Bad Day...

OK, good news!

I went and grabbed a standard 5.6v 1w axial zener from the electronics store - pretty ugly and not a 3W (hoping that means it will be more likely to blow in the future....)

Anyway - I can power up from 24v now, 5v power is OK, no more low voltage warning.
All inputs are working again
Spindle control is working again.

Thanks for the assist Peter - this seems to be a testament to how well these boards are designed.

Don't worry Cam,

These things happen.

I managed to kill 2 Mesa 7i96 boards (one through 24v short when I cut the wrong probe wire, one from the plasma EMI into the outputs).

And don't get me started on the amount of Arduino boards I have killed through a wrong wire or stray connection as I'm wiring them at midnight after working all day.