I've had a long and hard think about this, the simplest way is to get 1 relay, and use the contacts to provide an input to the cnc controller (via an optoisolated input to avoid electrical noise).
All you do is get a relay which has a coil the same voltage rating as your current compressors pressure differential switch (it will probably look something like this).

The wiring is essentially the same they have a dual pole switch (3-pole if 3 phase.... but then you would not have a lack of air availability if it was 3 phase) internally which will switch the live and neutral (or if you are in america and running 220V, it will switch the two hot wires) that run from the mains to the motor.
What you do is simply wire a relay between the two contacts and then on the relay you use the normally open contacts to switch an input to the cnc controller (uc300/400/100 etc).
This way the "ok to run" signal will only be active when the compressor is not running (i.e. when the reciever is fully charged and when the reciever is being discharged).
Yes, the "ok to run" signal will also be active when the compressor was turned off..... but normally the plasma cutters have an internal pressure switch so that if the incoming air pressure is too low they won't attempt to fire (the spark won't travel across the gap, and if its a blowback start the gap wont be formed.... so it wont start the arc).
So..... all you need is a single relay of whatever COIL voltage your compressor is supplied with.
Rob
Einstein ― “If you can't explain it to a six year old, you don't understand it yourself”
...working my way through the 1000+ ways things don't work to find the one that does
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