Servo as a spindle motor replacement - what to look out for?

[Doddy]

Okay, so here's something different.

I've been driving this spindle servo using a UC300eth with step/dir signalling from UCCNC, with the two pins from the £5 Chinese BoB driving two separate TTL->RS485 adapters (£3 boards, based on MAX485 chips) - these to provide a differential drive to the PP+/PP- and PD+/PD- position inputs on the servo controller. This worked out of the box from first attempt, so many months ago. I should say there's ~10 ft of 4-core dual-twisted-pair signal cable between the control box and the servo controller.

Nothing has changed, wiring wise.

Today, no spindle. Nada.

I hate working on this machine as the controller is built into the bench as a rack-unit (I'm never doing that again). But, stripped it all down, and noticed that when scoping the step/dir signalling to the servo controller, if I (accidentally) grounded the PP- with the scope ground on the probe, that the servo kicked into life. I've checked this all the way back to the MAX485's, so it's nothing to do with the wiring or connector, but that the differential drive output from the '485 can no longer drive the Opto-inputs on the servo controller. Annecdotally, this appears to be the same on the direction input as well as the step input.

QUESTION: There's nothing other than wiring on the typical Chinese servo drivers to set Unipolar/Differential drive, is there? - pretty sure it's just a case of driving the opto-input accordingly.

So, I have a quandary.

1) Replace the 485 drivers with same (need to order some) - but I'm loathed to do that - there's no reason to doubt that the replacement will also fail at some stage.
2) Replace the 485 drivers with an open-collector transistor drive - I've plenty of trannies to hand, and that allows the unipolar drive iaw the manual. But it's a strip-down of the control box.
3) But the servo controller input is just an opto-isolator input configured for 0/5V signalling. The easiest solution is to drive the servo from the BoB output pin, and ground the PP- input at the servo controller input. This is the lazy solution.

QUESTION: What signalling method do others use for reliably driving the step/pulse input on servo controllers?