New to MESA

Actually, I am. I setup one that will work this way on a Mesa 7i77 setup. On my set up through a wiring error (that I can't seem to track down) the 12v field power connected to my 7i77, just happens to float at the machine's ground level (and the field power's ground is at -12v to ground). So this machine will work perfectly well using just a wire direct from the touch off plate to the input with no wire attached to the tool. However i don't trust the electrical contact through the spindle bearings and I still clip on a 12v supply wire to the tool.

Had my machine frame been at 0v I was going to wire it like this. 12v field supply -> 10K ohm resister -> input connector <- wire to touch off plate. Then a ground clip on the tool. The input pin should read high normally and be pulled low when the tool contacts the plate.

I gave up trying to get the field power 0v to be at ground level, when I found that when I connected it to ground everything shut down, shorted out my little wall wart power-supply, and I could not see why. Besides it made connecting the touch-off-plate easier.

As an alternative you can still use an ordinary parallel port and breakout board alongside a Mesa card.

If you have a 7I76E, 5I25, 6I25, 7I80DB, or 7I92, you can use the (normally unconnected)
second port and a low cost parallel port breakout for additional 5V inputs suitable for probes etc