Ok, I haven't given up on this and I'm finally in a position to use the oscilloscope on the inverter (isolation transformer, USB isolator).
Also, while waiting for the USB isolator to arrive I've painstakingly mapped all the ADC pins on the MCU.
I've put probes on all the ADC inputs and none appear to exhibit the pattern reported by the "AC Plate voltage" value reported by MODBUS when I run the compressor
![Sad :(](https://www.eevblog.com/forum/Smileys/default/xsad.gif.pagespeed.ic.L3FGyzQrjB.png)
That said, there is another characteristic that might provide some clues; when powering off the inverter there's enough power in the capacitors that the 3.3v rail holds for another 15 seconds or and continues to power the MCU and I can continue to read the values for this period. The DC voltage drops in a fairly linear fashion until about 160v at which point the MCU cuts out
BUT the reported "AC Plate voltage" drops immediately to 190v and stays exactly at that until the MCU cuts out. In fact, regardless of what input voltage I feed the inverter (using a step up/down transformer I have been able to generate 250v, 217v and 210v in addition to my current line voltage of 227v) and consequently where the "AC Plate voltage" starts when the power is cut (it always seems to read 14v lower than I measure at the terminals with a DMM), it always drops abruptly to exactly 190v for the remaining duration before the MCU cuts out.
One reason this behavior is useful is that I have probed
every pin of the MCU while powering off the inverter from idle and the only two pins that show any real change between power off and the MCU cutting out are pins 6 and 9.
Pin 6 (ADCINA7) drops from about 1.6v in a linear fashion to about 0.8v before the MCU cuts out (pin 6 is connected to DC+ through 4 x 470kΩ resistors and I'm fairly confident this is where the DC voltage measure is taken)
pin 9 (ADCINA2) drops from about 2v much faster, more of an exponential decay. Maybe not directly converted to the "AC Plate voltage" but certainly suggests its it involved.
My guess is that because the "AC Plate voltage" does not directly track any input it is some sort of calculated value (with a floor of 190v) but I'm at a bit of a loss as to where next?
I was thinking that maybe I should I try manipulating the voltage on pin 9 with an external source and see what it reports as the "AC Plate voltage"? What sort of supply would be suitable for this?