This is just from the top of my head:
The optocoupler is in a feedback loop from the demultiplexer output to the
antilog amp.
If I remember correctly there are two unused "slots" in the DAC (and hence
two "unused" multiplexer outputs). But those two "slots" are always at the
same fixed value (I think). By using such a feedback loop it should be
possible to account for any changes to the circuitry (temperature,
voltage...) because this output would carry the changes and hence counter
them in the optocoupler.
I haven't checked what exactly the second "unused" multiplexer output does.
That's the one that leads back to vicinity of the 4051 in front of the
antilog amp (or the one on KLM-396 for old production boards).
At 6:15 Uhr -0800 02.03.2014, Bob Grieb wrote:
> One thing I don't understand is how the 4051's are being used to change the
> behavior of the circuit. And what is the fcn of the opto-coupler? Any
> thoughts?