The input module is used to receive and collect input signals. There are two types of input signals: one is the switching input signal provided by pushbutton switches, travel switches, digital dip switches, proximity switches photoelectric switches, Reed relays, etc.; the other – the continuously changing analog input number sent from potentiometers, thermoelectric, tacho motors, various transmitters. The input module also needs to convert these different level signals into digital signals that can be received and processed by the CPU.

The role of the output module is to receive the digital signal processed by the central processor, and convert it into a signal that can be received by the field execution components, used to control contactors, solenoid valves, control valves, speed control devices, etc., control of another class of load is the indicator, digital display and alarm devices, etc.. In order to improve the anti-interference ability, the general input / output units are opto-isolated devices. In digital IO modules are widely used by light-emitting diodes and phototransistors composed of opto-couplers, in analog IO modules are usually used in isolation amplifiers.