the nth image data (Ty), the CPU  300  starts a series of processes for transmitting the (n+1)th image data. 
     The difference between  FIG. 3  of the present exemplary embodiment and  FIG. 9B  is as follows. For example, in  FIG. 9B , the C color control data ( 9503   c _ n ) is output after the Y color control data ( 9501   c _ n +1) is output. In contrast, according to the present exemplary embodiment, the control data for any one of the colors ( 401   c _ n  to  404   c _ n ) for the nth print medium is output before the control data ( 401   c _ n +1 to  404   c _ n +1) for the (n+1)th print medium is output. In addition, according to the present exemplary embodiment, after the data processing performed by the second data processing unit  356  based on the control data (e.g.,  401   c _ n ) for the nth print medium is completed, the control data (e.g.,  401   c _ n +1) for the (n+1)th print medium is set by the CPU  300 . For example, the timing at which the setting of the control data (e.g.,  401   c _ n +1) for the (n+1)th print medium is delayed by the time period TD 1  behind the timing (indicated by “(n+1)” in  FIG. 3 ) in the existing technique illustrated in  FIGS. 9A and 9B  in which the control of the present exemplary embodiment is not performed. For example, in the case of Y color, the generation of the trigger signal  401   b _ n +1 by the CPU  300  is delayed by the time period TD 1 , and the generation of the trigger signal  400 _ n +1 for the image formation on the (n+1)th print medium is also delayed by the time period TD 1 . The same applies to M, C and K. 
     According to the present exemplary embodiment, control is performed so that communication of the control data for the succeeding print medium starts at the transmission end timing (Ty, Tm, Tc, Tk) of the image data. However, for the first sheet of a job (also referred to as a “first print medium”), communication of control data may be started at any time if communication of the control data is completed before the instruction to generate the reference timing signal is transmitted. In addition, the reference timing signal for the first print medium is generated after the image forming apparatus  100  completely enters a print ready mode. 
       FIG. 4  illustrates control data  500  according to the present exemplary embodiment. The control data  500  includes control data corresponding to each of Y color data, M color data, C color data, and K color data. The control data is set for at least each of the sizes of recording media. The control data may be set for each of the types of the recording media (e.g., the basis weight/thickness). The control data  500  for each color includes data related to the lengths of an image formed on a print medium in the main scanning direction and the sub-scanning direction and the image forming position on the print medium. The data forms the size information area of the control data  500 . In addition, the control data  500  for each color includes correction data used to correct an image (partial magnification correction data). The image formation area of the optical scanning device  104  (refer to  FIG. 1B ) is divided into 32 sub-areas in the main scanning direction, and the partial magnification correction data is used to corrects the partial magnification for each of the sub-areas. More particularly, the control data  500  includes the partial magnification correction data corresponding to each of 32 sub-areas from the partial magnification 0 to the partial magnification 31, and the partial magnification correction data constitutes a correction information area of the control data  500 . Furthermore, the control data  500  for each color includes the time period from the time of the reference timing signal to the start of the transmission of the image data calculated from expression (1) (i.e., the image data transmission start time), and the time period constitutes the time information area. Note that the control data  500  may include other information. However, the lengths of the same print medium in the main scanning direction and in the sub-scanning direction remain unchanged for all of the colors. The partial magnification correction data has different correction values for the optical scanning devices  104 Y,  104 M,  104 C, and  104 K. According to the present exemplary embodiment, the transmission start time of image data is 0 for Y and Td 1 , Td 2 , and Td 3  for M, C, and K, respectively. 
     According to the present exemplary embodiment, the CPU  300  functioning as a controller for switching setting of the magnification correction data in accordance with the size of the print medium in the above-described manner performs control as follows. That is, the CPU  300  outputs, to the second data processing unit  356  via the common signal line  380 , the magnification correction data for the first image data and the magnification correction data for the second image data at different timings based on the delay amount corresponding to the distance between the transfer positions. In this manner, the CPU  300  switches the magnification correction data that are set in the second data processing unit  356 . In addition, according to the present exemplary embodiment, the CPU  300  functioning as a controller for switching the setting of the position correction data in accordance with the size of the print medium performs control as follows. That is, the CPU  300  outputs, to the second data processing unit  356  via the common signal line  380 , the position correction data for the first image data and the position correction data for the second image data at different timings based on the delay amount corresponding to the distance between the transfer positions. In this way, the CPU  300  switches the position correction data that are set in the second data processing unit  356 . 
