IEC 61131-3 PLC Programming in 2026: ST, FBD, LD, SFC, IL, CFC Compared

PLC Fundamentals · Engineering Notes · Updated 2026-06-30

By KOEED Engineering Team · 2026-06-30 · 8 min read · Engineering Notes

If you maintain or specify PLC code, IEC 61131-3 still defines the lingua franca. The standard's five normative languages — Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC) — cover almost every controller you will see on a plant floor, and PLCopen's Continuous Function Chart (CFC) adds a sixth for continuous processes. This 2026 refresh walks through each language, what changed since the 2024 edition of this article, and which language to reach for in a modern brownfield retrofit on Allen-Bradley ControlLogix, Siemens SIMATIC, or Mitsubishi MELSEC iQ-R hardware.

> AI Summary

• IEC 61131-3 defines five normative PLC programming languages — LD, FBD, ST, IL, SFC — with CFC as a PLCopen extension for continuous control.
• The third edition (2013) deprecated IL, and the fourth edition (IS 2025) reinforced the recommendation to use ST instead; no new project should start in IL in 2026.
• ST is now the default high-level language, with object-oriented features (methods, namespaces, structured types) refined in the 4th edition.
• LD and FBD remain dominant for discrete I/O and process loops; SFC structures batch, mode-of-operation, and any state-driven sequence.
• Vendor IDEs (Studio 5000, TIA Portal V20, GX Works3, Sysmac Studio) now treat ST as a first-class citizen with full OOP, refactoring, and OPC UA companion-spec support.

In 2026 the six IEC 61131-3 languages still cover the same control problems, but the recommended mix has shifted: ST for math, LD for wiring, FBD for loops, SFC for sequences, with IL on legacy support only.

What changed since 2024

Two industry shifts matter if you learned these languages before 2024:

1. IL is fully deprecated. The third edition (IEC 61131-3:2013) already marked Instruction List as legacy, and mainstream IDEs have stopped generating IL by default. Treat existing IL as maintenance-only and port to ST on the next service window.
2. The fourth edition of IEC 61131-3 was published as an International Standard in 2025, adding timestamp and security annexes, tightening the OOP extensions in ST, and aligning with the OPC UA companion specification. The five normative languages are unchanged, but the "ST first, LD for wiring, FBD for loops" workflow is now the vendor consensus.
3. Vendor tooling consolidated around ST. Studio 5000 v36+, TIA Portal V20, GX Works3, and Sysmac Studio now expose ST with proper OOP, namespaces, and refactoring tools on par with C# or Java. SFC gained clearer branch and step-action semantics in the 4th edition, which removes a long-standing source of confusion for new users.

The six languages, 2026 perspective

1. Ladder Diagram (LD)

Ladder remains the workhorse for discrete I/O. Electricians and maintenance technicians read it without a programmer's eye, which is why it stays mandatory in brownfield work. In 2026, ladder supports inline ST expressions inside contacts and coils, so a complex interlock can be a single rung with a Boolean ST call rather than ten rungs of ANDs. Use it for motor starters, valve manifolds, safety circuits, anything a wireman will fault-find at 02:00 with a printout. See the live stock list for Allen-Bradley and Mitsubishi CPUs that run LD natively.

2. Function Block Diagram (FBD)

FBD is the language of choice for process loops, drives, and any code that benefits from visual data flow. Each block has named inputs and outputs and a defined function, which makes FBD programs self-documenting. In 2026, vendor libraries ship thousands of certified blocks (PID, motor, filter, signal conditioning) that you wire rather than write, so a closed-loop PID becomes five connected blocks instead of a 200-line ST routine. For Schneider M580 and ABB AC500 deployments, browse Schneider modules ready for FBD-based libraries.

3. Structured Text (ST)

ST is now the default for everything that isn't a wire diagram. It looks like Pascal or a subset of C, with IF / CASE / FOR / WHILE, functions, function blocks, methods, namespaces, and structured data types. The third edition (2013) added the OOP extensions; the fourth edition (IS 2025) refined them. If you need math, parsing, recipe management, MQTT payloads, or any algorithmic logic, write it in ST.

