Controlled engineering has evolved over a long period of time. in the Past human were the controlling device for a system. More recently electricity has been used for control and the early control is done by the help of electrical relays. The relays allow the supply can switched ON and OFF electrically without out a mechanical switch. Nowadays relays are common to make simple logical control decisions. The development of low cost computes discovers the most recent control system, Programmable Logic Controller (PLC).The advent of PLC began in the 1970’s.Today PLC is most common for controlling processes. The PLC is used as a standard in the field of automation for control. It replaces earlier relay controls and taken over many additional functions.
The PLC is designed by a computer to operate in industrial environment and arranged with special input, output and a control programming language for use in machine.
The PLC is designed to controlling a system, but it is finding many and more complex applications. The structure of PLC and computer architecture are constructing based on the same principles, so the relay performing not only the relay switching but also counting ,processing ,calculating and comparing of analog signals.
PLC is a microprocessor based computer, which is user friendly and performs control functions of different types and of different levels of complexy. A PC can be used as PLC with input sensing devices such as push button , switches ,.etc; input according some conditions. A program is needed to process Input and the PLC turn ON and OFF load devices by means of Ladder Diagram. It essential to draw a Ladder Diagram of controlling of devices with laptop or PC. The drawing is converted into computer language and runs as user rogram. 
