Depending on the probe that you have, you should have two wires coming from the probe. One of those two wires will need to be connect to an input pin as shown in this wiring diagram (the diagram show the probe connected to pin I4):
The other pin is connected to V-. Please make sure that you have 24V connected to the V- and V+ on that side of the Mach3 USB board.
The way the probe works is when the probe is in contact with an object, a specific spring mechanism will cause the input pin to be connected to the V- (close the circuit). When Mach3 senses an active low on the input pin, mach3 knows that the probe has made contact (closed the circuit).
If your probe is used as a Z-Axis zero plate, then the concept is the same, but the end mill is connected to V- and the plate is connected to the input pin. If the plate comes in contact to the end mill, then the circuit is closed and mach3 will sense an active low on the input pin.
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Mach3 USB interface board. Could you please give me the correct wiring configuration for tool prob?