Although there are guidelines for many programming languages, such guidelines are nearly non-existent for the important area of industrial control, e.g. IEC 61131-3 and its PLCopen extensions. Nevertheless, the software in the industrial environment is becoming more and more important, the software projects are larger, and the costs of errors increase. Software nowadays absorbs half of the initial project costs, and between 40 and 80% deals with maintenance over the life cycle costs of the software. In order to deal with the complexity of larger programs one needs modern software development processes supporting a structured approach and creating re-usable code. Also needed is increased efficiency in coding via re-use of pre-defined functionalities to help to better understand the program over the life cycle.
PLCopen supports this goal via their Software Construction Guidelines initiative.
Now the first official version of the Coding Guidelines is released (visit www.plcopen.org), containing 63 rules for software programming spread over different sections like: Naming Rules, Comment Rules, Coding Practice, Languages, and even Vendor-Specific IEC 61131-3 Extensions.
These rules are very useful to train users and can be a good basis for universities to help them teach IEC 61131-3 programming more efficiently, as well as to define company-internal guidelines.
Eelco van der Wal, Managing Director PLCopen, describes how this development might affect packaging machinery.
“The application software in a packaging machine spans a whole life cycle. During this life cycle there will be updates and new wishes, making changes and add-ons a certainty. Unfortunately, such changes have to be dealt with by people who did not write the original software. This can be a daunting and expensive task unless there is software methodology, including coding guidelines.
“With the modern functionalities of a packaging machine, the development of the application program is a group process. It includes people with different backgrounds who all have to cooperate through different stages of the process. In order to make this cooperation efficient, a set of rules has to be followed. The PLCopen Coding Guidelines are those rules. The coding guidelines are part of a larger set of guidelines aimed at both the software creation process and at creating a more transparent and reusable application software that can be maintained and updated with much more ease during the life-cycle of the packaging machine. It will be a standard methodology to enhance existing packaging machines in the field, to cope with new materials, products, and sizes. And this all comes down to software.”
Since the release of the first documents, further work was done on the following topics:
• Creating PLCopen compliant Function Block Libraries
• Structuring and decomposition via SFC (do’s & don’ts)
Both areas are close to publishing their first results. The libraries are giving guidance and examples, and templates are provided for the two basic Function Block operation differences: edge-triggered or level-triggered. This basis is extended with time-out functionality and a timer to spread an operation over multiple operating cycles. The included examples show also the basic code, also in the object oriented approach as provided by the 3rd edition of the IEC 61131-3 standard.
Additional working groups active under the Software Construction Guidelines will focus on the following topics:
• Guidance for documentation in software programs
• Software quality issues and software consistency
• Software development process
• Object orientation in the 3rd edition
The results of the different subgroups will be made available on the PLCopen website on an on-going basis.
