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Comparing IEC 61131-3 programming Languages: Which One Should You Use?

IEC 61131-3 is the international standard for programming languages used in industrial automation. If you work with PLCs, DCS, or SCADA systems, you’ve likely come across one or more of these languages. But which one is right for your application?

In this article, we break down the five programming languages defined in IEC 61131-3, compare their features, and help you choose the best one based on your use case.

What Is IEC 61131-3?

IEC 61131-3 is a part of the IEC 61131 standard that defines how programmable logic controllers (PLCs) should be programmed. It introduces a unified model that includes:

  • Data types and structures
  • Program organization units (POUs)
  • Task scheduling
  • Five standardized programming languages

This standard promotes hardware-independence, interoperability, and modularity across automation platforms.

The Five IEC 61131-3 Programming Languages

1. Ladder Diagram (LD)

Best For: Electricians, maintenance techs, traditional relay logic.

Description:
LD is the most visual and widely used language, resembling electrical relay logic diagrams. It’s ideal for digital I/O and boolean control logic.

Strengths:

  • Easy to read
  • Great for discrete processes
  • Broad vendor support

Limitations:

  • Not suitable for complex algorithms or data handling

2. Function Block Diagram (FBD)

Best For: Process engineers, continuous control, analog logic.

Description:
FBD uses blocks (functions) connected by lines to represent signal flow. It’s intuitive for analog control loops and reusable logic.

Strengths:

  • Visual flow of signals
  • Great for PIDs, calculations, timers
  • Encourages modular design

Limitations:

  • Can become messy in large programs
  • Not ideal for state logic or sequential operations

3. Structured Text (ST)

Best For: Software engineers, data handling, complex math.

Description:
ST resembles high-level programming languages like Pascal or C. It’s suitable for tasks that require loops, conditions, and algorithms.

Strengths:

  • Powerful for data manipulation
  • Ideal for batch processing, array handling
  • Easy integration with modern automation software

Limitations:

  • Steeper learning curve
  • Less visual for troubleshooting

4. Instruction List (IL) (Deprecated)

Best For: Low-level control, legacy systems.

Description:
IL is a low-level, assembly-like language. As of IEC 61131-3 (3rd Edition), it’s deprecated but still supported in older systems.

Strengths:

  • Lightweight and compact
  • Easy to execute on limited hardware

Limitations:

  • Hard to read
  • Poor scalability
  • Phased out in most modern systems

5. Sequential Function Chart (SFC)

Best For: Batch processes, step-based control, safety sequences.

Description:
SFC breaks a process into steps and transitions. It’s ideal for state-based logic and recipe-driven processes.

Strengths:

  • Great for sequencing and interlocks
  • Easy to visualize process flow
  • High maintainability

Limitations:

  • Requires integration with ST or LD for detailed logic
  • Can grow complex if misused

Comparison Table – IEC 61131-3 Language Overview

LanguageBest ForVisualComplexityCommon Use Case
LDElectricians, Digital I/O✅ High🟢 LowMotor start/stop logic
FBDAnalog Control, Engineers✅ Medium🟡 MediumPID control loops
STData Processing, Complex Logic❌ None🔴 HighBatch control, calculations
ILLegacy Systems❌ None🔴 HighLegacy machine code logic
SFCSequence Control✅ High🟡 MediumSafety interlocks, batch recipes

How to Choose the Right Language?

✅ Use Ladder Diagram (LD) if:

  • Your background is electrical
  • You need quick troubleshooting
  • The logic is mostly on/off

✅ Use Function Block Diagram (FBD) if:

  • Your application involves analog inputs
  • You’re building control loops
  • You value modular design

✅ Use Structured Text (ST) if:

  • Your task involves calculations or large data sets
  • You’re comfortable with high-level programming
  • You need compact, flexible code

✅ Use Sequential Function Chart (SFC) if:

  • Your process has clear step transitions
  • You work in batch or state-based systems
  • You require strong visualization of states

IEC 61131-3 in Modern Automation Platforms

Modern PLCs and PACs like Siemens, Allen-Bradley, Beckhoff, and Schneider Electric support multiple IEC 61131-3 languages. Platforms like Codesys, TwinCAT, and Studio 5000 let you mix LD, FBD, ST, and SFC in one project.

Using the right language—or a combination—can improve debugging, code reusability, and system performance.

Real-World Example

Use Case: A batch reactor system with:

  • Motor controls
  • Analog temperature loops
  • Sequential step transitions

Solution:

  • LD: For motor start/stop logic
  • FBD: For PID temperature control
  • SFC + ST: For recipe-driven batch steps

This hybrid approach minimizes troubleshooting time while keeping the control logic organized and efficient.

Final Thoughts

IEC 61131-3 isn’t about picking one language—it’s about understanding when to use which.

  • LD and FBD are easy to start with.
  • ST is best for advanced logic.
  • SFC is unmatched in sequencing.

Pro Tip:

When starting a new project, outline your process requirements and pick languages accordingly. A hybrid approach often leads to the best results.

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