PLC Programming Languages 2026: IEC 61131-3 4th Edition Field Guide

PLC Engineering Notes · IEC 61131-3 · 2026 Update

By KOEED Editorial · 2026-06-30 · 12 min read · Engineering Notes

Programmable controllers still speak the five IEC 61131-3 languages engineers learned two decades ago — but the 2024 4th edition, ratified and entering deployment in 2025–2026, has quietly changed the rules around namespaces, POUs, and how ladder logic co-exists with object-oriented structured text. This engineering note walks through each language, what actually changed since 2024, and how to pick the right one on a modern Allen-Bradley, Siemens, Mitsubishi, Omron, or Schneider line.

AI Summary · Key Takeaways

• IEC 61131-3 4th edition (2024, deployment 2025–2026) keeps the five core languages but expands POU/namespace handling and harmonizes object-orientation in Structured Text.
• Ladder Diagram (LD) and Function Block Diagram (FBD) remain the dominant field languages on Allen-Bradley ControlLogix, Siemens S7-1500, Mitsubishi iQ-R, Omron NX/NJ, and Schneider Modicon M580.
• Instruction List (IL) was deprecated in the 3rd edition and is removed in deployment guides — ST and LD have replaced it on every major platform.
• Sequential Function Chart (SFC) is still the cleanest way to model batch, state-machine, and machine-step control on lines with 8 or more process steps.
• Choosing a language in 2026 means matching the language to the maintainer, not the controller — most lines now run a mixed ST + LD project.

Pick the IEC 61131-3 language by who has to maintain the line in five years, not by what the programmer likes today — ladder for the technician on shift, structured text for the algorithm, SFC for the batch process.

What is a PLC programming language in 2026?

A PLC programming language is a vendor-neutral formalism defined by IEC 61131-3 that maps discrete, analog, and motion I/O onto a deterministic scan cycle. The 2024-published 4th edition of the standard keeps the five languages you already know — Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC), and the formally deprecated Instruction List (IL) — but tightens the rules on Program Organization Units (POUs), namespaces, and how object-oriented extensions behave in ST. In plain shop-floor terms: the languages are the same, the way you structure a project is not.

The reason KOEED still sees the same five languages on every RFQ — whether the customer is replacing an Allen-Bradley 1747-ASB, a Siemens 6ES7 315-2EH14, or a Mitsubishi Q02HCPU — is that IEC 61131-3 is the lingua franca every controller family compiles down to. Even vendor-proprietary editors like Studio 5000, TIA Portal, GX Works3, and Sysmac Studio emit IEC 61131-3-compliant POUs under the hood.

The five IEC 61131-3 languages, still in use in 2026

Here is what each language does today, where it fits, and the platforms where KOEED customers most often see it.

1. Ladder Diagram (LD)

Ladder is still the default language on discrete-control lines. It models relay logic as horizontal "rungs" between two vertical power rails — the same visual metaphor electricians have used since the 1970s. A maintenance technician who can read a wiring diagram can read a ladder rung. On a 2026 line that means Studio 5000 on Allen-Bradley ControlLogix 1756-L73 / L74 / L85, TIA Portal on Siemens S7-1500, GX Works3 on Mitsubishi iQ-R, and Sysmac Studio on Omron NX/NJ — all of them open with a ladder editor by default.

LD is the right call when:

• The control logic is boolean-first (start/stop, permissives, interlocks, e-stops).
• The on-shift troubleshooter is an electrician, not a programmer.
• The plant already has a maintenance team trained on ladder from a previous generation (SLC 500, S7-300, MELSEC-A, SYSMAC CS1).

2. Function Block Diagram (FBD)

FBD is the signal-flow cousin of ladder. Instead of relay rungs, you wire boxes — each block is a function (PID, scale, analog-in, comparator) and lines carry signals between them. FBD is the cleanest way to model a process loop with analog I/O, scaling, and alarm limits, and it is the language most engineers reach for on a Schneider Modicon M580 or on the analog-heavy channels of a Siemens ET200SP. FBD also plays well with object-oriented extensions in the 4th edition: each block can be a POU with its own namespace and methods.

