Programmable Logic Controllers (PLCs) are the micro processor controller devices situated at the remote sites of the plant or at the field of the process systems. PLCs purpose is to monitor the process parameters located at the field and adjust the outputs based on the inputs received by the PLCs. PLCs will operate on any systems which will have input field devices that are On-Off type (discrete or digital ) or analog input devices. Similarly it will operate on the field devices which will be discrete or analog output type.
In simple words, Hence PLC acts as interface between the input and output devices at the process side of the industries. It monitors the inputs receive from the input devices and takes necessary output control functions by executing the programs stored in its memory.
The term Logic is used in Programmable Logic Controllers because all the program inside the PLC will be of Logic programming (eg: Ladder diagrams ).
In industrial applications hard wired relay logics were replaced by Programmable Logic Controllers due to its reliablity, simplicity, low cost, easier programming and many other functions