need to turn on & off, a physical pin with one of the un- unused M code.

Need additional M code to operate a hydraulic cylinder valve. Already using M3 & M5 also M7 & M9. need a third on/off M code.
Using Jon Elsons stepper board.
davidcnc thanx

Is there a reason you can't setup and use a couple of the user definable M-codes (M100-199)
linuxcnc.org/docs/html/gcode/m-code.html#mcode:m100-m199

M62/M63 and M64/M65 by default provide 4 digital outputs you can turn on and off but this can be extended to up to 64.
linuxcnc.org/docs/html/gcode/m-code.html#mcode:m62-m65
The link in that section tells you how to extend the number of pins that can be controlled.