-10V to 10v circuit

[EddyCurrent]

Assuming the OP still wants -10 to +10.
With the method proposed previously by Jonathan there exists a problem, if the reference wire to the servo driver goes open circuit, the drive would go to full speed reverse.
Would it be possible to use a standard spindle board with 2 relays such that the output would always be 0-10v but one relay would reverse the polarity of the 0-10v output ? opto isolators would need to be used to prevent grounding issues. Within Mach3 the user would use S for speed and M3,M4 for direction.
In additiaon a Mach3 Brain could be used to read the Spindle Speed DRO and only operate the direction relay outputs if the speed was above a particular figure, this would give a deadband around the zero position.
Then the logic could also include a test to see if the requested speed (via S command within Mach3) was odd or even, if it was even this would indicate forward, if odd it would indicate reverse.
The speed would then be programmed in Mach3 such that a value ending with zero would be forward and ending in 1 would be reverse i.e. S1000 would be 1000rpm forward, S1001 would be 1000(almost)rpm reverse. The Mach3 Brain would operate the polarity switching relays automatically. It's clear that the code could move from full forward to full reverse e.g. S10000, S10001 implying that an integrator be used but this is normally a function of the VFD or Servo Driver that employ ramps.