     Communication Timing of Control Data 
       FIG. 5  is a flowchart illustrating the process of determining whether the timing at which the communication of the control data  500  is performed during continuous printing overlaps the timing at which the control data for another color is communicated and the process of controlling the communication timings of the control data based on the determination result, accordance with the present exemplary embodiment. As illustrated in  FIG. 3 , the process illustrated in  FIG. 5  is performed by the CPU  300  when the transmission end timing (Ty) of the Y image data is reached and the communication timing of the Y control data is reached. Note that the CPU  300  refers to the timer at the timing when the reference timing signal is generated and manages the elapsed time from the time the reference timing signal is generated to each timing described below. In step  601  (hereinafter referred to as “S 601 ” for simplicity), the CPU  300  refers to the timer and determines whether the time period tp has elapsed since the time of the reference timing signal for the nth sheet and the timing at which communication of the control data  500  for Y, which is a predetermined color, has been reached. If, in S 601 , the CPU  300  determines that the communication start timing of the Y color control data  500  has not been reached, the processing of the CPU  300  returns to S 601 . However, if the CPU  300  determines that the communication start timing of the Y color control data  500  has been reached, the processing of the CPU  300  proceeds to S 602 . In S 602 , the CPU  300  determines whether an (n−1)th sheet P preceding the nth sheet (a preceding print medium) is found. If the CPU  300  determines that no preceding print medium is found, the processing of the CPU  300  proceeds to S 607  since the communication timing of the control data  500  does not overlap the processing time of the preceding print medium. In S 607 , the CPU  300  refers to the timer and stores the current time in the RAM  302 . Thereafter, the CPU  300  transmits the control data  500 , and the processing returns to S 601 . 
     If, in S 602 , the CPU  300  determines that no preceding print medium is found, the processing of the CPU  300  proceeds to S 603 , where the CPU  300  performs process A, which is described later with reference to  FIG. 6 . Process A is a process of comparing the communication time of the control data  500  for the preceding (n−1)th sheet for each color with the current time. By performing process A, the CPU  300  determines whether the communication timings of the control data  500  overlap. 
     Process for Determination of Overlapping of Control Data Communication Timings 
     Process A illustrated in  FIG. 6  is described below. In S 651 , the CPU  300  calculates the timing Tm at which communication of the M color control data  500  for the preceding print medium is started (hereinafter, such timing is referred to as “communication timing”). The communication timing Tm is calculated by adding the time period Td 1  to the communication time of the Y color control data  500  stored in the RAM  302  for the preceding print medium. The communication time of the Y color control data  500  of the preceding print medium is based on the data stored in the process performed for the preceding print medium in S 607  illustrated in  FIG. 5 . That is, the communication time is the time stored in the RAM  302  at the communication start timing of the Y color control data  500  after the transmission end timing of the Y color image data of the preceding print medium has passed. In S 652 , the CPU  300  refers to the timer, calculates the absolute value of the difference between the current time and the communication timing Tm for the M color calculated in S 651 , and determines whether the calculated absolute value is smaller than the predetermined value TD 2 . Note that the transmission timing of the Y color control data  500  for the current print medium may be earlier or later than the transmission timing of the control data  500  for each color for the preceding print medium. Accordingly, the absolute value of the difference is calculated. If, in S 652 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tm for M color is smaller than the predetermined value TD 2  (|current time−Tm|&lt;TD 2 ), the processing proceeds to S 658 . In this case, the communication timing Tm of the M color control data  500  for the preceding print medium is close to the communication timing of the Y color control data  500  for the succeeding print medium on which an image is about to be formed. Therefore, in S 658 , the CPU  300  determines that both timings overlap and, thus, ends process A. Thereafter, the processing returns to the process in  FIG. 5 . However, if, in S 652 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tm for M color is larger than or equal to the predetermined value TD 2  (|current time−Tm|≥TD 2 ), the processing proceeds to S 653 . 
