Programmable logic controllers — history and basic overview

Tibor Kasas 2 min read

During history in manufacturing processes, people used thousands of relays, timers, closed-loop controllers, and many other controllers. This type of manufacturing process was complicated to update. Due to its complicity, these processes were also very expensive and time-consuming. Because of this conundrum in the 1960s engineers have invented the Programmable Logic Controller and since then PLCs have been used in industrial control systems.


A PLC is an industrial digital computer. It has been adapted for the control of manufacturing processes. These processes include assembly lines or robotic devices which require high-reliability control and ease of programming and process fault diagnosis.


Today there are five types of programming languages for programmable control systems. Those five are function block diagram, ladder diagram, structured text, instruction list, and sequential function chart. The most common language out of the mentioned ones is the ladder diagram. This type of programming is most widely used because of its simplicity. It uses symbols that are interconnected with lines that show the flow of current through relays.


Basic components of the PLC consist of the Central Processing Unit, the power supply, the programming device, and the input/output section. Programmable logic controllers can be modular (rack-mounted), or compact (integrated). Their main difference is their size and capabilities which correlate with the number of inputs and outputs they have. In modular PLC’s you can add components to the rack while on the compact PLC the inputs and outputs are already fitted with the CPU.


In conclusion, the Programmable logic controller has been a major invention for manufacturing processes. They are cost-effective and adaptable. With PLC’s you can control complex systems and their big advantage is that they can be easily reprogrammed for a different application.