Ricoh FT4430 Service Manual page 38

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 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 34 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.
The center section of LEDs turns on and off as a block, and the outer LEDs
turn on and off in pairs.
2-11
16 April '88