Last Updated: April 23, 2024


Leena Sanap


Programmable Logic Controllers PLCs are extremely popular in varied industries like manufacturing, automobiles, pharmaceuticals, packaging, construction, energy, chemicals, food, and the list is endless. The reason why PLCs are so popular is that they have varied applications and are smart computers for rugged environments.

One can observe that many medium and large scale industries are hiring a PLC programming company and investing heavily to introduce PLC in their mainstream operations. Thus there is a great boom and a flourishing career ahead for PLC programmers.

This brings us to the central idea of this article which is to understand the prevailing types of PLC programming languages in the market and the ones that should be followed along with the reasons why they are trending and sought after.

How are PLCs programmed?

In order to understand the types of PLC programming languages, it is necessary to understand the format and what constitutes the actual programming of a PLC.

PLC has various modules. The significant part of which is the I/O module. A PLC program is coded for running the programmed logic on the basis of the instructions received through the input devices and carry out the execution via the connected output devices.

Basically, the PLC program is an intermediate link between the input and output devices connected to the PLC. The following are the types of PLC programming languages:

  • Ladder diagram (LD)
  • Function Block Diagram (FBD)
  • Sequential Function Charts (SFC)
  • Structural Text (ST)
  • Instruction List ( IL)

The above 5 types of programming languages are officially recognized by the IEC International Electrotechnical Commission. Each of these languages is defined by the international standard code (IEC 61131-3).

Understanding the Types of PLC programming languages

Ladder diagram (LD) or the Ladder Logic Programs:

This is the most popular and preferred type of PLC programming language. Since PLCs are programmed with the basic relay logic, the ladder logic uses simple normally open (NO) normally closed (NC) and other lines of code instructions.

One can actually witness the process logically through the “Rungs” and data arriving as inputs to the components of PLC.

Function Block Diagram (FBD):

FBD programming language is used for multiple repetitive functions like starters, closed-loop control, PID loops, etc. Each component of the FBD is connected to the preceding component’s output.

In this, the data flows in a nested manner from one block to the other virtually. The identifiers, keywords, variables, and data types need to be taken care of in this type of programming.

Sequential Function Charts (SFC):

The SFC similar to LD and FBD, and one of the graphical programming languages for PLC. SFC is used to write concurrent programs between steps and transitions. These conduct actions based on logical conditions.

This programming language has logic like that of a flow chart. The program is divided into steps. And, the steps can either be active or inactive.

Structural Text (ST):

This programming is very similar to that of high-level languages like Basic, Pascal, or C. This is a great choice for implementing complex algorithms and mathematical logic. It is very easy to make modifications in this program as per the standard format.

The common set of instructions in this programming language are IF, WHILE, CASE, RETURN, FOR, REPEAT, etc.

Instruction List (IL):

This is an assembly language type of PLC programming language. It has a high level of execution speed. As compared to others, IL has very low memory consumption. It makes use of the concept STACK. The mnemonic codes are usually used for IL-based PLC programming like:

  • ANB
  • ORB
  • LDA
  • ANI

In Conclusion:

PLC range from small modular devices that can be carried around to big machines with thousands of input and output ports that are of the size to fill up a room. The reason why PLCs are preferred in industries is that they can endure tough environments and provide the application of an intelligent computer.

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