Yes, I guess there's a chance that it's a switch. I bought a box of 10 cheap from China. One was dead on arrival, one failed soon after fitting, and one was completely the wrong type (only two wires was a bit of a give-away). They've been on there for 3-4 years now but even if the switch is good,wiring could be the issue. I've done a fair bit of electronic wiring and building in my day, as well as being in IT for my working life, and this one smells to me like software. I reckon that it's a bit of confusion about limits going on somewhere on the Mach3-IP/M boundary and both my problems are aspects of the same thing - just that one shows up as a hard limit trip and one as soft limits. It's not a million miles from the problems with tool-height setting during an M6 where the later versions of Mach3 get their coordinates in a twist.

Whatever - no-one's going to fix it but if I can understand how to reliably work around it, I'll be happy. Although i was looking at the UC300/UCCNC manuals earlier and I hadn't realised that they've sorted the master-slave homing problem as well. Maybe next year Santa will bring me a UC300/UB-1, I can try to remember how I wired the IP/M originally, and I'll do a heart transplant on the control box.

Thanks for your comments - haven't found the answer but it's helped me think about it differently instead of just cursing every time it goes wrong when I'm in a hurry to do something