powermax 600 wiring for cnc control

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I have a powermax 600 with the pac123 torch that I would like to wire for cnc control. I want to put a relay across the trigger wire but I'm sort of confused on which is the right one. There three wires, purple, orange and blue. I think I should be putting the relay across the purple wire. I beleive that the orange is power input and the blue is return from the (parts in place) switch and therefor the purple is return from the trigger switch? I'm I right? Does any know for sure before I start cutting wires.
thanks

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Ok I answered my own question, it's the purple wire that you want to put a relay across.