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IEC 61131-3

Structured Text and SFC simulator for IEC 61131-3 practice

IEC 61131-3 is the international standard for PLC programming languages. EDWartens Lab lets you practise three of them, ladder, Structured Text and Sequential Function Chart, in one free browser studio, each running against a simulated machine.

Open the lab freeSee the whole lab

Free with a free EDWartens account. Runs in your browser. The lab is in beta.

PLC lab Structured Text editor with a three-line motor program running beside the live tag table and the 2D plant
A Structured Text program, live

Three IEC 61131-3 languages in one studio

Ladder suits relay-style logic. Structured Text is a text language that reads a little like Pascal, and it suits arithmetic, decisions and anything that would take twenty rungs. Sequential Function Chart draws a process as steps and transitions, which fits a machine that moves through states such as idle, running and tripped.

In the lab you can choose the language for each job. The Structured Text editor shows a short program running beside the live tag table and the plant. The SFC editor highlights the active step as the machine moves from one to the next.

PLC lab Sequential Function Chart editor with Idle, Running and Tripped steps and the active step highlighted
A sequential function chart, running

One set of tests, whichever language you write

Your program is judged by the same numbered acceptance tests whichever language you use. That makes it a fair way to compare them. Write the pedestrian crossing as an SFC and the steps map onto the light sequence almost one for one. Write the tank pump in Structured Text and the dry-run protection becomes a few clear lines.

The tag table, the I/O list and the plant model are shared, so switching language does not mean relearning the machine.

PLC lab with the ladder editor showing energised green rungs beside the running 3D pump plant and simulator
Ladder logic running next to the plant

What the simulator covers and what it does not

The lab implements the three languages above. It does not cover every part of the standard, and it uses its own engine rather than a vendor runtime, so treat it as practice for the thinking and the structure of IEC 61131-3 programs. For full-featured development environments, tools such as CODESYS are worth learning once you have the basics.

How it works

  1. Open a PLC lab and choose ladder, Structured Text or SFC.
  2. Write the program from the brief and the I/O list.
  3. Run it and watch the tag table, the active step or the energised rungs.
  4. Try the same job in a second language, then run the FAT.

Labs to open for this

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Questions people ask

Is there a free Structured Text simulator online?

Yes. EDWartens Lab includes a Structured Text editor that runs your program against a live plant model in the browser. It is free with a free EDWartens account, and nothing needs to be installed.

What is IEC 61131-3?

IEC 61131-3 is the international standard that defines PLC programming languages: ladder diagram, function block diagram, structured text, instruction list and sequential function chart. EDWartens Lab practises ladder, structured text and SFC.

What is an SFC in PLC programming?

Sequential Function Chart (SFC) describes a process as steps and transitions. Only the active step runs, and a transition condition moves the process to the next step. It suits machines that run in clear stages, such as a crossing or a batch.

Which should I learn first: ladder or Structured Text?

Most learners start with ladder because it resembles relay circuits, then add Structured Text for calculations and complex decisions. Try the same lab job in both, so you can compare them directly.

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