Digital input/output module provides computer monitoring of digital input and digital output signals and control of industrial devices through solid-state relays. It allows a single host computer to control thousands of I/O points from one serial port. The digital inputs accept a wide signal level input, up to +30Vdc without damage, and have TTL logic input switching levels. The digital outputs are open-collector transistors offering the most flexibility for almost any application. It is ideal for controlling solid-state relays.

A digital input typically consists of a power supply (voltage source), a switch and a voltage-sensing device (analog-to-digital converter). Depending on the switch's open/closed status, the sensing device detects a voltage or no voltage condition, which in turn generates a logical 0 or 1, on or off, alarm or normal or similarly defined state.