If you are using a floating head with a micro switch to set the torch height prior to each pierce and can fire the torch with a relay, I'm not sure why you need an arc ok signal. THC systems may require an arc ok signal but the wiring would be between the plasma cutter and the THC controller not the G540. You didn't say what model of plasma cutter you are using but specific wiring for the arc ok signal to the THC controller would depend on your plasma cutter. I suppose you could wire the arc ok signal to a pin on the G540 to halt movement in the event of lost arc or if the cutter doesn't fire, but with a basic setup and no THC it may be more trouble than it's worth.