Status IEC 61131-3 standard

The international standard "IEC 61131-3" was originally released in 1993 and, since its adoption, has become widely accepted by the international user and vendor community. Today, it is the worldwide recognized standard for programming and configuring industrial control devices.

Enhanced IEC 61131-3: Proven standard fit for future

The popularity of IEC 61131-3 in the industry is evident. The world’s leading industrial control manufacturers have adopted the programming model for a wide range of controllers; various software companies offer compliant development tools; and the standard is certainly the world’s leading paradigm for industrial control languages. The success of PLCopen and its increasing number of members prove this too.

Why then revise a successful standard ?

There are, however, several reasons why the standard had to be revised: First of all, since 1993 a great deal of practical experience was gained in which a number of inconsistencies and contradictions were detected. These included inconsistent definitions and features which are needlessly complicated. Also many users of the standard proposed revisions and enhancements. These can be found in the Addendum and in the Corrigendum as belonging to the standard. In addition, the demands on industrial control systems and their engineering environments have considerably changed over the years, where the most important item is the acceptance of object oriented programming (OOP) in the diverse areas of the modern world, including applications ("apps") on mobile phones and tabs. With a shortage of engineers, it is absolutely necessary to adopt to their world, and so include the aspects of object oriented programming in IEC 61131-3, including namespaces to fit areas like OPC UA (see Communication). Only through this, long-term company investments in control systems can be assured.

The 3rd Edition - list of changes

This standard is fully compatible with IEC 61131-3 : 2002 (= second edition). The third edition of 2012 has a long list of changes and enhancements with probably the object oriented extensions as major enhancement. These include Class, Methods and Interfaces, as well as Namespaces.

The Annex B of the 3rd edition of the IEC 61131-3 list the following changes and extensions:

  • Editorial improvements: Structure, numbering, order, wording, examples, feature tables
  • Terms and definitions like class, method, reference, signature
  • Compliance table format

New major features

  • Data types with explicit layout
  • Type with named values
  • Elementary data types
  • Reference, functions and operations with reference; Validate
  • Partial access to ANY_BIT
  • Variable-length ARRAY
  • Initial value assignment
  • Type conversion rules: Implicit – explicit
  • Function – call rules, without function result
  • Type conversion functions of numerical, bitwise Data, etc.
  • Functions of concatenate and split of time and date
  • Class, including method, interface, etc.
  • Object-oriented FB, including method, interface, etc.
  • Namespaces
  • Structured Text: CONTINUE, etc.
  • Ladder Diagram: Contacts for compare (typed and overloaded)
  • ANNEX A - Formal specification of language elements

Deletions (of informative parts)

  • ANNEX - Examples
  • ANNEX - Interoperability with IEC 61499


  • Octal literal
  • Use of directly represented variables in the body of POUs and methods
  • Overloaded truncation TRUNC
  • Instruction list (IL)
  • “Indicator” variable of action block

What about this ‘6’ in IEC 1131 ?

The International Electrotechnical Commission, IEC, is a world wide standardization body. Nearly all countries over the world have their own, national standardization bodies. In Germany for instance this is the Deutsche Elektrotechnischen Kommission, DKE. These commissions have agreed to accept the IEC approved and published standards. At local publication, often after translation, the standard was published under a local number. This local number often had no match to the number of the IEC published standard. For a standardization body this looked awkward. To harmonize this, they searched for a world wide numbering system that was available to use. This is where the famous ‘6’ came in. And so IEC 1131-3 became IEC 61131-3, without any changes to the standard itself.

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