I managed to hook up a piece of blank PCB as a touchprobe for zeroing the z axis to my NC-studio controlled router.
After making sure that the machine was earthed correctly. The power socket earth wasn't connected at all so I grounded the chassis and the power supply and verified that there was no voltage between the PC earth and the chassis of the machine.
Then I opened up the IO state tab in NC-studio:
and verified that the calibration input changed state if a wire on pin 1 of the 15 pin cable was grounded - it did:
Then I soldered a 1" square of blank PCB to a long wire connected to the NC studio breakout board in the machine and verified that the "calibration" input changed state if I touched the copper surface of the PCB to the tool tip - it did.
In the "manufactory" menu there's a setting for mobile calibrator thickness - I set this to 1.6mm - the thickness of the PCB. The default password for access to the manufactory menu is "ncstudio" BTW.
Then I placed the PCB under the tool and pressed "mobile calibration". NC-studio lowers the z axis slowly until it contacts the PCB then raises 5mm. The Z reading is now 6.6mm which is the 5mm it raised + the 1.6mm thickness of the PCB.
here's a really boring video of the tool touching the PCB:
I'd feel happier if the input was optoisolated - but this seems to work.
If you try this and there's no continuity between the tool tip and ground - then this won't work. I've seen one person rig 2 wires - one to a PCB and the other to an alligator clip which he clipped to the tool when calibrating.
My plan is to swap the machine over to mach3 but this makes the machine usable until I get around to it.