Newbie (ageing rapidly!)

[m_c]

A couple miniature relays, and some simple logic could be used.

Something like this flow should work-
Initiate homing.
Axes move towards home sensors at usual speed.
When first homing sensor is triggered, it powers a relay which disconnects the step signals to that axis.
When second homing sensor is triggered, it does the same, but both home sensor signals are also summed to trigger the IP/M homing input.
Once the IP/M input is triggered, both relays get de-powered.
Homing then reverses the axes, and the process repeats.

Admittedly just stopping the step signal may lead to a bit of an abrupt stop, but you could switch the drives enable signal on the higher speed approach, so each axis coasts to a stop, then switch the step signal on the retract, which should happen at a much slower pace so an abrupt step stop shouldn't be a problem.

For the relays, something like a EC2-5NU (5vdc coil, or EC2-24NU for 24vdc) would work nicely for switching the step signals.
You'd probably need a couple more for the logic, but that with a couple flip-flop type configurations, and independent homing should be implementable externally with the controller thinking it's just homing normally.
If you can't figure out the required wiring, let me know, and I'll add it to my list of things to do.

[JAZZCNC]

A couple miniature relays, and some simple logic could be used.

Yes your right M_C that's exactly how I do it and was going to post all this but really this is not the place and distracting from the OP question so think it's best left alone or if somebody's bothered enough start another thread.

[ManicMetal]

Yippeeee!

Finally I have electrons going in and out of the right places on the NVUM motion controller.
It was the plugin. The supplier kept assuring me they had provided the correct one. Big surprise, they hadn't. Got hold of the v2.1 plugin and it leapt into life. I've offered to tell them how to get it working if they settle my invoice for about 5 hours of my time!

The bit of signal conditioning I referred to earlier is simply due to the highZ inputs. The opto inputs only sink about 1.8mA and switch on at around 2v, that could be a problem from noise pickup. So a 680R pullup and 100nF capacitor to GND plus a 3v zener to raise their transition voltage should make them much more immune to spurious signals. As observed before there are no isolation or common mode problems with opto inputs.

Now just need to connect it to the rest of the gubbins....... (and find out if USB is as flaky as suggested).

MANY MANY Thanks for all the help and suggestions.