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PLCopen adds independent schemes to IEC 61131-3


 
Opening up the development environments
by specifying XML formats for IEC 61131-3

Background information

Since the release of the IEC 61131-3 programming standard, users want to be able to exchange their programs, libraries and projects between development environments. Although this was not the intent of the standard itself, it was a task that the independent organization PLCopen committed itself to. IEC 61131-3 is focused on the software development environment. As such it is just a part of a total solution. The other parts are a structure of tools like: network tools, debug tools, simulators, documentation tools, etc. Therefore PLCopen has decided a few years ago to realize interfaces towards these support tools. This resulted in a workgroup named TC6 for XML (eXtended Markup Language). This committee defined an open interface between all different kinds of software tools, which provides the ability to transfer the information that is on the screen to other platforms. This screen information does not only contain textual information, but also graphical information, like where the blocks are and how they are connected.

User Pespective

PLCopen wants to accomplice that, without much additional effort, a project can be transferred from one development environment to another, without loosing information, even when it is incomplete, e.g. not compilable without errors. This of course is also valid for the POUs, and especially for User Derived Function Block libraries. This means that the program itself has to remain the same, so not be altered during this transfer. The big variety of possibilities has to be brought under one umbrella. And XML provides the right technology for this. The result of the PLCopen XML work opens the development environment by providing interfaces to other software tools. As such it will be more than an export / import tool from one development environment to another. From the moment onwards that this format is available, it is just a small step to feed for instance a documentation tool with the information. Actually, it is not important where this XML-code is coming from as long as it is recognizable and useable. So it could be generated by other tools like simulation and modeling tools, and consumed by verification, documentation, and version control tools. In principle all relevant information will be exported. The importing tool has to be intelligent in filtering which parts of this information is useful and needs to be imported and used. With this PLCopen creates a complete new market, in which the focus is on reusability of software developments up to whole projects.

Short introduction into XML

XML stands for extended Markup Language, providing the basis for the well known HTML (Hyper Text Markup Language) which is used extensively on the internet.
XML has several advantages:
  1. It is extendable
  2. The data included can be checked for consistency with the scheme provided
  3. Different schemes provide a possibility to check the incompatibilities
The W3C consortium calls XML "a common syntax for expressing structure in data." Structured data refers to data that is tagged for its content, meaning, or use. For example, whereas the <H1> tag in HTML specifies text to be presented in a certain typeface and weight, an XML tag would explicitly identify the kind of information: <BYLINE> tags might identify the author of a document, <PRICE> tags could contain an item's cost in an inventory list--all the way down to <DOGFOODBRAND> if that's the level of detail required. By separating structure and content from presentation, the same XML source document can be written once, then displayed in a variety of ways: on a computer monitor, within a cellular-phone display, translated into voice on a device for the blind, and so forth. It'll work on any communications devices that might be developed; an XML document can thus outlive the particular authoring and display technologies available when it was written. Check www.xml.org for more information.

PLCopen Mission

The work as done by the PLCopen Technical Committee 6 fits well into the PLCopen mission: We want to be the leading association resolving topics
related to control programming
to support the use of international standards in this field.

Programming in its environment

As mentioned before, the programming tools are just a part of the whole suite of software tools that can be used to make an application work. The results of the PLCopen XML work opens the development environment by providing interfaces to other software tools. In the figure below, one can identify the interfaces at the end of the blue circle.



Definition of use cases

The participants identified 4 areas where the specification fits ideal:
  1. Exchange format between all five languages;
  2. Interface to producers of graphical and logical information, like modeling tools;
  3. Interface to consumer of graphical and logical information, like documentation generator and management; and version control;
  4. Distribution format for function block libraries.
Presented in a graphical overview, this looks like:

What is included

With the scope of whole projects, a lot of information needs to be stored in the XML format. And since we do not know upfront which tool will be the consumer of the information, we have to export all potential relevant information. These include items like:

Textual Programming Languages – IL and ST
Graphical Programming Languages – LD, FBD
Structural Language – SFC
Graphical Information, like place and position, and routing of connections
Comments
Program Organization Units – (User Derived) Functions and Function Blocks, Programs
(User Derived) Datatypes
Project information (layered structure)
Mapping information
Supplier specific information

This means that the exported file contains ‘everything’, including supplier specific information. The consumer side, with the parsing function during input, must contain the intelligence to select the relevant information, and to check on the consistency of this data. For this the tool can use a different scheme. Combining both schemes provide the possibility to check the incompatibilities, and get the relevant information more easily. This clearly shows the benefits of using XML schemes.

Status of the PLCopen TC6 – XML work

PLCopen published he XML schema and documentation in June 2005 as version 1.01. After that, feedback came in from companies starting to implement this specification, not only from Europe but also from Japan.

With a new initiative called AutomationML we have come to a new phase. The companies Daimler, ABB, KUKA, Rockwell, Siemens, netAllied and Zühlke together with the University of Karlsruhe and the University of Magdeburg jointly define and standardize the Automation Markup Language (AutomationML™) as an intermediate format for the Digital Factory. For more information check www.automationML.org.

One of the formats that have been accepted in this group is PLCopen XML for the sequencing. In order to optimize this interface additional changes to the specification were proposed.
These proposals resulted in a new release of the PLCopen XML schema, version 2.0, in December 2008. This schema also includes previous feedback on this schema from other parties involved. With this release an optimization between the simulation and realization is created, creating more efficiency in automation. 

PLCopen members using this technology can be easily identified by the following logo: