Theory of Operation 4
E4416A/E4417A Power Meters Service Guide 77
The recorder is a 12 bit DAC driven by the microcontroller. The DAC output is
buffered, filtered and scaled to give a 1 V full scale output with a nominal 1 kΩ
output impedance.
Circuitry for the keyboard driver includes some damage protection, but it is
basically a direct connection from the keypad row and column matrix to the
microcontroller's control lines.
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 for analogue to digital
conversion. The microcontroller uses this function to perform a self test.
A6 measurement assembly to microcontroller communication is conducted on
a parallel expansion bus. This bus is connected to the bi- directional serial
port on the microcontroller via buffers. The 20 MHz 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 TTL outputs are connected to the A6 processor assembly via in line
resistors located on the A4 interface assembly. A TTL logic level is output from
the processor assembly when the user defined windows based limits are
exceeded.
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