PLC is a digital computer used for automation of electromechanical processes in plants. The PLC is designed for multiple inputs and outputs arrangements, so we can get the data from the sensors, work with it and command the actuators.
The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968, when the company were looking for an alternative to replace complex relay control systems.
The new control system had to meet the following requirements:
- Simple programming
- Program changes without system intervention (no internal rewiring)
- Smaller, cheaper and more reliable than corresponding relay control systems
- Simple, low cost maintenance
The function of an input module is to convert incoming signals into signals that can be processed by the PLC, and to pass those signals to the central control unit. The reverse task is performed by an output module.
This converts the PLC signal into signals suitable to operate and invoke the plant actuators. The actual processing of the signals is undertaken in the central control unit and with respect to the program stored in the memory.