Hey I have had simular experiences, here is how I did the work around.
IN your Ports and PINS: DISABLE your spindle motor output!! (I suspect that the SS is fighting over this with your VFD).
Once you have disabled your Spindle motor output, in PP, then goto "Brains". I am assuming you have an Analog module in your PLC??, I am going to assume it is a 12 bit module, change to your situation.
In Brians Put:
Input spindle percent, DRO(99)->formula: (A*4095) -> (your modbus Serial Plugin ->modbus Config, and Local var termination here). DRO 99 ALWAYS shows 0-1.00 (0-100%) of your current Pully range. Just remember also, for tying in your M3/M4/m5 across the Brians, you will need to make sure that you DO enable "Relay control" of M3/M4, and tie each of them, and limit them to Outputs 1-6. The M3, M4 LEDs will only Lite up if tied to these output ranges, then in Brians, be watching the LED for M3, and on another rung, the LED for M4 to send out to your PLC for dirction. NOTE: Mach3 will take care of "interlocking" M3/4 out from eachother, and also will turn them off on m5 calls.
ALSO: check to make sure that your SS is NOT competing with ANY other IO, that your PLC is using, it WILL win in the battle for IO.
scott