3. Structured Text (ST)

ST is the high-level text language of IEC 61131-3. Its syntax deliberately echoes Pascal and C: IF / THEN / ELSE / CASE / FOR / WHILE. ST is where the heavy math lives — recipes, motion profiles, energy optimization, statistical process control, anything that would be painful to express as rungs. In 2026 most greenfield lines carry 60–80% of their code in ladder and 20–40% in ST, with ST taking the data-handling and algorithm burden.

A small ST snippet — scaling a 4–20 mA input on a 6ES7 analog module:

// TIA Portal SCL — scale a 4-20 mA AI to engineering units
IF AI_Raw > 27648 THEN AI_Raw := 27648; END_IF;
IF AI_Raw < 0      THEN AI_Raw := 0;      END_IF;
AI_EU := (AI_Raw / 27648.0) * (EU_MAX - EU_MIN) + EU_MIN;

4. Instruction List (IL) — deprecated, but still on legacy lines

IL was the assembly-language cousin of the family in the 2nd edition, and it was formally deprecated in the 3rd edition (2013). The 4th edition (2024) does not bring it back. Every major IDE — Studio 5000, TIA Portal, GX Works3, Sysmac Studio, EcoStruxure Control Expert — has dropped the IL editor from its default install. IL still ships inside the firmware of older controllers (SLC 500, S7-300, MELSEC-A, SYSMAC CS1) and KOEED still sources those modules, but any new project should be written in ST or LD instead.

5. Sequential Function Chart (SFC)

SFC is the only one of the five that is not really a language in the traditional sense — it is a state-machine editor that calls POUs written in LD, FBD, or ST as actions and transitions. Each step and transition is its own named block, which makes SFC the cleanest tool for batch, recipe, and machine-step control. If your line has eight or more named process steps (CIP clean-in-place, sterilization, fill, cap, label, pack, palletize, dispatch), draw it in SFC first, then implement the actions in ST.

What changed since 2024

The original article on this topic was published in March 2024, just before IEC TC 65/SC 65B finalized the 4th edition. Three years later, the things that matter on the shop floor:

• POU namespacing is now mandatory. Every POU lives in a namespace, which means Line3.Conveyor.Motor_Start rather than Motor_Start. The standard finally caught up to what Studio 5000 and TIA Portal had already been doing.
• Object-oriented ST extensions are harmonized. Methods, interfaces, and inheritance now look the same across vendors, which makes porting a Structured Text library between Allen-Bradley, Siemens, and Schneider much less painful than it was in 2024.
• IL is officially out. No vendor's 2026 IDE installs an IL editor by default. The 4th edition leaves IL in the standard for reference but every vendor migration guide has been updated to direct customers to ST.
• OPC UA and MQTT publish/subscribe are now part of the standard's data-exchange layer. A 2026 PLC project can expose its tags as an OPC UA server without a separate gateway, on platforms like the S7-1500, ControlLogix v33+, and the iQ-R.
• Functional safety languages (IEC 61131-3 / IEC 61508 alignment) have clearer guidance on which languages are allowed inside SIL 2 / SIL 3 POUs — ST and FBD are preferred, SFC is allowed for the orchestration layer, IL is excluded.

None of those changes alter the five-language lineup. They tighten the rules around how you structure the POUs those languages live inside.

How to pick a language for a 2026 project

Choosing a programming language for a programmable controller is not a religious question. Match the language to the maintainer, the safety class, and the I/O mix — not to the controller on the bench today.

