4 Theory of Operation
128 Keysight 34420A Service Guide
Front Panel
The front panel circuits (schematic shown on Chapter 8, "Display and Keyboard
Schematic") 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 bi-directional serial
interface. The main CPU, U500, can cause a hardware reset to processor U602 by
signal IGFPRES. The front panel logic operates from -13 volts (logic 1) and -18
volts (logic 0). The front panel logic high supply (-13 volts) is produced by the -18
volt supply and voltage regulator U609. The four serial communication signals
are level shifted by comparator U607 from the floating logic O V to 5 V levels to
the -18 V to -13 V levels present on the front panel assembly. U608 acts as the
serial shift register interface for processor U602.
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 168). Keyboard scanning is accomplished through a
conventional scanned row-column key matrix. Keys are scanned by outputing
data at microprocessor U602 port pins P0.0 through P0.4 to poll each key column
for a key press. Column read-back data are read by the microprocessor at port
pins P1.0 through P1.3 for decoding and communication to the floating logic
circuits.
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