Quote Originally Posted by EddyCurrent View Post
Just asking on behalf of andy586 as he may come across this requirement later and I didn't want to steal your thunder by replying myself.
No your being a smart Arse Eddy but just for the sake of others and Andy586 I'll answer it.

To be 100% sure touch probe will work then you should use Input (+) & (-) terminals with Clip to spindle and Plate for touch off. Yes Spindle may be connected to Earth thru the BOB but you don't know that for sure and actually better quality BOB's or Motion control cards with direct I/O (Like CSLAB's products) don't actually allow this to protect there inputs and protect against Noise etc so force you to use dedicated Negative so they know every thing is at the same potential.

Just because it works doesn't mean it's correct way or actually safe for the BOB.!