It's come to my attention that UL now requires EVSEs to perform continuous GCM testing at all times, not simply when the (or any in this case) contactor is closed I was lamenting how to accomplish this, as it was going to require changes to the logic board and firmware, but I had an epiphany: Since GCM testing is supposed to occur at all times now, there's no reason it has to interact with the logic board at all. If the GCM test gates power to the logic board, then cutting power to the entire logic board would shut everything down including the contactors (if they were closed).

So that actually simplifies things somewhat. There will be a new v6.0 power board that will support continuous GCM. It will use the same primary-side circuit as the old relay test did, but connected just after the supply fuses rather than to a relay test terminal. On the secondary side of the optoisolator, it will be treated as an open collector, which will bring down a cap that is pulled up by a resistor to 5v. That cap will be the gate of a P MOSFET that gates power to the FFC to the logic board. There will also be a "GCM OK" LED to diagnose things (a dead logic board won't display anything, of course). You'll know the GCM has failed when you see the PWR LED turn on, but not the GCM OK LED.

This also gives us an opportunity to simplify the relay test. For that, an AC optoisolator (which simply means it has two LEDs connected anode-to-cathode so that it lights up with either polarity) and series resistor between the two lines of each relay test terminal is sufficient for the primary. And for the secondary, the same open-collector, cap and pull-up is enough to supply a logic level to the board - low is energized, high is not. In both the relay test and the GCM case, the cap is required to filter out zero-crossing drop-outs.