Erase Lamp Circuit - Ricoh FT5540 Service Manual

4.5 ERASE LAMP CIRCUIT

This copier uses 80 LEDs in the erase lamp unit. The large number of LEDs allows precise
control of the side erase margins.
Shift register drivers control the LEDs. Initially, the ENABLE signal (LOW active) sets all shift
registers to OFF. Then the main board sends ERASE DATA to the shift registers from
CN102-A17.
After the ENABLE signal goes back to HIGH, the shift registers start accepting data. The data
bit for LED 1 is set at each CLOCK pulse. If the data line is HIGH at the time of the CLOCK
pulse, 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 each clock pulse; so, it
takes 80 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. Simultaneously, the shift registers turn on all LEDs for which data bits have been set.
To turn off the erase lamp unit, the ENABLE signal is reset to LOW.
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31 August 1989