Ricoh FT4430 Service Manual page 24

OPERATION PANEL CONTROL
The CPU controls all functions of the operation panel. The CPU sends data to
programmable 1/0 devices which form a multiplexing circuit consisting of 8
scan lines and 17 segment lines, Transistors drive all the scan lines at 12 volts
to reduce effects from noise.
The multiplexing circuit lights all displays and monitors all keys of the
operation panel. The door switch is also monitored by this circuit and acts like
any other operation panel key. The counters are also controlled by the
operation panel multiplexing circuit and act like the operation panel lamps.
16 April '88