IEC 61131 vs. IEC 61499: What Is the Real Difference for Automation Engineers?

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In industrial automation, two IEC standards are frequently mentioned: IEC 61131 and IEC 61499.
Engineers encountering them for the first time often ask:

“Aren’t they both PLC standards? What’s the actual difference?”

This article explains the real, practical differences in a way that automation engineers can understand—and confidently explain to customers.

A Simple Analogy (Software Perspective)

If we compare automation to software development:

  • IEC 61131 → Writing a single-machine program in C

  • IEC 61499 → Designing a distributed, service-oriented system (similar to microservices)

Both control machines—but they are built for very different system architectures.

1. Fundamental Design Philosophy

StandardCore ConceptEngineering Implication
IEC 61131Cyclic scan (Scan Cycle)Logic executes repeatedly in a fixed order
IEC 61499Event-driven executionLogic runs when events occur

  • IEC 61131 assumes a single controller executing deterministic cyclic logic.

  • IEC 61499 assumes a distributed system, where logic reacts to events across devices.

2. Programming Model and Structure

IEC 61131: Traditional PLC Programming

IEC 61131 defines five programming languages:

  • LD – Ladder Diagram

  • FBD – Function Block Diagram

  • ST – Structured Text

  • IL – Instruction List

  • SFC – Sequential Function Chart

Typical structure:

Configuration
 └── Task
     └── Program

This model is optimized for single PLC-based control systems and tightly coupled hardware.

IEC 61499: Function Block Network (FBN)

IEC 61499 uses a component-based, distributed model:

Application
 └── Function Block Network (FBN)
     └── Function Blocks (FBs)

Function Blocks are categorized as:

  • BFB – Basic Function Block

  • CFB – Composite Function Block

Each Function Block has:

  • Event interfaces (control execution)

  • Data interfaces (inputs/outputs)

This structure feels much closer to modern software engineering than classic PLC programming.

3. Execution Model

IEC 61131: Cyclic Scan Execution

  1. Read inputs

  2. Execute logic

  3. Write outputs

  4. Repeat next scan cycle

Advantages

  • Highly deterministic

  • Easy to reason about timing

Limitations

  • Poor scalability

  • Inefficient for large, distributed, or dynamic systems

IEC 61499: Event-Driven Execution

  • No fixed scan cycle

  • Function Blocks execute only when triggered by events

Advantages

  • Better real-time responsiveness

  • Naturally suited for distributed systems

  • Easier functional reuse and system expansion

Challenges

  • Higher implementation complexity

  • Steeper learning curve for engineers

4. Deployment Architecture

AspectIEC 61131IEC 61499
DeploymentSingle PLCMultiple distributed devices
System viewController-centricSystem-centric
ScalabilityLimitedDesigned for scale

  • IEC 61131 follows a “one PLC = one program” mindset.

  • IEC 61499 supports distributed deployment across controllers, edge devices, and intelligent nodes.

This aligns naturally with IIoT and Industrial 4.0 architectures.

5. Typical Use Cases for IEC 61499

IEC 61499 is especially suitable for:

  • Smart factories

  • Distributed robotic systems

  • Edge computing + device-level coordination

  • Energy management systems

  • Building automation with multi-device interaction

6. Reusability and Portability

  • IEC 61131

    • Limited portability

    • Strong vendor-specific implementations

  • IEC 61499

    • Designed for cross-device, cross-platform reuse

    • Encourages modular, portable automation software

In simple terms:

  • IEC 61131 → Traditional PLC user programming

  • IEC 61499 → Portable, component-based industrial software

7. Industry Adoption Status

  • IEC 61131

    • Industry standard and mainstream

    • Supported by Siemens, Omron, Mitsubishi, Rockwell, etc.

  • IEC 61499

    • Newer and still evolving

    • Adopted by edge-computing and next-generation automation platforms

    • Examples:

      • Eclipse 4diac (open source)

      • nxtControl

      • Beckhoff TwinCAT (partial support)

Industry Trend:
The market is gradually moving from 61131 toward 61499, but 61131 will not disappear anytime soon.

Final Summary

  • IEC 61131

    • The traditional PLC software standard

    • Best for fixed-function, tightly integrated machines

  • IEC 61499

    • A distributed automation software standard

    • Designed for Industrial Internet, edge control, and multi-device collaboration

They are not competitors, but complementary standards:

IEC 61131 = Single-controller automation
IEC 61499 = Distributed industrial software architecture

In future large-scale systems, both standards will often coexist—each used where it fits best.