"DiagDac On with Resolution = 2^xx" would suggest that the output is being generated by a digital-to-analogue converter, and that the PES voltage is being scaled to fit within the DAC's range.
The DAC appears to have an 8-bit resolution:
"... the high byte of the shifted result is sent out to AMUX"
AIUI, this is the layout of the 16-bit PES:
Code:
sign num tracks partial track
|----|--------------|---------------------------|
| 15 | 14 : 13 : 12 | 11 : 10 : 9 : : 0 |
|----|--------------|---------------------------|
+/- 4 2 1 1/2 1/4 1/8
Let's say that the PES has a low value, eg ...
Code:
|----|---------|---------------------------|
| 0 | 0 0 0 | 0 1 0 1 0 1 1 0 x x x x |
|----|---------|---------------------------|
| | |
|------------------------|-----------------|
| high byte to DAC | ignored |
|------------------------|-----------------|
In this case the PES will only have an effective resolution of 3 bits. We can scale up the PES by shifting it left by 4 bits.
Code:
|----|---------|---------------------------|
| 0 | 1 0 1 | 0 1 1 0 x x x x 0 0 0 0 |
|----|---------|---------------------------|
| | |
|------------------------|-----------------|
| high byte to DAC | ignored |
|------------------------|-----------------|
ICBW, but that's how I understand the docs.
BTW, the 7200.12 PCB has two sets of pads on the underside. Some of these pads will probably be assigned to JTAG, but perhaps one of them is your AMUX pin.