Digispeed to Variable Frequency Drive problem

Peter Homann has been working with me on this problem (he's a
pleasure to deal with and I don't want anyone to take this as a
negative comment), but we're both currently stumpped.

I'm using his Digispeed speed control board to control an Omron
(Yaskawa) G5+ VFD with Mach3.

Supply voltage (from VFD) to Digispeed (through pot) varies from 11.3
at full stop, 11.37-11.52 depending on speed setting (PWM) (I think
it was higher voltage at lower speed), so that's OK.

I have valid PWM output from Mach2 which varies as I adjust spindle
speed.

Digispeed LED behaves properly (double blink at zero spindle speed,
fast blink when PWM is calling for another speed.

There is no voltage out of Digispeed when connected to VFD. If
Digispeed is "primed" by starting tool with control voltage wire
disconnected from VFD and then reconnecting it, Digispeed controls
spindle speed (until spindle is stopped, and then won't control it
without removing wire, starting PWM signal and then re-attaching
wire).

I'm not sure what electronics are involved, but it seems like the
state the the Digispeed finds that the VFD in when everything begins
to happen is pulling the output low and clamping it there.
If the Digispeed is connected afterwards, whatever is doing this has
no effect and the Digispeed operates properly.

LED on Digispeed is working properly, PWM output is working
properly. VFD is working properly. Wiring is correct (both control
and power). Voltage feeds (both control and power) to Digispeed are
correct.

Sorry to make this so long, but wanted to explain completely what's
going on.

Any clues?

Regards,
Jeff