PLCs are microcontroller circuit boards that are frequently used in a wide variety of industrial control and automation applications. The PLC described here is implemented in software. A complete PLC is simulated on a standard Windows PC. Using an external connection box, the PC can also sense and control many kinds of real items.The abbreviation ‘PLC’ stands for ‘Programmable Logic Controller’. These devices are also sometimes referred to as ‘freely programmable controllers’ in the trade. This term refers to an electronic circuit that can be used to automate machinery and processes. A PLC thus can replace a conventional relay logic circuit.A PLC always consists of a CPU module and several digital input and output modules. The number of input and output modules depends on the size of the process to be controlled.One of the major advantages of a PLC is that it makes it very easy to extend or modify the process. Consequently, PLCs have for years been an indispensable element in the automation of industrial systems.