Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Front Panel
Front Panel
Unless otherwise noted, components in this discussion are located on the
A2 circuit assembly (34970-66502).
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
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through a 2-wire bi-directional serial interface. The front-panel logic
operates from -13 volts (logic 1) and -18 volts (logic 0). The two serial
communication signals are level shifted by comparator U6 from the
floating logic 0 V to 5 V levels to the -18 V to -13 V levels present on the
front panel assembly. The front panel logic high supply (-13 volts) is
produced from the -18 volt supply by voltage regulator U7.
Display anode and grid voltages are +18 volts for an on segment and -18
volts for an off segment. The -11 V cathode bias for the display is
provided by the main pc boards filament winding center tap bias circuit
A1CR108, A1R106, and A1C106.
Keyboard scanning is accomplished through a conventional scanned rowcolumn
key matrix. Keys are scanned by outputting data at
microprocessor U1 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 P2.0 through P2.3 for decoding and communication to the
floating logic circuits. Rotary knob quadrature inputs are read directly
by the microprocessor port pins P2.6 and P2.7.
The standby power switch, S19, provides a low true signal to main
controller A1U205. In turn, A1U205 takes actions that either place the
instrument in the “standby mode” or “on” mode. In “standby”, both the
filament supply to the front panel and the +5V_BP supply to the
backplane, rear panel interfaces, and fan are turned off.
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