You might ask why, it's for my mill turn setup running on mach3 which has a vertical milling spindle (huanyang vfd), and a servo controlled horizontal turning spindle. I have it working already using swap axis to switch the servo spindle between spindle and 4th axis, but the vfd vertical spindle has to run on a plugin since the motor tuning for spindle is setup for the servo spindle. This causes some odd behavior during synchronized ops like threading since the rpm readout gets confused between the 2 spindles. I have a work around that is working, but not optimal.
I think the best solution would be to ditch the plugin and be able to run both spindles on step/dir. Matching steps per unit for the 2 spindles is not important since I can call a pulley selection to adjust ratio in my swap axis macro. I will have the proper units setup for the servo spindle and use pulley gearing to adjust vertical spindle to correct speed.

Does anyone know of a step/dir to +/-10v converter I can use for this? I found the yapsc which I think would work, but can't find one for sale. Not sure if he even makes them any more. I've searched and found quite a few links from years ago, but having trouble actually finding something for sale.