1.  Field of the Invention
The present invention relates to an image forming apparatus.  More particularly the present invention relates to an image forming apparatus that includes both: a color image forming apparatus and an image forming apparatus that can simultaneously read both surfaces of a monochrome document. 
2.  Description of the Related Art
Some image forming apparatuses have a configuration for reading both surfaces of a document.  Two configurations can realize this.  First, provide a single scanner unit and pass the document under the scanner unit two times: once to read one surface and then to read the other surface.  Second, provide two scanner units: one for reading one surface and another for reading the other surface. 
Because the running cost of a color copier is still considerably high, there is a trend to put up with monochrome scanner. 
Moreover, recently, a technique has been used to read a color document with a color scanner and send the read data to a personal computer (PC) or the like via a local area network (LAN) for outputting at the PC.  Because, a configuration for forming images is not prepared in the color scanner, it is possible to lower the overall cost. 
For example, in Japanese Published Unexamined Patent Application No.  2003-163792 discloses a conventional image forming apparatus.  This image forming apparatus is provided with a color information reading sensor unit having laser diode (LD) light sources that emit lights with respective colors of R, G, and B, respectively, and a monochrome information reading sensor unit having a laser emitting diode (LED) light source that emits light to read monochrome information.  According to this image forming apparatus, when reading color information of a document, only one side of the document is read, and when reading monochrome information of a document, both sides of the document are read by a one-time reading scan. 
Japanese Published Unexamined Patent Application No.  H11-331493 discloses another conventional image forming apparatus.  This image forming apparatus is a both-sided color document reading device having a configuration in which color image sensors by a light-source sequential lighting method are arranged opposite each other and that is capable of simultaneously reading both sides of a document.  This is realized by inputting, into an image processing device, color signals with three colors of one line of respective surfaces of the image sensor unit in sets while switching the same by turns for the front and rear, both surfaces of a document are simultaneously read. 
Meanwhile, a color scanner output image data on three channels of RGB, whereas a monochrome scanner output image data on only one channel.  In both of the conventional arts described above, an image processing is executed for the front page first (front image processing) and then executed for the rear page (rear image processing).  In addition, because front and rear image effective image areas are different in the arranging position of a charge-coupled device (CCD) and a contact image sensor (CIS), image effective gate signals are differently inputted.  Therefore, it is necessary for a color scanner and a monochrome duplex scanner to employ different circuit configurations between when reading color image information and when reading both surfaces in monochrome, and it is necessary to separately provide dedicated circuits when both scanners are used.