Depends on your plasma interface. If it can take 5V logic signals, you can map a normal PMDX-424 output pin to Mach4's "spindle on" output signal, then use M3/M5 to turn the plasma on and off. Presuming you have one of the 3 outputs available to use. If all of your outputs are in use, including the relay, then yes, the 407 could (probably) be used to turn the plasma on and off. The 407 also has the advantage of isolating your plasma on/off signal from the rest of the PMDX-424, since the 407 uses solid-state relays for its two control outputs.
