Programmable Logic Controllers (PLCs) are a type of industrial computer system that is used to control and automate processes in manufacturing, assembly lines, and other such industrial automation settings. PLCs are designed to be rugged and reliable, capable of operating in harsh environments with high levels of noise, vibration, and temperature. They consist of a central processing unit (CPU), input and output (I/O) modules, and a programming interface.

PLCs can be programmed to perform a wide range of control functions, such as monitoring sensors, controlling motors and other actuators, and executing logic-based instructions. They use a programming language based on ladder logic, which is similar to the electrical wiring diagrams used in traditional relay-based control systems.

I/O stands for Input/Output and refers to the process of transferring data to and from the PLC, which allows the PLC to receive input signals from sensors and other devices, and send output signals to control actuators and other devices. There are two main types of I/O modules used in PLC systems: digital and analog. Digital I/O modules are used to interface with devices that produce binary signals, such as on/off switches, push buttons, and proximity switches. Analog I/O modules, on the other hand, are used to interface with devices that produce continuous signals, such as temperature sensors, pressure sensors, and flow meters.