Basics of Programmable Logic Controllers

A programmable logic controller is widely employed in Electrical industries for controlling various operations and processes. In this post you'll learn:

  • What is the programmable logic controller
  • Parts of PLC
  • Programming languages for PLC
  • How to learn PLC

What is a Programmable Logic controller

A programmable logic controller or PLC is a special type of computer which is used in control and operations of manufacturing industries.

A PLC takes an input from sensors and controls the behavior of connected components across its output. This behavior of PLC is referred as I/O or input/output capability.

Construction of PLC

The actual construction of PLC depends on its size. PLCs are available in numerous sizes. It can be as smaller as a brick or it can be 2 to 10 X larger than your normal PCs.

The figure below displays a small sized PLC which is manufactured by CLICK. PLCs like this have all I/O, communication ports and processers built inside one single box.
basics of programmable logic controllers
The large industrial systems have separate racks where various cards are mounted. Processors, I/O points, communication ports and other functions are separately mounted in such PLC systems.

The figure below displays a PLC where I/O points and other processors are separately mounted on three different racks.
Pic credits: Under CCA 4.0 Tony R Kuphaldt

Programming Languages

A PLC is a very powerful device, but its real power lies in its programming capabilities. A PLC can use any of the five different types of programming languages:

  1. Function Block Diagram (FBD)
  2. Instruction List (IL)
  3. Ladder Diagram (LD)
  4. Structured Text (ST)
  5. Sequential Function Chart (SFC)
All of us are at least familiar with instruction list programming (IL) is C/C++ language. However the 

most popular language in PLC world is Ladder diagram programming.

LD represents the I/O in terms of visual circuits so that electricians and technicians should easily understand it.

In Electrical Industries, it is often desired to simplify the processes and to display only the required information. Due to this usually, HMI panels or dedicated computers with some software is installed.

As a result of this, a technician or Engineer can directly control and access the required info from PC of HMI panel without even touching the PLC itself.

How to Learn PLC

So that was all about Basics of PLC. The above information is quite enough for anyone to develop an overview of PLC. You can join some PLC course or can get a book on PLC and your own PLC trainer.
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