Abstract: A probe arm for locating a probe on a machine includes a signal conditioning circuit which produces probe status signals to be sent to the machine. The signal conditioning circuit includes two LEDs mounted on the arm and which are differently coloured to provide a visual indication of probe status e.g. probe ready or probe triggered. One of the LEDs has twice the efficiency of the other and therefore to maintain consistent illumination one is driven with twice the current of the other. Thus by monitoring the current through the LEDs in a sensing circuit at the output terminals of the signal conditioning circuit the status of the probe is communicated to the machine. Power to the probe and signal conditioning circuit is provided from the machine via the output terminals so that only two wires are required to connect the arm to the machine.