PLC programming languages

What are the PLC programming languages?

There are five languages according to IEC-International Electrotechnical Commission, Section 61131-3
they are,

  1. Ladder Diagram (LD)
  2. Sequential Function Charts (SFC)
  3. Function Block Diagram (FBD)
  4. Structured Text (ST)
  5. Instruction List (IL)

A.Ladder Diagram

  • Ladder diagram is similar to Relay logic system which actually collective response of switches and mechanical relays.
  • These physical control devise are replaced by program logics in ladder diagram mostly (there must be control relays need to be attached with PLC,but its number can be significantly  reduce while implementing PLC compared to previous relay logic technology )
Fig:Expample for Ladder diagram in RSLogix
  • the name Ladder Diagram came from as it looks like ladder with two vertical rails, and the horizontal RUNGS
  • Each rung similar to a relay logic schematic, which represent the electrical connections
  • Left hand side of the Rungs are the Input side and right-hand side represents outputs or the logical conditions

Advantages of Ladder Diagram

  • Easy to follow similar like electrical schematics, no need for Coding knowledge
  • Allow description for input and output variables considered
  • Editing would be easily compared to other method

Disadvantage of Ladder Diagram

  • Making complex logic is difficult because of unavailability of default instructions
  • Programming become too lengthy compared to other languages for the same set of logics

B.Sequential Function Charts (SFC)

  • It is similar to a flow charts except, it can take multiple path

C.Function Block Diagram (FBD)

  • An FBD also a graphical representation of PLC language, it connects input and output through line to a block which having predefined Functions or default program.
  • It is mostly using for commonly repeating task such Timer,PID,Counter…


  • It is possible to include a program having several lines into a single functional block.

C.Structured Text(ST)

  • It is a textual language just like C, C++, Basic…
  • It is useful for implement complex logic
  • Those who are already are familiar with C language coding are happed to be easier this language.

Its really time saving, and can be implemented more easily with lesser time consume

E.Instruction List(IL)

  • For compact coding and time critical application this language is preferred.
  • Now a days it is very rare to see Instruction list implementing

As a summary, in my own experience ladder logic is the popular language this may be by the reason it showing a technological transition from electrical relay schematic to plc programming.

4 thoughts on “PLC programming languages

  • April 9, 2021 at 5:32 am

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