User`s guide

Chapter 4 Theory of Operation

Earth-Referenced Logic

Microprocessor U1 handles GPIB (IEEE-488) control through bus interface chip U6

and bus receiver/driver chips U3 and U11. The RS-232 interface is also controlled

through microprocessor U1. RS-232 transceiver chip U19 provides the required level

shifting to approximate ±9 volt logic levels through on-chip charge-pump power

supplies using C3 and C9. Communication between the earth referenced logic

interface circuits and the floating logic is accomplished through an optically-isolated

bi-directional serial interface. Isolator U5 couples data from U1 to processor U19.

Isolator U2 couples data from U19 to microprocessor U1.

Front Panel

he 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.