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Client v 1.1
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Creating PLCopen
Compliant Libraries
v0.99 RfC

Safe Motion v0.99
Release for Comments


Extensive report on
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Status of the Standard

Enhanced IEC 61131-3: Proven standard fit for future

The international standard "IEC 61131-3" was released in 1993 and, since its adoption, has become widely accepted by the international user and vendor community. Today, it is, as such, the worldwide recognized standard for programming and configuring industrial control devices. 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 must be revised: First of all, since 1993 a great deal of practical experience has been gained in which a number of inconsistencies and contradictions have been detected. These include 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, incl. 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 TC4). 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 61131-3:2003 (= second edition). 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.
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 61131-3 became IEC 61131-3, without any changes to the standard itself. Moreover, during the current transition phase, you have to order the IEC 6-1131 standard to get a publication that clearly has on its front cover ‘IEC 61131-3’. As this might be confusing to non-insiders, we decided to wait for a new edition of the standard to migrate to the new number. In this way it is coupled to change. How to order the Standard ?