Ricoh FT7770 Service Manual page 66

30 April, '89
Erase Lamp Circuit
3.
This copier uses 88 LEDs in the erase lamp unit. The large number of
allows fine control of the side erase margins.
Shift register drivers control the LEDs. Initially, the RESET signal (LOW ac-
tive) turns off all the shift registers. Then the main board sends erase lamp
data to the shift registers from CN201-A8.
After the RESET signal goes back to HIGH, the shift registers start accept-
ing data.
The data bit for LED 101 is set each time a CLOCK pulse ar-
rives.
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 posi-
tion at the start of each clock cycle; so, it takes 88 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.
LEDs for which data bits have been set.
To turn off the erase lamp unit, the RESET signal is sent again
At this time the shift registers turn on all
2-10
LEDs