IEC 61131-3 Explained

IEC 61131-3 is the international standard that defines the programming languages used for PLCs (Programmable Logic Controllers) and industrial controllers. It gives engineers a common, vendor-neutral foundation for writing control software.

The languages

  • Ladder Diagram (LD) — graphical, relay-logic style; familiar to electricians and widely used for discrete logic.
  • Function Block Diagram (FBD) — blocks wired together; well suited to signal processing and process control.
  • Structured Text (ST) — a high-level textual language, ideal for calculations, loops and complex algorithms.
  • Sequential Function Chart (SFC) — for structuring step-by-step sequences and state machines.
  • Instruction List (IL) — a low-level textual language, now deprecated in the latest editions.

Why it matters

Code written to IEC 61131-3 is more portable, more maintainable and easier to hand over between teams, because it follows a recognised structure rather than one engineer’s personal style. It also makes it easier to move between platforms such as CODESYS, Siemens, Beckhoff and OMRON.

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