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

[Muzzer]

I expect the "backward" drive of the opto input through the antiparallel diode reduces the recovery time of the LED, or at least overcomes its capacitance. The propagation delay of a typical opto can be around 1us or so and the input capacitance may be as much as 200pF or more.

You may have noticed that some drives show a higher PPR for differential mode than for single ended and this can only be accounted for by the LED being flipped more quickly.

Have a look at what's inside any of the controllers you have to hand. Many of them are simple, single-ended (open collector) drivers and the rest are what you used. Not much help to you, I know....

EDIT - my Yaskawa Sigma servo (4th axis) has SN75174 drivers https://www.ti.com/lit/ds/symlink/sn...oogle.com%252F

These are probably a good "golden reference".