The CIRCUIT MODULE-DIO 300 is probably a replaceable circuit board for digital interfaces that can handle a variety of analog and digital input/output signals. Its purpose is to work with a certain line of rail equipment products. In order to monitor status metrics and regulate electromechanical components, it would interact with a variety of sensors and control systems within an EMD diesel locomotive. It can be connected to a variety of signals, such as pressure transducers, relay actuators, temperature probes, voltage sensors, current sensors, and switch sensors. The circuit board would handle an analog voltage in addition to digital signals that were either one or zero logic.