This one's frustrating because they clearly have the code to do it in the IP/S - maybe the IP/M processor is at the limit of its memory? Maybe it's a marketing decision to make people buy bigger?

I did wander outside the box. One solution would be to add in a little bit of electronics to do the homing. However, that would have to sit between the IP/M and the EM806 and receive/transmit differential signals. That's not a big deal but it's just a little bit more complicated than "just" sticking an Arduino in there with 5V single-ended signalling and it wasn't worth the effort. I also looked at setting up two different machine profiles, with a second one that treated the two slaved axes as separate axes. You can configure Mach3 to home these simultaneously, and I think that this would have worked except that it looks as if you can only have one IP/M profile that loads for every Mach3 profile. Because the IP/M profile knows about slaving, you can't do it.

Current workaround is home the machine normally, then hit e-stop which takes power off the steppers. Manually turn the slave axis pulley watching the LED on the home proximity switch, and in effect manually go through the Mach3 homing sequence. Not as accurate as it probably only goes to the nearest whole step (because of stepper cogging effects) but given that that's max 5/200mm error (5mm pitch ballscrew), about a thou in old money, and at one end of a 1000mm gantry, that's not a whole lot out of square.

But if there's a better way - I'm not too proud to nick other people's ideas!