Keysight E3631A, Service Manual

Page: 109 / 131

Theory of Operation 4
Keysight E3631A Service Guide 105
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 U17 can cause a hardware reset to front-panel
controller by signal IGFPRES. The front panel logic operates from -11.4 volts
(logic 1) and -16.4 volts (logic 0). The front panel logic high supply (-11.4 volts) is
produced by the -16.4 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 -16.4 V to -11.4 V
levels present on the front panel assembly. U6 acts as the serial shift register
interface for the front-panel controller U7 on the front panel board.
Display anode and grid voltages are +16.4 volts for an "on" segment and -16.4
volts for an "off" segment. The -10.2 V cathode bias for the display is provided by
filament winding center tap bias circuit VR3, R43, and C32 on the top board.
Keyboard scanning is accomplished through a conventional scanned row-column
key matrix. Keys are scanned by outputing data at front-panel controller U7 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.