HP E4418A Service Manual page 75

LED
Standby
On
Power Ref
The LCD controller on the A3 front panel assembly is configured as a
memory mapped peripheral, and as such requires only to be fed with the
appropriate address, data and control lines from the microcontroller
circuits. The bias voltage for the LCD is produced by a DC to DC converter
that takes the +5 V (DIST) voltage and converts that to a nominal +21 V.
The DC to DC converter is adjusted by a combination of the contrast
control signal from the microcontroller and the temperature sense voltage
that is generated on the A3 front panel assembly. The temperature sense
voltage helps to compensate for the normal variation of LCD contrast with
temperature. Current to the LCD's LED backlight array is sourced from a
constant current circuit that compensates for variation in LED knee
voltage. A control input to this circuit from the front panel temperature
sense voltage allows for de-rating of the maximum LED current at high
operating temperatures. A control input from the microcontroller allows
the optimum backlight brightness to be factory set. The temperature
sense voltage, LED current and LCD bias voltage are all measured at self
test.
An analog multiplexer provides means by which several diagnostic points
can be switched to the A6 measurement assembly(s) for analogue to
digital conversion. The microcontroller uses this function to perform self
test.
A6 measurement assembly(s) to microcontroller communication is
conducted on a serial interface bus, the Internal Serial Bus (ISB). The ISB
is connected to the bi-directional serial port on the microcontroller via
buffers. The system clock and some control signals are connected to the
measurement modules. The measurement module interface is completed
with the distribution of switched and direct power (+12 V, -12 V and +5 V).
The microcontroller circuits that control all the above functions, and
provide platform for the system software to run on, comprise the
microcontroller itself, memory, and clock and logic circuits. The logic
circuits have the function of ensuring the correct sequencing and decoding
HP EPM-441A/442A Service Guide
A2 Processor Assembly
+12 V +5 V
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Theory of Operation
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