Image forming apparatus

An image forming apparatus comprising:

• a first photoconductive drum, a second photoconductive drum, a third photoconductive drum and a fourth photoconductive drum;
• an exposure unit that includes a first laser unit which emits a laser beam for exposing the first photoconductive drum, a second laser unit which emits a laser beam for exposing the second photoconductive drum, a third laser unit which emits a laser beam for exposing the third photoconductive drum and a fourth laser unit which emits a laser beam for exposing the fourth photoconductive drum and performs main scanning and sub-scanning of the respective photoconductive drums by the laser beam emitted from the respective laser units;
• a laser detecting unit that detects the laser beam emitted from the first laser unit as a main scanning reference position;
• an integrated circuit that includes a first image processing circuit, a second image processing circuit, a third image processing circuit and a fourth image processing circuit which process input image data in accordance with the input first clock signals and output the processed image data at a predetermined timing based on the main scanning reference signals which is input;
• a first laser control circuit, a second laser control circuit, a third laser control circuit and a fourth laser control circuit which hold the output image data momentarily from the respective image processing circuits, output the held image data in accordance with the input second clock signals, generate laser control signals for controlling the operation of the laser units in accordance with the main scanning reference signals and output the laser control signals;
• a first data processing circuit, a second data processing circuit, a third data processing circuit and a fourth data processing circuit which are operated in accordance with the laser control signals output from the laser control circuits, convert the image data output from the respective laser control circuits into serial data signals then output the signals, output signals synchronized with detection of the laser detecting unit as the main scanning reference signals, and output clock signals having predetermined frequency as the second clock signals;
• a selection section which selects any one of the main scanning reference signals output from the data processing circuits and supplies the signals to the respective image processing circuits;
• a first gate circuit, a second gate circuit, a third gate circuit and a fourth gate circuit which allow and block the input of the first clock signals to the respective image processing circuits by opening and closing;
• a fifth gate circuit, a sixth gate circuit, a seventh gate circuit and a eighth circuit which allow and block the input of the second clock signals to the respective laser processing circuits by opening and closing;
• and a selection circuit which has a first selection mode which respectively selects the laser control signals output from the laser control circuits and supplies the selected signals to the respective laser units and the respective data processing circuits, a second selection mode which selects the laser control signals output from the fourth laser control circuit and supplies the selected laser control signals to the first laser unit, the fourth laser unit, the first data processing circuit and the fourth data processing circuit, and is capable of setting the first selection mode or the second selection mode selectively;
• and
• a control section that sets the first selection mode in the selection circuit while opening all of the gate circuits in a case of a color image forming mode using the first, second, third and forth image holding bodies and sets the second selection mode in the selection circuit while closing the remaining gate circuits whereas opening only the fourth gate circuit and the eighth gate circuit in a case of a monochrome image forming mode using only the fourth photoconductive drum.