Well, that was fun! Couple of minutes to wire from CSMIO to socket on case. Then rather longer to work through the CSMIO manual to get the Mach3 and plugin configuration sorted. Seemed to be about 4 different config screens and tabs to find all the relevant places to configure things while remembering that "output" means something different to Mach3 and CSMIO, and that pin numbers are not necessarily output numbers. All good fun, but now the forward relay comes on when commanded from the screen and a quick check with a meter says that the PWM analogue output voltage is about right for the commanded spindle speed. In fact, with these settings, it looks as M3 or M4 will both actually switch the spindle forwards.

Now to do the equivalent with the VFD... Good luck, Neil - it's easy when you've done it once