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System Functional Descriptions
6-12
Audio Functions
The M3500B/M5500B has 2 types of audio output: tones, and voice prompts.
Both are generated and controlled by the Control PCA, which also amplifies
the signals and passes them directly to the speaker via a dedicated connector
and wire pair.
Display Functions
All display functions are handled by the Control PCA. Display formats,
graphics, waveforms, numeric values, and messages are all generated and for-
matted by the Control PCA, using either data it has or data it receives from
other parts of the unit.
Contrast - manual control
The display contrast is controlled by pressing front panel keys. These key
presses are transferred from the keys to the Interface PCA via a flex circuit.
The key presses are detected and processed by the Interface PCA and then
passed to the Control PCA via a large ribbon cable.
Contrast - automatic control
The display contrast is also controlled automatically by the Control PCA
based on ambient temperature. The temperature sensor is located on the dis-
play; it is connected to the Control PCA via a dedicated connector and wire
pair.
Backlight
The backlight on the display is controlled automatically by the Control PCA
based on ambient light levels. The light sensor is located on the IRDA/Light
Sensor break-off PCA; its signal is routed via a soldered (non-removable) flex
circuit to the Interface PCA. From there it is routed to the Control PCA via a
flex cable.
The Control PCA then decides whether the backlight is needed (or not), and
sends the appropriate signals to the backlight control circuits (also on the
Control PCA), which turn the backlight on or off..
When the unit is powered up, the backlight turns ON initially, then turns OFF
if it isn't needed.
There is a non-replaceable 0.25A fuse on the Control PCA. If there is a fault
in the backlight, it may blow this fuse. Therefore, if the backlight stops work-
ing and replacing the display doesn't correct it, replace the Control PCA as
well.
Theory of Operation
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