Figure 1: Data can be published via symbol configuration using the OPC UA server.

© Codesys

The original aim of OPC and OPC UA is to make controller data accessible to other participants. This is done via a corresponding server, either as additional software for a controller or - in the case of Codesys-compatible controllers - integrated in the PLC runtime system. If a user wants to make the current PLC values known to other devices, he inserts the 'Symbol configuration' object in the Codesys project and specifies which variables are to be published. Publishing can be done object by object, in data structures or individually. If required, the data can be structured in different symbol groups and the access rights of individual variables or complete groups can be defined in fine granularity. A valid X.509 certificate is also required on the controller for secure communication. It can either be issued by a certification authority and imported to the control unit or created directly in the device itself.

OPC UA Client - various use cases

Figure 3: In the data source manager, the OPC UA client enables the insertion and use of PLC data from a connected controller.

© Codesys

The client is the remote station to the server: It searches the data structures provided and enables their use. In the past, as today, machines and systems are visualized using the PLC data via the client. With the visualization integrated in Codesys, users create suitable user interfaces in the development system parallel to the development of the PLC application - or completely independently of it. Almost all users use such visualizations to operate machine units, systems or as a convenient tool for debugging or commissioning the application on compatible controllers. By integrating an OPC UA client into the visualization runtime under Windows and Linux, such user interfaces can be used to visualize any controllers with OPC UA servers. To do this, the visualization user connects the data source manager in the project with the server URL of the OPC UA server on the controller. The client displays the published data and enables visualization or animation.

Figure 4: Convenient data exchange through paired use of OPC UA server and client.

© Codesys

In addition to the original use case of visualization, OPC UA communication via server client is suitable for data exchange between devices. The data source manager in Codesys can also be integrated into "normal" PLC applications in the same way. Once the connection has been established from the client to a controller with OPC UA Server, its variables are available in the development system in exactly the same way as if they had been declared locally in the project - regardless of the manufacturer of the connected controller. For standardized data exchange, it is therefore advisable to implement OPC UA server and client functionality in pairs on suitable controllers.

The data source manager is automatically based on a library whose blocks can also be called and used directly to implement the client connection within the application. This gives experienced developers of PLC applications additional degrees of freedom when implementing their logic functions.

OPC UA PubSub - for real-time communication

Figure 5: Calling the PubSub communication modules for Writer and Reader.

© Codesys

As convenient as client-server communication is, there is a catch: the server in particular requires a considerable amount of CPU power and memory. This is not a problem for most of the now "very well motorized" industrial controllers with Windows or Linux as the operating system. However, if data exchange is to be realized with low-cost field devices, this protocol is no longer suitable. To remedy this, the OPC Foundation has defined PubSub, a lean alternative that enables communication via different protocols: Codesys users simply use an available library for communication. To send and receive messages, the object-oriented blocks are called cyclically within the PLC application. Data is currently exchanged on the basis of UDP (User Datagram Protocol). However, the library is designed in such a way that time-synchronized communication protocols can be added at any time. The future implementation of a real-time-capable OPC UA-based fieldbus system is thus prepared. This makes OPC UA PubSub suitable not only as an alternative for communication between less performant devices, but also in future for data exchange between controllers in real time via protocols such as TSN or even with cloud systems.

The author: Roland Wagner is Head of Product Marketing at the Codesys Group.

© Codesys

Codesys users can therefore use the OPC variant best suited to their application. This is possible thanks to the seamless integration into the IEC 61131-3 development system. After a simple configuration, integrated data is available just as if it had been declared in the local controller. Whether OPC UA Server, OPC UA Client, OPC UA PubSub or all three variants simultaneously on a compatible device.