Quote Originally Posted by Kitwn View Post
Next move will be to connect the inputs to the PCB headers and see if the extra 0.4V of headroom clears the problem. I don't always get spurious trips so this might be enough.
Interesting analysis. Re. the PCB headers, the only caution in a high noise environment is the protection of the '14 inputs to >5V spikes. The IC should have clamp diodes that should work to protect but worth bearing in mind.

I have to admit all my cheap boards have had opto isolated inputs which I think can only help for noise immunity for systems like this.