     In S 653 , the CPU  300  calculates the communication timing Tc of the C color control data  500  for the preceding print medium. At this time, the communication timing Tc is calculated by adding the time period Td 2  to the communication time of the Y color control data  500  stored in the RAM  302  for the preceding print medium. In S 654 , the CPU  300  refers to the timer and calculates the absolute value of the difference between the current time and the communication timing Tc for C color calculated in S 653 . Thereafter, the CPU  300  determines whether the calculated absolute value is smaller than the predetermined value TD 2 . If, in S 654 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tc for C color is smaller than the predetermined value TD 2  (|current time−Tc|&lt;TD 2 ), the processing proceeds to S 658 . In this case, the communication timing Tc of the C color control data  500  for the preceding print medium is close to the communication timing of the Y color control data  500  for the succeeding print medium on which an image is about to be formed. Therefore, in S 658 , the CPU  300  determines that the timings overlap and, thus, ends process A. Thereafter, the processing returns to the process in  FIG. 5 . However, if, in S 654 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tc for C color is larger than or equal to the predetermined value TD 2  (|current time−Tc|≥TD 2 ), the processing proceeds to S 655 . 
     In S 655 , the CPU  300  calculates the communication timing Tk of the K color control data  500  for the preceding print medium. At this time, the communication timing Tk is calculated by adding the time period Td 3  to the communication time of the Y color control data  500  stored in the RAM  302  for the preceding print medium. In S 656 , the CPU refers to the timer and calculates the absolute value of the difference between the current time and the communication timing Tk for K color calculated in S 655 . Thereafter, the CPU  300  determines whether the calculated absolute value is smaller than the predetermined value TD 2 . If, in S 656 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tk for K color is smaller than the predetermined value TD 2  (|current time−Tk|&lt;TD 2 ), the processing proceeds to S 658 . In this case, the communication timing Tk of the K color control data  500  for the preceding print medium is close to the communication timing of the Y color control data  500  for the succeeding print medium on which an image is about to be formed. Therefore, in S 658 , the CPU  300  determines that the timings overlap and ends process A. Thereafter, the processing returns to the process in  FIG. 5 . If, in S 656 , the CPU  300  determines that the absolute value of the difference between the current time and the communication timing Tk for K color is larger than or equal to the predetermined value TD 2  (|current time−Tk|≥TD 2 ), the processing proceeds to S 657 , where the CPU  300  determines that the timings do not overlap and ends process A. Thereafter, the processing returns to the process in  FIG. 5 . Note that the predetermined value TD 2  used in the determination in S 652 , S 654 , and S 656  is the time required for transmission of the control data  500  and corresponds to a in  FIG. 9B . 
     The CPU  300  determines whether the timings overlap based on the timing of starting transmission of the control data  500  for the (n−1)th sheet, the timing of starting transmission of the control data  500  for the nth sheet, and the time required to transmit the control data  500 . In this manner, the CPU  300  functions as a determination unit for determining whether a first timing at which transmission of the Y color control data  500  for the nth sheet starts overlaps the second timing at which the control data  500  for at least one of the colors for the (n−1)th sheet is transmitted. 
     Referring back to  FIG. 5 , description of the flowchart continues. In S 604 , from the result of determination made in S 603 , the CPU  300  determines whether the communication timing of the control data  500  for the preceding print medium and the communication timing for current print medium overlap. If, in S 604 , the CPU  300  determines that the timings do not overlap, the processing proceeds to S 607 . In S 607 , the CPU  300  stores, in the RAM  302 , the current time, which is used for determination in process A for the succeeding print medium, and performs communication of the control data  500 . Thereafter, the processing returns to S 601 . 
