Ricoh FT4460, Service Manual, Page: 80 / 413

28 February 1989
4.2 ERASE LAMP CIRCUIT
This copier uses 120 LEDs in the erase lamp unit.
Shift register drivers control the LEDs. Initially, the RESET signal (LOW active)
turns off all the shift registers. Then the system control PCB sends erase lamp
data to the shift registers.
After the RESET signal goes back to HIGH, the shift registers start accepting
data. If the data line is HIGH at the time of the CLOCK pulse (rising edge), the
shift register is set to turn the LED on; if it is LOW, the shift register is set to
keep the LED off. The data in the shift registers is shifted right one position at
the start of each clock cycle; so, it takes 120 CLOCK pulses to set the shift
registers for all the LEDs.
After all data bits have been sent, the main board sends the LATCH signal to
turn on the erase lamp unit. The latch signal sets the shift registers which turn
on all LEDs for which data bits have been set.
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