TechBlog

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 <