Front Panel
The front panel circuits (schematic shown on page 9-16) consist of
vacuum fluorescent display control, display high voltage drivers, and
keyboard scanning. Communication between the front panel and
floating logic circuits is accomplished through a 4-wire bidirectional
serial interface. The main
CPU, U500, can cause a hardware reset to
processor U600 by signal IGFPRES. The front panel logic operates from
+13 volts (logic 0) and +18 volts (logic 1). The four serial communication
signals are level shifted by comparator
U602 from the floating logic 0 V
to 5 V levels to the 13 V to 18 V levels present on the front panel
assembly. The front panel logic low supply (+13 volts) is produced by the
+18 volt supply and 4.7 V zener
CR606. Level shift outputs for the front
panel receive data are clamped to the 13 volt supply VLB.
Display anode and grid voltages are +18 volts for an “on” segment and
–18 volts for an “off” segment. The –12 V cathode bias for the display is
provided by filament winding center tap bias circuit
CR556, R556, and
C559 on the power supply schematic (see page 9-15). Keyboard scanning
is accomplished through a conventional scanned row-column key matrix.
Keys are scanned by outputing data to shift register
U601 to poll each
key column for a key press. Row read-back data are parallel loaded into
shift register
U601 and shifted back into processor U600 for decoding and
communication to the floating logic circuits.
5
Chapter 5 Theory of Operation
Front Panel
107
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