If your line looks like…	Pick	Typical platform
Discrete I/O, electricians on shift, conveyor / e-stop / permissives	Ladder Diagram (LD)	Allen-Bradley 1756-L73, Siemens S7-1500, Mitsubishi iQ-R
Analog loops, PID, scaling, alarm management	Function Block Diagram (FBD)	Schneider M580, Siemens ET200SP, Omron NX
Recipes, math, motion profiles, energy optimization	Structured Text (ST)	Any modern controller (ST is universal)
Batch / sequence / 8+ named process steps	Sequential Function Chart (SFC)	Mitsubishi iQ-R, Siemens S7-1500T, Allen-Bradley
Legacy 1990s / 2000s IL project on a replacement controller	Convert IL to ST, then keep LD for I/O	SLC 500 → CompactLogix, S7-300 → S7-1500

Choosing a controller to run those languages on

The five IEC 61131-3 languages run on every controller KOEED stocks. When a customer RFQ comes in with a language requirement, the matching decisions are usually about which controller family and which firmware revision is on the bench:

• Allen-Bradley ControlLogix (1756-L73 / L74 / L85, Studio 5000 v33+) — full LD / FBD / ST / SFC; IL editor removed.
• Siemens S7-1500 (6ES7 5xx, TIA Portal V18+) — LAD / FBD / SCL (ST) / GRAPH (SFC); STL (IL) hidden behind an optional install.
• Mitsubishi iQ-R (R04CPU / R16CPU, GX Works3) — LD / FBD / ST / SFC; IL still in the editor for legacy imports only.
• Omron NX / NJ (Sysmac Studio) — LD / FBD / ST / SFC; IL not supported.
• Schneider Modicon M580 (EcoStruxure Control Expert) — LD / FBD / ST / SFC; IL deprecated.

If you need a particular CPU module to host a particular language on a hard-to-find part number, send the BOM to Moritta@KOEED.COM and the team will cross-check active stock, EOL alternatives, and current firmware.

Programming software in 2026

Every vendor's IDE has tightened up around the 4th edition. Studio 5000 v33 dropped the IL editor entirely and added namespace enforcement. TIA Portal V18 enforced POU-level versioning. GX Works3 added multi-language import (LD ↔ ST) for the iQ-R. Sysmac Studio and EcoStruxure Control Expert followed suit. The practical implication: a language you wrote in 2022 still compiles in 2026, but the project structure is no longer optional — you have to use namespaces, and you have to declare your POUs before you call them.

Related on the KOEED blog

• Daily PLC News — vendor firmware updates and language editor changes.
• Siemens S7-1500 / S7-300 stock — for ST and SFC projects.
• Allen-Bradley ControlLogix & CompactLogix stock — for ladder + ST mixed projects.
• PLC Error Code Database — language-editor fault codes by vendor.
• AI Diagnostic Tool — describe your line and the controller, get a language recommendation.

Frequently asked questions

Is Instruction List still supported in 2026?

No. IL was deprecated in IEC 61131-3 3rd edition (2013) and is not installed by default in any 2026 vendor IDE. Convert legacy IL blocks into Structured Text before porting to a current controller.

Which IEC 61131-3 language should I learn first?

Ladder Diagram if you maintain discrete lines, Structured Text if you write algorithms. Most working PLC programmers carry both. SFC is a separate skill worth learning once you touch batch or recipe control.

Can I mix languages in one PLC project?

Yes — this is the standard pattern in 2026. A typical project uses ladder for I/O and permissives, ST for math and recipes, and SFC for the top-level state machine. Each POU is one language, called by another.

What changed in IEC 61131-3 4th edition (2024)?

Mandatory POU namespacing, harmonized object-oriented Structured Text extensions, official removal of IL, OPC UA / MQTT in the data layer, and clearer IEC 61508 alignment for safety.

Do all PLC brands support the same five languages?

Yes — every KOEED-stocked brand (Allen-Bradley, Siemens, Mitsubishi, Omron, Schneider, plus Yaskawa, Panasonic, KEYENCE controllers) supports LD, FBD, ST, and SFC. IL is the only one with brand-level variation, and it is fading out everywhere.

How do I migrate a legacy IL project to a 2026 controller?

Convert each IL instruction block into a named Structured Text function first, then bring it into the new project. For SLC 500 → CompactLogix or S7-300 → S7-1500 moves, send the source part numbers to Moritta@KOEED.COM and KOEED will quote the replacement controller and the migration CPU in one BOM.