The 5 PLC Programming Languages of IEC 61131-3 Compared

1.What IEC 61131-3 governs

IEC 61131-3 is the international standard for programming programmable logic controllers. It defines not only languages but also data types, block types (function, function block, program) and the execution model.

The practical benefit: programming to the standard produces code another programmer can read and understand on a different controller. That does not make it portable in the copy-paste sense – vendor dialects differ too much. But the concepts are the same, and in practice that is the more important part.

2.Standard names and Siemens names

This is where the most common confusion arises – including in technical articles. The standard defines five languages; Siemens uses its own names for the same languages:

One further subtlety: SFC is strictly speaking not a programming language but a structuring element. Inside the steps and transitions of an SFC, the code is again written in one of the other languages. In practice SFC is nevertheless counted as the fifth language, and the standard covers it in the same part.

3.The five languages compared

3.1.LD – Ladder Diagram

Ladder mirrors circuit diagram logic: contacts in series form an AND, in parallel an OR. For maintenance staff with an electrical background this is the language with the lowest barrier to entry – an invaluable advantage when someone without programming knowledge has to follow an interlock at night.

The limit shows up with scale and arithmetic. A network with twenty contacts and three parallel branches is wider on paper than the screen, and scaling an analogue value turns into a chain of blocks.

3.2.FBD – Function Block Diagram

FBD shows signals as blocks with inputs and outputs. Where LD emphasises the current path, FBD emphasises the data flow – a better fit for analogue processing, controllers and anything that transforms values rather than switching them.

3.3.IL – Instruction List

IL is the language closest to the machine, essentially an assembler for the PLC. In the third edition of IEC 61131-3 (2013), IL was classified as deprecated; it is no longer recommended for new projects.

It stays relevant for one reason: legacy plants. Anyone taking over an S5 or S7 Classic program regularly meets IL – often without symbols. Being able to read it is not nostalgia during migrations, it is craft.

3.4.ST – Structured Text

ST is the language for everything that becomes cumbersome graphically: loops, case distinctions, calculations, string handling, generic library blocks.

// Example: scale an analogue value with range checking
FUNCTION FC_ScaleAnalog : REAL
VAR_INPUT
  iRaw        : INT;    // raw value 0…27648
  rMin, rMax  : REAL;   // target range in engineering units
END_VAR

IF iRaw < 0 THEN
  FC_ScaleAnalog := rMin;          // wire break / underflow
ELSIF iRaw > 27648 THEN
  FC_ScaleAnalog := rMax;          // overflow
ELSE
  FC_ScaleAnalog := rMin + (rMax - rMin) * INT_TO_REAL(iRaw) / 27648.0;
END_IF;

The same functionality in LD would be a multi-rung chain of blocks whose intent only becomes clear on close inspection.

A second, often underrated advantage: ST is text. That makes it meaningfully versionable in Git, reviewable line by line, and searchable with ordinary tools. Graphical languages are stored in binary formats – a diff there shows "block changed" at best.

3.5.SFC – Sequential Function Chart

SFC represents sequences as steps and transitions. The big gain is visibility: the plant's current state can be read directly online instead of being reconstructed from a dozen memory bits.

For step sequences – batch processes, cleaning cycles, operating mode changes – that is the decisive advantage during troubleshooting. For a simple interlock it is overhead.

4.Which language for what: a decision guide

No single language is best at everything. In practice this split has proven itself:

5.Mixing is allowed – and sensible

A widespread misconception is that you have to commit to one language. The opposite is true. The standard explicitly provides for each block being written in the language that suits its task.

A typical project then looks like this:

OB1 (Main)                         → LD, calls only
  ├── FB_ModeControl               → SFC (operating modes as a step chain)
  ├── FB_ConveyorControl           → ST (logic + timing)
  │     └── FC_ScaleAnalog         → ST (calculation)
  ├── FC_SafetyInterlocks          → LD (must be readable without tools)
  └── FB_AlarmHandler              → ST (library block)

What matters is only that the choice is justified and stays consistent across the project. Five languages mixed at random are worse than one language applied consistently.

6.Conclusion

• IEC 61131-3 defines five languages: LD, FBD, IL, ST and SFC. SCL is Siemens' name for ST, not a sixth language.
• IL has been deprecated since 2013 – read it yes, write it new no.
• ST carries most of a modern project, because it allows calculations, libraries and versioning.
• LD stays for safety-relevant and maintenance-critical logic, because readability during a fault decides downtime.
• Mixing languages is standard-compliant and sensible – as long as the choice is justified and consistent.

7.Further reading

• PLC Programming: 8 Best Practices for Clean, Maintainable Code – how the language choice fits an overall structure
• PLC Documentation: What Belongs In It and What Matters at Handover – which language creates which documentation burden
• PLC Troubleshooting: Narrow Down Systematically – why SFC speeds up diagnosis