Model 78 and 78-1
Technical Manual
Section 7
Ludlum Measurements, Inc.
Page 7-3
March 2021
S1 (FUNCTION)
S1 is a 16-position binary rotary switch that selects the programmable parameters for
the Model 78. The switch selects the parameters using the hexadecimal numbering
system via buss lines
SW
1-
SW
4.
S2-S4
S2-S4 are pushbutton switches that enter/change the variables for each of the 16
parameters.
LCD Drive
U111 and U211 are serial input 32-bit LCD drivers. The data is loaded serially into
the 32-bit shift registers (internal) via the “D” IN input. The
LOAD
input instructs the
shift register to receive data while the
CLOCK
input shifts the data through the 32-bit
registers. After all the data is loaded, the
LOAD
line is pulsed by the µP, instructing the
registers to transfer the data to the LCD drivers. The backplane (BP) signal from
U211 provides the reference signal (approximately 125 Hz at 5 Vdc) to the LCD
(DSP1) BP connection. When a segment is illuminated, the signal to that segment will
be out of phase with the BP signal. If the segment is
OFF
, the signal will be in-phase
with the BP signal.
Backlight Drive
Depressing the
BKLITE
button instructs the µP to set the
BACKLIGHT
line, pin 31 on
µP, to "low" for the predetermined backlight
ON
time. (Refer to main board
schematic for details.) A "low" condition on pin 31 causes Q212 to conduct sending
+3 V to P8-3 on display board with +3 V at P8-3 (refer to display board schematic).
The backlight oscillator Q011, T011, and related components start to oscillate,
producing a 2.5 kHz, sine wave signal. The signal is amplified by T011 to 150 volts,
peak-to-peak, to drive the LCD backlight.
Refer to the Cal/Switch
Board schematic for
the following:
Refer to Display
Board schematic
for the following:
