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. Several of these inconsistencies strongly prevent one of the most important objectives of PLCopen, namely the language portability to various PLC types. In order to remedy this, 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 migration of large centralized control systems towards distributed systems. IEC 61131-3 exclusively applies to local controllers including their communication interfaces to other local systems, not the programming of distributed systems. That is why the reciprocal integration of the IEC 61131-3 programming languages with IEC 1499 (the new architectural model for distributed industrial process measurement and control systems) is absolutely necessary. Only through this, long-term company investments in control systems can be assured.
This is the reason why IEC 1131 is now being revised in three stages, each summarized below:
1) “Corrigendum”: correction of errors.
2) “Amendment”: more consistent structuring of the standard accompanied by additional implementation of features required in everyday controls situations.
3) Harmonization between IEC 61131-3 and IEC 1499.
In the course of this, the major goal to maintain upward compatibility for all amendments. This means, a control program which complies with the previous standard is also expected to comply with the new standard without conflicts.
Stage 1: Corrigendum
The initial stage exclusively eliminates hard errors and non-compliances within the existing standard. In addition to correcting misprints, this especially resolves semantic discrepancies in the main section and inconsistencies between the main section and appendices. During last four years, the IEC task force collected, evaluated and adapted the relevant modifications. This occurred with the active assistance of numerous experts, especially from the PLCopen group. None of the changes shall cause incompatibility.
Stage 2: Amendment
The second stage extends beyond a pure revision of errors and defines improvements to the standard. These include two main points.
Firstly, the chapter concerning the programming language “Instruction List” has been reworked in order to remove existing inconsistencies.
Secondly, the definition of functions is revised. The possibilities of using parameters with functions are too inflexible in the existing IEC-1131, sometimes resulting in awkward constructions. Moreover, the usage of “Enable Input/Output” has caused substantial confusion. This will be revised in the new release, and a serious stumbling block will have been overcome in the effort toward portable control programs.
A number of less significant amendments still complement these two main points. They are, for the greatest part, upwards compatible with the present standard.
The main objective of these amendments is to increase the acceptance of 1131-3 languages by the application programmers through unified and practice-orientated language constructs. In this relationship, enhanced utilization of the control hardware is to be achieved. In addition, IEC increases actual semantic portability for control programs, eliminating interpretation latitude for particular language constructs.
Stage 3: Harmonization with IEC 1499
The last stage of the intended revision concerns the desired harmonization with the architecture standard IEC 1499 – ”Function Blocks”. This standard for distributed industrial control systems does not define its own specific programming languages for algorithms. The languages contained in IEC 61131-3 are used instead, since they are best suited in modified or enhanced form for many industrial application domains. Again, the most important guideline for developing these revisions even in this case, is, to ensure as much upward compatibility as possible. Harmonization with IEC 1499 will open many opportunities for IEC 1131 and, thus, continue to increase its acceptance.
Current state
The corrections and amendments were reviewed and modified in the first half of 1998 by the international IEC 61131-3 task force. It circulated as Committee Draft in 1999. It collected many comments, and will be circulated as Committee Draft for Vote (CDV). The CDV spans a 5 month period. A revised version is published as Final Draft International Standard, FDIS, for 4 months. If approved, it will be published as International Standard, IS, within 2 month. The total time from CDV till publication takes about 1 year. This looks long compared to de-facto standards, but during half the time the standard should be stable already, and so usable to develop products. Parallel to it, a French edition has to be made and circulated.
Overall, a new edition of this standard has been published as International Standard in 2003.
The third proposed stage is in a much less mature development stage. This planned harmonization work can lead to a third edition of the standard.
To keep them alive, standards are always subject to change and undergo evolutionary changes in regard to technology advancements and market needs. On the other hand, the essentials of a standard must be established on a solid and long-lasting basis. For this reason, caution must always be applied to all exercised amendments: The investments of the industrial end users and hardware and software control vendors are always expected to be the primary concern. The approach described here represents an appropriate compromise to this concern.
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.
