TROUBLESHOOTING AND REPAIR
DIGITAL CONTROLLER
Display PCA 6B-27.
The A1 Display PCA provides a readout of the programmed modulation, frequency,
amplitude parameters, and status information. This displayed information and the
bright digit are controlled by the A13 Controller PCA under the direction of the
instrument software. The display is comprised of two vacuum fluorescent displays and
their associated control circuitry. The two displays are refreshed as four groups of nine
display fields (usually a digit) each. The four groups share the digit (grid) strobes but
have individual segment (anode) strobes.
Data Communications 6B-28.
Display data is sent through a byte-wide bidirectional data bus from the Controller
PCA and is latched by U1 through U5, and U19. The front panel latch select signals
DIG1L, DIG2L, SEG1L, SEG2L, SEG3L, and SEG9L are decoded by U20. These
latch select signals determine which latch receives the data. Level-shifting buffer
drivers U6 through U10 interface the latches directly to the +37V grids and anodes of
the vacuum fluorescent displays.
Display Filament Voltage 6B-29.
The 6.0V AC filament voltage for the display is derived from a center-tapped winding
on the Power Supply PCA transformer (T1). The AC filament voltage is biased at
+6.2V above ground by circuitry on the A14 Power Supply PCA to provide a cutoff
potential for the displays.
Bright-Digit Effect 6B-30.
The bright-digit effect is achieved by providing three extra refresh cycles (strobes) to
the specified digit. A grid current-limiting resistor (R3) ensures uniform digit
brightness by controlling electron depletion from the display cathode filaments.
Switchboard Interface 6B-31.
The digit strobe data latched by U1 is buffered by open-drain inverters (U13 and U15)
and strobes the front panel switch matrix. The switch columns are strobed in unison
with the display fields. The switch matrix status is read by the tri-state buffer (U14).
Remote Footswitch 6B-32.
The rear panel AUX connector has inputs that accept remotely generated sequence up,
sequence down, and bright digit field (frequency or amplitude) commands. The
requests are generated by momentarily grounding the signal of interest. The pinout of
this connector is provided in Appendix I.
Electromagnetic emission considerations dictate that the rear panel control inputs are
static. Gates from U12, U13, U15, and U16 convert the static rear panel inputs into
strobed key requests. The software services the requests in the same manner as all of the
strobed keys.
The front panel display can be turned off by special function. Turning off the display
also stops the switch matrix strobes, so all strobed keys become totally inactive. The
front panel key is excluded from the switch matrix and is connected to
circuitry similar to the rear panel control signals. This allows the key to remain active
when the display is off so it can be used to enable the display.
6B-10
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