What is PLC : Brief Description

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines.  It is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact.

The first PLC was introduced in the late1960s to replace relay logic controls in the automotive industry. It’s work is to control machine operation are typically stored in battery-backed or non-volatile memory. It is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.Hence, It is a specialized computer used to control machines and processes.



Therefore PLC shares common terms with typical PCs like central processing unit, memory, software and communications.  It is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.

The components that make a PLC work can be divided into three core areas.

  • The power supply and rack
  • The central processing unit (CPU)
  • The input/output (I/O) section

 Since a PLC is a dedicated controller it will only process this one program over and over again. 





  • Troubleshooting is Easier and Faster.
  • Easy to develop Programs by offline simulation.
  • Less amount of Space Needed
  • Less Maintenance required.
  • Easy to Monitor Inputs and Outputs by HMI devices….and from PC’s.
  • Using a PLC eliminates the need for numerous timers to control many machines.




PLC’s are industrial computers, rather than basic home or office type personal computers. “Machine-Information-Systems.com” refers to the PLC as an “unsung hero controlling a massive range of equipment”.Based on their programming , which is controlled via one of the five languages.

 PLC units are actively working in the following industries: manufacturing, aerospace, travel, food, textile, film, hospital, leisure, foundry, agriculture, plastics and printing, among others.



Programmers may chose from among five PLC languages.

  • Ladder diagram (LD) is best for programs controlled by multiple files, subroutines and code sectioning.
  • Sequential function charts (SFCs) are used to program systems that are more advanced than those run by LD.
  • Function block diagram (FBD) is a graphical language that drives data from inputs to outputs by sending through blocks of nested data.
  • Structured text (ST) resembles Basic or Pascal programming languages, in that it uses statements such as “If-Then-Else,” “While” and “Repeat.”
  • Instruction list (IL) uses mnemonic instructions from the ladder diagrams and sends the instructions to the PLC via a programming terminal.


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