After all the effort of creating a separate speed control for the wire feeder ...
Now the TinyG motion control board is not activating a command to start the motor.
The strategy was to use the coolant on / off pin to run this circuit.
Commands M7 / M9 should be switching on / off 3 volts at the pin.
G code commands M 3 / M5 are used to switch the welder on / off with the spindle pin on the board.
The welder turns on / off as expected.
When the wire feeder is connected to the spindle pin all is fine.
Hooking two circuits to one pin out didn't seem like a good idea ... but ... I tried it anyways.
Both circuits switch on but won't switch off.
Operating both circuits from one pin isn't going to work.
A solution ? My first guess ... a firmware change or upgrade will need to happen.
Happy to say the feeder drive board has evolved away from bread board testing to a soldered connection prototype board. Also changed the potentiometer to a 10 turn precision version for finer speed tuning.