Well, does Mach3 have support for a straight motion controller protocol, and is the specification published?
Yeah are individual pulses actually being send out on the modbus, rather than a motion command (50 steps on this motor with this frequency)?
I could always just send G-code to the controller via a modem protocol with on/off flow control. That's complicated to implement because there's quite a few less common G-codes that might come up (automatic arcs, drilling cycles, etc). Sucks since a drilling cycle is a number of moves that COULD be conveyed as simple moves. This whole idea does bypass much of Mach3's functionality though, a lot of which is helpful. Acceleration profile for one. Actually at this point Mach3 probably wouldn't be involved at all if I'm just sending G-code over with a terminal program, losing all the benefits of Mach3's control panel and position tracking and all.
Ideally I'd love to find out that Mach3 had a motion control plugin that sent compressed data with a few basic commands (513 pulses on this motor with this frequency) and all the drills and auto-arcs were changed into this format, and some sort of feedback on the link that would prevent the buffer from overrunning or underruning if there were some minor differences in the time Mach3 thinks it takes to complete each task and the time the controller actually needs to complete each task.