What input should I use for a probe fault.

The 7i76 has 8 analog inputs and I'm successfully using 3 for limit/home switches.

oldtech wrote: The 7i76 has 8 analog inputs and I'm successfully using 3 for limit/home switches.

7i76 should have 4 analogue inputs when set to mode 2, not 8, but in any case double check that.

oldtech wrote: The 7i76 has 8 analog inputs and I'm successfully using 3 for limit/home switches.

That reminds me, you should have mentioned you have an 7i76 in the first post, wasted 10 minutes looking where did i put those LM331 chips as i have 3 of them in a box waiting for some quality time .....

oldtech wrote: The 7i76 has 8 analog inputs and I'm successfully using 3 for limit/home switches.

Yes, you need to change modes to use as analog inputs. If you don't they are normal digital inputs. I use them for home/limits too.

Sorry, I will include a background in new questions. In any case, it looks like I will need an analog to digital chip for connecting to E-STOP from the probe.

oldtech wrote: Sorry, I will include a background in new questions. In any case, it looks like I will need an analog to digital chip for connecting to E-STOP from the probe.

Yes, you probably don't need to use a THCAD as you do not need isolation from the noise of the plasma environment. Tommy's chip should work nice. But you still need an encoder input.. There is not really any other way to do it.