     However, if, in S 604 , the CPU  300  determines that the timings overlap, the processing proceeds to S 605 . The CPU  300  starts the timer in order to measure the predetermined time period TD 1  in step S 605  and refers to the timer in S 606 . Thus, the CPU  300  determines whether the predetermined time period TD 1  has elapsed. The predetermined time period TD 1  is a time period set based on a time period for which the time period required for transmitting the Y color control data  500  for the nth sheet and the time period required for transmitting the control data  500  for the color determined to overlap the timing for the (n−1)th sheet (the time period for which α and β overlap in  FIG. 9B ). That is, the predetermined time period TD 1  is a time period calculated based on the first timing, the second timing, and the time period required for transmitting the control data  500 . If, in S 606 , the CPU  300  determines that the predetermined time period TD 1  has not elapsed, the processing returns to S 606 . However, if the CPU  300  determines that the predetermined time period TD 1  has elapsed, the processing proceeds to S 607 . As described above, according to the present exemplary embodiment, if it is determined that the communication start timing of the control data  500  for at least one color overlaps the transmission timing of the control data  500  for the current print medium in process A performed in S 603 , control is performed as follows. That is, the CPU  300  shifts the communication start timing of the Y color control data  500  by the predetermined time period TD 1  (delays the communication start timing by the predetermined time). In addition, the CPU  300  instructs the image processing unit  320  to wait for the predetermined time period TD 1  and, thereafter, generate the reference timing signal to be output, which is used as the reference of the timing when image data of each color is transmitted. In S 607 , the CPU  300  stores the current time in the RAM  302  and performs communication of the control data  500 . Thereafter, the processing returns to S 601 . 
     As described above, according to the present exemplary embodiment, the CPU  300  stores, in the RAM  302 , the time at which communication of the Y color control data  500  is started. Thereafter, when communicating the Y color control data for the succeeding print medium, the CPU  300  calculates the communication time of the control data for each color by using the current time, the communication start time of the Y color control data  500  for the preceding print medium, and the time periods Td 1 , Td 2 , and Td 3  in Expression (1). Thereafter, the CPU  300  performs comparison. By using the result of comparison among these timings, the CPU  300  determines whether overlapping of the communication timings of the control data  500  occurs. If it is determined that the timings overlap, the CPU  300  delays, by the predetermined time period TD 1 , the communication timing of the Y color control data  500  for the succeeding print medium and the timing of instructing generation of the reference timing signal used to start transmission of the image data. Note that the predetermined time period TD 1  required for serial communication is obtained in advance and is stored in the ROM  301  as a fixed value. In this manner, overlapping of the communication timing of the control data  500  for a print medium and the communication timing of the control data  500  for the preceding print medium for which transmission of image data has already started can be prevented. 
     As described above, according to the present exemplary embodiment, the CPU  300  determines whether the output timing of the magnification correction data for the first image data and the output timing of the magnification correction data for the second image data overlap. If the output timing of the magnification correction data for the first image data and the output timing of the magnification correction data for the second image data overlap, the CPU  300  performs control as follows. That is, the CPU  300  outputs the magnification correction data for the second image data before the magnification correction data for the first image data is output. After the magnification correction processing performed by the second data processing unit  356  based on the magnification correction data for the nth print medium is completed, the CPU  300  outputs the magnification correction data for the (n+1)th print medium. Note that the first image data is data for forming a first electrostatic latent image for the (n+1)th print medium. The second image data is data for forming an electrostatic latent image for the nth print medium having a size smaller than the (n+1)th print medium in the conveyance direction of the print medium. In addition to the case where the data to be output is magnification correction data, the same applies to the case where the data to be output is, for example, position correction data. Accordingly, description is not repeated. 
     As described above, according to the present exemplary embodiment, the occurrence of image defects caused by overlapping of transmission timings of control data during continuous printing can be prevented. 
     Effects 
     According to an aspect of the embodiments, the occurrence of image defects caused by overlapping of transmission timings of control data during continuous printing can be prevented. 
     While the disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-115453 filed Jun. 9, 2016, which is hereby incorporated by reference herein in its entirety.