An apparatus for mutually converting signals between one or more modular printing devices and a document processing system, includes a device controller having a network interface for communicating with a system communications channel, memory for storing data and control code related to the operation of the modular printing devices, and a processor for running the control code, a digital input/output controller for receiving and sending document processing signals, a connector for receiving and sending modular printing device signals, and an optical input/output isolator for optically isolating the signals received via the connector.