Circuit Descriptions
5–40
1502C MTDR Service Manual
In operation, the controller is usually accessing the display memory and refreshing
the screen with the bit pattern data. At the rate of about 10 Hz, the CPU intervenes
in the refresh operation to update the bit pattern display memory. This operation
occurs as a block transfer of 4k X 8 from the CPU memory to the display memory.
This block transfer takes place in about 17 ms. During thew block transfer, the
controller cannot access display memory and, therefore, sends null data to the
screen.
Because the LCD display is non-emitting, a light source is needed for low light
applications. This source is provided by an electroluminescent (EL) backlight
behind the transflector. The EL backlight is a long-life device, requiring a 130 VAC,
400 Hz supply. This supply is routed from the Front Panel Board through the 40-pin
connector to pads on the Row Driver/Controller Board. The leads on the backlight
are then attached to these pads.
CAUTION. The pads for the EL backlight are exposed when the display module is
removed from the front panel. They operate from a high voltage source. Do not turn
on the backlight when the pads are exposed.
Because the LCD display response time slows down rapidly at temperatures below
+10° C, a heater is required to maintain the temperature of the LCD cell at +10° C
when the ambient temperature falls below +10° C.
The heating element is a resistive plating of indium tin oxide (ITO) on the back side
of the row pane. This plating has a resistance of about 64
W
. The power for the heater
is supplied through the 40-pin connector to pads on the Row Driver/Controller
Board, similar to those for the EL backlight.
A thermistor, RT1030, is attached to the lip of the row pane opposite the cable. This
thermistor is used to track the temperature of the LCD cell and turn on the heater
power (+16 VDC) when the temperature falls below +5° C. With a supply voltage
of +16 VDC, the heater dissipates about 4W. The circuitry to control the temperature
is located on the Front Panel Board. The thermistor leads are attached to pads on the
Row Driver/Controller Board, similar to the heater, and routed through the 40-pin
connector.
Electroluminescent
Backlight
Indium Tin Oxide Heater
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