How to use PubSub in Codesys?

Structure of a publisher's data transmission

© Kempten University of Applied Sciences

Codesys offers an object-oriented IEC library for implementing PubSub, which can be integrated as standard IEC code in any IEC programs on almost any Codesys-based controllers. To do this, the PubSub library must first be downloaded from the Codesys store and then integrated 'as normal' within the library manager. For test purposes, the library can be used in demo mode for 30 minutes - without any content restrictions - before the associated program has to be restarted. For permanent use of PubSub, a license is required, which must be purchased for each controller that is to use PubSub and installed on the respective device via the license manager.

The cost of a license is 50 euros (as of 02.12.2020). The PubSub library from Codesys contains all the necessary components to create publishers or subscribers. The structure for both variants is very similar: The code for data transmission consists of five function blocks that are to be interconnected.
The UADP.Connection block establishes the connection and therefore requires the most complex parameterization. In the UADP.WriterGroup block, several DataSet messages are combined into one network message. This is provided with additional parameters, such as a time stamp, in the UADP.Writer.

The project tree for publisher and subscriber communication

© Kempten University of Applied Sciences

For the subscriber, only the Writer Group and the Writer module are replaced by a Reader Group and a Reader module and the parameterization of the Connection module is adapted.
The assignment of data to a topic is carried out using freely selectable IDs in unsigned integer format instead of clear topic names. To transfer the data to the desired controllers, the IP addresses and ports of all participating devices are entered. A multicast IP can be selected for data transfer to several controllers.

A significant difference to known transmission protocols such as TCP/IP lies in the preparation of the data: All variables must be referenced and assigned in a data set function block. The latter is derived from a DataSet function block offered in the library according to the concepts of object-oriented programming and thus 'inherits' a certain, necessary basic structure. This provides for the references to the variables to be transferred to be stored in an array of the block. The PrepareValues method is available for the corresponding registration of the variables.

To ensure that publishers and subscribers belonging together use the same DataSet module, a version code is generated in the Init method, which must be identical for all devices involved in the communication. It should be updated or regenerated if the DataSet module content changes. If the version codes of a publisher and a subscriber differ, no data transfer can take place between them.

The PubSub implementation

Prof. Dr. rer. nat. Josef Griesbauer is Professor of Automation and Measurement Technology at Kempten University of Applied Sciences.

© Kempten University of Applied Sciences

With OPC UA PubSub, Codesys offers an entry-level system for data transmission systems that is also suitable for beginners with a little training. However, prior knowledge of object-oriented programming is required. If this is not available, this is reflected in the project planning time for the initial project. For advanced programmers with experience in object-oriented programming, it is quite possible to understand the requirements and implement them in a basic project within a few hours. For beginners, on the other hand, this can take several days. The example project provided in the library, which contains a basic structure for publishers and subscribers, is always helpful. The professional, thoroughly object-oriented implementation of this sample project can be challenging and sometimes time-consuming for newcomers in this field.

Florian Kast is a non-scientific assistant (former Master's student) at Kempten University of Applied Sciences.

© Kempten University of Applied Sciences

The naming of the internal variables of the library's function blocks takes some getting used to. In the Connection module, for example, it is not always clear at first glance which IP address must be assigned to which variable. In this case, the assigned comments are helpful.
It is advisable to create and test the communication separately, as the library uses the NBS namespace, which is also used for data transmission via TCP/IP in Codesys. This can lead to overlapping and therefore errors if PubSub and TCP/IP are used at the same time.

If the communication structure is faulty during the actual test of a PubSub project, this can be easily recognized by the PubSub modules used. These have three output variables that indicate the status, the activity and an error status. It should be noted here that an error status in the subscriber does not necessarily have to originate in the subscriber program. For example, another incorrectly parameterized publisher, which itself reports correct activity, can lead to connection problems in a subscriber. However, testing each transmission separately can prevent misinterpretation of a communication problem.

A summary

The ability to easily and quickly transfer data between a publisher and any number of subscribers makes OPC UA PubSub a powerful tool. Especially when it comes to realizing projects that aim to implement the Internet of Things in industry.

The first-time use of object orientation certainly entails difficulties when implementing a Codesys PubSub project. The slightly higher and object-oriented programming effort in the first project can be compensated for in subsequent projects thanks to the good reusability of the code. In conclusion, it should be noted: At the moment, there is no alternative to implementing PubSub in Codesys and: it works!