Theory of Operation 4
Keysight E3633A and E3634A Service Guide 109
Front Panel
The front panel circuits 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 controller U19 can cause a hardware reset to front-panel
controller by signal IGFPRES. The front panel logic operates from -12.4 volts
(logic 1) and -17.4 volts (logic 0). The front panel logic high supply (-12.4 volts) is
produced by the -17.4 volts bias supply and the voltage regulator U2 on the front
panel board. The four serial communication signals are level shifted by the
comparator U8 from the floating logic 0 V to 5 V levels to the -17.4 V to -12.4 V
levels present on the front panel assembly. U2 acts as the serial shift register
interface for the front-panel controller U5 on the front panel board.
Display anode and grid voltages are +17.4 volts for an ‘’on’’ segment and -17.4
volts for an ‘’off’’ segment. The -11.2 V cathode bias for the display is provided by
filament winding center tap bias circuit VR1, R18, and C25 on the main board.
Keyboard scanning is accomplished through a conventional scanned row-column
key matrix. Keys are scanned by outputting data at front-panel controller U5 port
pins P0.0 through P0.3 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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