This invention relates to a speed monitor apparatus for monitoring speed and acceleration of rotating turbine equipment (turbomachinery) and for operating an overspeed trip to shut down the equipment in the event that the speed or acceleration exceed predetermined thresholds. The invention provides a speed monitor module having an input conditioner comprising: a multiplexer; an amplifier; and a comparator; in which the multiplexer selects an input signal from the sensor, or an input signal via a feedback loop from the comparator in dependence upon a test input, such that the input circuit may be tested by confirming that the amplifier oscillates when the feedback loop is enabled by the test signal.