Circuit Descriptions
1502C MTDR Service Manual
5–27
The rotary binary switches provide a 4-bit binary value, indicating their position.
The outputs are tied to the inputs of the multiplexers. The position of the rotary
switches control the following functions:
H
FILTERING, SET REF, SET DELTA
H
HORIZONTAL GAIN (DIST/DIV)
H
V
P
COARSE
H
V
P
FINE
The switch multiplexers are U2024, U2025, U3025, and U3031. These dual
four-channel multiplexers multiplex the switch settings of the push button and
rotary switches onto the data bus. The control signal MUXCS
, in conjunction with
A
2
, selects the multiplexers while A
0
and A
1
determine which switch bank is placed
on the data bus.
The resistive shaft encoders R1022, R2024, and R3020 are dual-concentric, 360°
rotation potentiometers, with the wipers set 180° out of phase with respect to each
other. The wipers are tied to the analog-to-digital converter inputs of ADC U2023.
The three resistive shaft encoders control the following functions:
H
VERTICAL GAIN
H
VERTICAL POSITION
H
HORIZONTAL POSITION (Cursor)
The ADC, U2023, is an eight-channel analog-to-digital converter. It converts the
voltages on the wipers of the resistive shaft encoders to a digital value, depending
on the position of the encoders. It also converts the voltage on the display thermistor
(T
SENSE
) and the chart recorder thermistor divider circuits into digital values
representing the corresponding temperatures. The temperature data is used by the
processor to compensate the LCD drive voltage and chart recorder print parameters
for variations in temperature.
The control signal TRIG ADC is used to start a conversion; ADC RD reads the
value; and A
0
, A
1
, and A
2
select one of the eight channels for conversion. Control
signal EOC notifies the processor of a conversion completion, via the IR3 line.
The EL (electroluminescent) backlight is switched by software. Control signal
LIGHTCS
, with RD or WR, sets or resets (respectively) NOR latch U3020. The
output of the latch is applied to the + side of comparator U2020B; the – side is held
at 2.5 VDC. When the output of the latch is high, the comparator output is +16 VDC,
which turns off the gate of P-channel FET Q1030, turning off power to the EL power
supply, PS2030. When the output is low, the comparator output is 0V, which turns
on the FET, turning on the power to the EL power supply. R1031, C3030, and C3031
serve to filter noise introduced to the +16 VDC supply by the EL power supply.
Rotary Binary Switches
Switch Multiplexers
Resistive Shaft Encoders
Analog-to-Digital
Converter
Electroluminescent
Backlight Switch and
Power Supply
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