Electronic plug in units used for interfacing the i/p and o/p device in the machine or process to be controlled. I/P module receives data from i/p devices (Pushbutton, Switches, Transmitters) and send it to processor. The O/P module receives data from processor and send it to output device (Relay, Valves). Digital/Discrete :- Sends and Receives On/Off signal Analog :- Sends and receives variable input or output signals