The RadiCentre® (Pro) uses a hardware interlock plug (6,3mm ¼” stereo jack) which can be used as a safety interlock. The hardware interlock plug provides two floating contacts which require connection for the system to operate. By default the supplied interlock plug is shorted and must be inserted into the interlock input at the back side of the RadiCentre®. When used with an external interlock system, like a door switch, the wires need to be connected in series with your interlock system and/or emergency stop circuitry. In this case, please cut the RED wire and connect the open ends to your interlock system.