HP 8550 Service Manual - Page 194

The reader controller PCB sends the MTCLK signal, CW/CCW signal, and STEP_ANGLE through STEP_ANGLE3 signals to the scanner motor driver PCB to suit the scanning mode, distance, and reproduction ratio. In response, the motor control IC (Q301) on the scanner motor driver PCB generates motor drive pulses (SPA to SPE, SNA to SNE) to drive the scanner motor. The scanner motor is a five-phase stepping motor, and controls the direction and speed of scanning by varying the sequence and frequency of the motor drive pulses (SPA through SPE and SNA through SNE). The constant current control circuit controls the current flowing to the motor to a specific level to suit the speed of motor rotation. When the motor remains at rest in standby, the current flowing to the motor is cut in response to the MOVE signal to prevent heating of the motor. Changing the reproduction ratio The reproduction ratio in main scanning direction (drum axial direction) is changed by skipping image signals when writing them into the line memory (reduction) or by reading the same image signals when reading them from the line memory (enlargement). The reproduction ratio in sub scanning direction is changed by moving the mirror mount faster (reduction) or slower (enlargement). Figure 88. Changing the reproduction ratio 192 Chapter 5 - Theory of operation EN