A programmable logic controller (PLC) is a small, modular solid state computer with customized instructions for performing a particular task. PLCs, which are used in industrial control systems (ICS) for a wide variety of industries, have largely replaced mechanical relays, drum sequencers and cam timers. The main components of a PLC consist of a central processing unit (CPU), power supply, programming device, and input and output (I/O) modules. The CPU is the brain of the PLC and carries out programmed operations. These operations or outputs are executed based on signals and data provided from connected inputs.
Programmable Logic Controllers execute logic in a sequential manner. They are programmed in ladder logic, structured text, sequential function charts, function blocks or instruction lists. The programs designed by the engineers interface to the control system through various sensors that send digital and analog data. There are two types of PLCs- Compact PLC and Modular PLC. In Compact PLC, the capability of the I/O module is fixed. • In Modular PLC, the capability of the I/O module is not fixed.
PLC are divided into three types based on output namely Relay output, Transistor output, and Triac Output PLC. The relay output type is best suited for both AC and DC output devices. A typical PLC is connected to a power supply and consists of a central processing unit (CPU), a mounting rack, read-only memory (ROM), random access memory (RAM), input/output (I/O) modules, a power supply, and a programming device. There are 2 types of PLC Digital Output Modules – Solid State and Relay.
