Canon LBP 2460 Service Manual - Page 74

CHAPTER 2 a. Initial APC Initial APC is performed to increase the intensity of the laser beam from zero to near the target value during initial rotation, and to obtain the increase rate and between-page APC initial value. After the CPU sets the FORCED LASER ON signal (/LON) to "L", the CPU changes the APC1 signal (rough adjustment) and lets the LLCONT signal gradually rise. As the laser current increases, the laser diode (LD) begins emitting light. The intensity of the laser diode (LD) is detected by the photo diode (PD). After amplification, the output voltage is fed back to the CPU as the LASER BEAM INTENSITY DETECTION signal (PD). The CPU ends initial APC when the input value of the PD signal was over the target value stored in the CPU. b. Between-page APC Between-page APC is carried out for each page immediately before writing a VIDEO signal (VDO) onto the drum in order to obtain the target value based on the initial value, and the increase rate obtained by the initial APC. The CPU sets the values of the initial APC as the initial values. It calculates the increase rate to obtain the target value, from the increase rate obtained from the initial APC, and determine the output values of the APC1 signal (for rough adjustment) and APC2 signal (for fine adjustment). The CPU then sets the FORCED LASER ON signal (/LON) to "L", outputs the APC1 and APC2 signals, compares the input value of the PD signal with the target value, and optimizes the intensity of the laser beam. 3. Horizontal synchronization control The gate array (IC205) generates an UNBLANKING signal based on the BD INPUT signal (/BDI) sent from the BD PCB of the laser/scanner unit. IC205 sets the FORCIBLE LASER ON signal (/LON) to "L" in the unblanking period. When the /LON signal is "L", the laser driver turns the laser diode on. The laser beam emitted from the /LON signal strikes the small fixed BD mirror located in the light path where the laser beam scanning starts, and inputs to the BD PCB in the laser/scanner unit. The BD PCB detects the laser beam, generates a BD INPUT signal (/BDI), and sends it to the gate array on the DC controller PCB. The gate array generates the HORIZONTAL SYNCHRONIZATION signal (/BD) according to the /BDI signal and sends it to the video controller. 4. Laser diode emission control Laser diode emission is controlled by using the VIDEO signal (VDO) sent from the video controller to the printer and the paper size data. The paper size data is supplied to the CPU from the cassette-size sensing switch for cassette feed and by the paper size specification command from the video controller for manual feed. The VDO signal is sent from the DC controller to the laser driver. The CPU sends data to the gate array (IC205) to generate the IMAGE TOP ERASE AND BOTTOM ERASE signal (TOPE) and HORIZONTAL IMAGE MASK signal (MSK) according to the paper size data from IC205. The TOPE and MSK are used by IC205 and are not output externally. To inhibit the laser beam from writing on non-image area during the non-unblanking period, IC205 makes the VIDEO DATA ENABLE signal (/LENBL) "L" only when the TOPE and MSK signals are "L". The laser driver switches the laser ON when the /LENBL and VDO signals are "L". 2 - 28