Agilent Technologies 34970A, Service Manual, Page: 129 / 273

Front Panel
Unless otherwise noted, components in this discussion are located
on the A2 circuit assembly (34970-66502).
The schematics are
included in Chapter 8 starting on page 229.
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 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 shown on the power supply
schematic ( see page 224 ).
Keyboard scanning is accomplished through a conventional scanned
row-column 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.
5
Chapter 5 Theory of Operation
Front Panel
127