Inputs are wired to sensing devices such as button keypads, selector switches, photocells, motion detectors, etc. Ouputs are wired to switching devices such as lights, garage doors, heating systems, and other household appliances.
Outputs switch the supplied control voltage that energizes or de-energizes the switching devices.
If an output is turned ON by the CPU, the control voltage is switched to activate the switching device. Within the CPU are the digital processor, memory, and power supply. These components interact to solve application logic and pass control signals to the outputs. The CPU reads the converted input signals, executes the user logic program stored in its memory, then writes the appropriate output signals to the switching devices. The application program is written in a language called ladder logic.