Also - The axis velocity might be able to be used to accept input from the THC. With LinuxCNC, I can program of minimum actual velocity, say 80%, of programmed velocity as a threshold. If the the machine's velocity slows down below 80% (like when programmed for cutting a smaller hole), LinuxCNC will ignore input from the Proma until the velocity returns to a speed greater than 80%.

I think the Proma's ARC-OK signal is sort of a pseudo signal based on measured voltage. Not 100% sure about that as I haven't used it.