Programmable Logic Controller or generally known as PLC only, is a device that replaced the sequential relay circuits for controlling various kind of automatic small and very large machines and various plants. The PLC works by managing its inputs and depending upon their state, turning on/off its outputs as per the logics feed into PLC. The PLC programmers enters a program using various Machine languages like FBD,Ladder,SFC,LI and so on, usually via software, that gives the desired results

Automation world is full of PLCs ,running almost every production field without human help.From simple tube mill to hightech botteling plant,PLC's play very important role by involving PLC's into production everyone is making profit.e.g manual boring machine which was need atleast 2 operators,now help the end user to eliminate human need and saving lot of money and precious time , on the same time the company that was manufacturing and selling this machine ,maked profit by selling this automatic machine at higher rates.If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a plc.

For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.

As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time.

This site gives you enough information to be able to write programs far more complicated than the simple one above. We will take a look at what is considered to be the "top 20" plc instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence.
That's right, more than 80%! Of course we'll learn more than just these instructions to help you solve almost ALL your potential plc applications .