Ladder, FBD, ST, SFC: 4 PLC Languages Engineers Still Use in 2026

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

By KOEED Engineering Team · 2026-06-30 · 7 min read · Engineering Notes

The four core PLC programming languages — Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), and Sequential Function Chart (SFC) — still cover almost every controller KOEED ships. They are the four normative languages from IEC 61131-3 that every modern vendor IDE (Studio 5000 v36+, TIA Portal V20, GX Works3, Sysmac Studio) implements, plus a handful of vendor-specific dialects that are still useful to recognize on legacy and brownfield lines. This 2026 update walks through what each language is for, what changed since the 2024 edition of this article, and which one to reach for on a Allen-Bradley ControlLogix 5580, Siemens S7-1500 / S7-1200 G2, or Mitsubishi MELSEC iQ-R retrofit.

What changed since 2024

Two shifts are worth noting if you learned these languages before 2024:

1. ST became the default in vendor IDEs. Studio 5000 v36+, TIA Portal V20, GX Works3, and Sysmac Studio now expose Structured Text with full OOP, namespaces, and refactoring tools on par with C# or Java. SFC gained clearer branch and step-action semantics in the 4th edition of IEC 61131-3 (IS 2025), which removes a long-standing source of confusion.
2. Vendor-specific dialects are now clearly branded. Allen-Bradley calls its ladder dialect Ladder Diagram (LD) and supports the same IEC 61131-3 constructs in Studio 5000. Siemens S7-SCL is essentially ST with Siemens extensions. Mitsubishi's GX Works3 ST, Omron's ST on Sysmac Studio, and Schneider's ST in EcoStruxure Control Expert all follow the IEC 61131-3 base — code ports with a renaming pass.

The four core PLC programming languages, 2026 perspective

1. Ladder Diagram (LD / LAD)

Ladder Logic, also written as LAD in Siemens and AB literature, is the most widely used of the four. It is a graphical language designed to resemble the electrical schematic of a relay panel, with two vertical rails (power) and horizontal rungs (logic). Contacts represent inputs, coils represent outputs, timers and counters are standard blocks, and a 2026 IDE lets you drop an inline ST expression inside a contact or coil when the Boolean logic gets dense.

LD is the right choice for applications dominated by discrete logic: on/off control, motor starters, valve manifolds, and any safety circuit a wireman will fault-find at 02:00 with a printout. Modern ladder rungs can call ST functions and FBD blocks, so the discipline of "keep the wiring sheet in LD" survives in 2026. For the current Allen-Bradley and Mitsubishi line-up, see Allen-Bradley ControlLogix and Mitsubishi MELSEC iQ-R CPU stock.

2. Function Block Diagram (FBD)

FBD is a graphical, data-flow language: blocks with named inputs and outputs, wired together to form the program. Each block is a function (PID, scaling, motor, filter) and the wiring shows how data flows between them. FBD is the language of choice for process loops, drives, and any code where you want the program to be self-documenting on a single screen.

In 2026, vendor libraries ship thousands of certified FBD blocks (PID, motor, filter, signal conditioning, OPC UA client), so a closed-loop PID becomes five connected blocks instead of a 200-line ST routine. FBD is also the natural target for Schneider Modicon M580 process work and for EcoStruxure Control Expert, where the FBD library is one of the most complete in the industry.

3. Structured Text (ST)

Structured Text, often called SCL on Siemens platforms, is a text-based, high-level language that looks like Pascal or a constrained C. It has IF / CASE / FOR / WHILE, functions, function blocks, methods, namespaces, and structured data types, and is now the default for everything that is not a wire diagram. The third edition of IEC 61131-3 (2013) added OOP extensions; the fourth edition (IS 2025) refined them.

ST is the right choice for math, parsing, recipe management, MQTT payloads, alarm engines, and any algorithmic logic. The snippet below is a typical 2026 ST method on a function block, the kind you would write once and reuse across multiple Omron NX/NJ or Allen-Bradley ControlLogix projects.

// Structured Text: OOP method on a function block (IEC 61131-3 ed. 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 a program as steps, transitions, and actions — a state machine in graphical form. Each step does work; each transition waits for a Boolean condition; actions attached to a step run while the step is active. SFC is the right tool for batch processes, mode-of-operation logic, and any sequence with a finite number of named states.

The 4th edition of IEC 61131-3 (IS 2025) tightened the rules around simultaneous and alternative branches, which used to trip up new users. For a CIP clean-in-place sequence, a palletizer mode change, or a mill rewind cycle, SFC is faster to draw and easier to validate than the equivalent ST.

Vendor-specific dialects worth knowing in 2026

Choosing the right language in 2026

A practical rule of thumb 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. All four are now first-class citizens in Studio 5000 v36+, TIA Portal V20, GX Works3, and Sysmac Studio, so mixing per routine is the standard 2026 workflow, not a workaround.

For runtime math, recipe work, and Modbus commissioning, the Hex to Float Converter and Modbus CRC Calculator pair well with ST routines. For module identification and EOL cross-reference, the AI Diagnostic Tool reads a nameplate and returns a modern equivalent.

Related on KOEED Blog

• Engineering Notes — long-form technical deep dives for maintenance engineers.
• Daily PLC News — firmware releases and EOL notices across the 9 brands KOEED supports.
• FAQs — selection, sourcing, and cross-reference questions answered by the engineering team.
• Siemens SIMATIC modules — S7-1500 / S7-1200 G2 / ET200 stock.