OK, I switched to the DLL version on the download page - for the probe, I am using a normally closed switch, probe is wired to X, Y, and Z latch, digitize input is set to the Z latch input, active low box is checked, digitize input lights up when I touch the switch. Calling G31 makes the machine move as expected, however, sometimes, the motion stops when the switch is hit, sometimes the motion stops when the switch is hit, then released (motion stops on the release), sometimes, it doesn't stop at all - I can't figure out any pattern to it...