Theory of Operation 5
Keysight 34970A/34972A Service Guide 135
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 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 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.
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