// IEC 61131-3 Structured Text: OOP method on a function block (edition 3+)
FUNCTION_BLOCK PneumaticCylinder
  VAR
    ExtendOK  : BOOL;
    RetractOK : BOOL;
    Position  : REAL := 0.0;
  END_VAR
  METHOD MoveTo : BOOL
    VAR_INPUT
      Target : REAL;
    END_VAR
    Position := Target;
    MoveTo := (Position = Target) AND ExtendOK AND RetractOK;
  END_METHOD
END_FUNCTION_BLOCK

4. Sequential Function Chart (SFC)

SFC models state machines: steps, transitions, actions. It is the right tool for batch processes, mode-of-operation logic, and any sequence with a finite number of named states. The fourth edition (2025) tightened the rules around simultaneous and alternative branches, which used to trip up new users. For a CIP clean-in-place sequence or a palletizer mode change, SFC is faster to draw and easier to validate than the equivalent ST.

5. Instruction List (IL)

IL is a low-level, accumulator-based language that resembles assembly. It was deprecated in the third edition (2013) and is no longer taught in vendor training. Most 2026 IDEs still accept IL for legacy loading, but new code should be written in ST. If you are maintaining a system with significant IL — common on 1990s Siemens S5 or Omron CV-series lines — plan a phased port to ST rather than a big-bang rewrite.

6. Continuous Function Chart (CFC)

CFC is a PLCopen-defined extension of FBD, intended for continuous processes where execution order matters. Unlike FBD, you can place feedback loops without artificial delays, and you set explicit execution priorities on each block. CFC is common in chemical, water/wastewater, and HVAC control. It is not part of the IEC 61131-3 normative list, but every major IDE supports it.

Comparison at a glance (2026)

A practical rule of thumb for 2026

For a new project, start with three languages: LD for the wiring sheet, FBD for loops and drives, and ST for everything else. Add SFC the moment your state count exceeds four or five. Treat IL as a read-only artifact of legacy code. Reach for CFC only when you genuinely need explicit execution order on a continuous process — rare outside of chemical and primary-metals plants.

Tools that pair with the languages

Once you have the right language picked, the next decision is which controller and IDE to run it on. KOEED's AI Diagnostic Tool can identify a nameplate module and recommend a modern equivalent in seconds, and the PLC Error Code Database covers fault codes from Siemens, Mitsubishi, and Omron. For runtime math, the Hex to Float Converter handles IEEE 754 swaps that bite every Modbus project.

Related on KOEED Blog

• Daily PLC News — firmware releases and EOL notices across the 9 brands KOEED supports.
• Engineering Notes — long-form technical deep dives for maintenance engineers.
• FAQs — selection, sourcing, and cross-reference questions answered by the engineering team.
• Allen-Bradley ControlLogix modules — current and legacy stock for the 1756 / 1769 / 1747 families.

Frequently asked questions

Is Instruction List (IL) still supported in 2026?

Yes, for legacy loading only. The third edition of IEC 61131-3 (2013) deprecated IL, and the fourth edition (IS 2025) reinforced that recommendation. Major IDEs no longer generate new IL. Treat existing IL as maintenance code and port to Structured Text on the next service window.

Which language should I learn first in 2026?

Start with Structured Text (ST). It is text-based, the default in TIA Portal V20, Studio 5000 v36+, GX Works3, and Sysmac Studio, and it covers roughly 70% of real control code. Add Ladder second for I/O wiring and SFC third for state-driven sequences.

What changed in the IEC 61131-3 fourth edition?

The fourth edition (IS 2025) keeps the same five normative languages but tightens the OOP extensions in ST, clarifies SFC branch and step-action rules, adds a security annex covering signed code and role-based access, and aligns with the OPC UA companion specification. Existing programs remain compatible.

Is CFC part of the IEC 61131-3 standard?

No. Continuous Function Chart is a PLCopen specification that extends FBD with explicit execution order and feedback paths. It is widely supported by Codesys-based IDEs, EcoStruxure Control Expert, and SIMATIC PCS 7, but it is not normative in IEC 61131-3.

Can I mix ladder, FBD, and ST in one project?

Yes, and you should. All major IDEs allow mixing per routine. A common 2026 pattern is Ladder for I/O wiring, FBD for the process loop, and Structured Text for recipe and alarm logic, all inside the same project.

Where can I get the standards document?

IEC 61131-3 is sold by IEC and national bodies (ANSI, BSI, DIN). PLCopen publishes free technical reports and coding guidelines online. For module-level error lookup, see the PLC Error Code Database on KOEED.