Hi,
Just an update on progress.
I really really need this issue resolved, so I will continue until I find a solution. Since I imagine there are plenty of people out there successfully using Mach3, Pokeys 57CNC via USB and servo motors (with just step/dir mode), then there must be a solution. Not forgetting also that I can get perfectly smooth operation under some conditions.
Whilst testing Mach3, the laptop I was using for Mach3 died, so I have replaced it with a new PC based on a motherboard from MSI (H110M pro-vd) and using the same HDD. And I still have the same issue. So whatever the problem, it would appear not to be PC HW based.
Just to explain what's happening with the jerking. If I use G0X10 to move 10mm, it seems to split the move into about 3 segments and the motor ramps up and down for each segment. This gives the jerky motion. Now I don't know how Mach3 works, nor how Polabs USB driver works for the 57CNC. So I can't see whether Mach3 is splitting the move command into smaller segments and the Pokeys board is just following what it's told, or Mach3 is passing the correct move command to the Pokeys board and it is causing the segmentation and jerky movement. I still need to check with Polabs.
I did also try an old fix using RMClock, which stops the CPU from going into sleep mode by always giving it something to do. This also made no difference. I just wondered if the CPU was regularly going into sleep mode (to save power for example) and this was causing Mach3 to split up the move into segments. But I'm sure others would have had this same issue and I would have read something about this on the board.
regards
Craig