I built the first prototype and found a couple of errors on it:
- The quad NAND gate has the wrong pinout.
- There was no connection from the Pi GPIO header for the GFI test line
Both of those have been corrected and in addition, I replaced 3 diode-plus-pull-up level shifters with a triple bus buffer chip. It may be slightly pricier, but placing a chip and a bypass cap beats 6 passives. I also replaced the dual JK flip-flop with a single D flip-flop chip. The fact that it's a D vs JK doesn't matter in our case because we only use the set/reset functionality as a latch.
I was able to test the pilot generator and ADC and they work properly. Also the relay test input works (though there wasn't much chance it wouldn't).
On to v1.1!