Protocol overview
OPC UA Pub/Sub defines different configuration parameters for the various components. They define the behavior of Publisher and Subscriber. Configuration of these parameters can be performed through the OPC UA Information Model for PubSub or through vendor-specific mechanisms, which is, in case of the product TF6105 TC3 OPC UA Pub/Sub, the TwinCAT XAE environment.
The following diagram depicts the different configuration components and their relation to each other. The WriterGroup, DataSetWriter and PublishedDataSet components define the publisher whereas the ReaderGroup, DataSetReader and SubscribedDataSet define the subscriber.
For more detailed information about the individual components, we recommend to consult the OPC UA specification, part 14 (Pub/Sub).
When using the TwinCAT XAE environment to set up the configuration, the engineering workflow can be visualized as follows:
Step | Description |
|---|---|
1 | A new device is added to the TwinCAT I/O configuration. |
2 | From the list of devices, you can then select the “Real-Time OPC UA Device”, which will be the entry point for all OPC UA Pub/Sub settings. |
3 | Publishers, subscribers, datasets and variables are added as well as their corresponding parameters. |
The following chapters provide an overview of the different components and link to more detailed documentation articles.
Device
The OPC UA RT (Realtime) Device is the entry point of the OPC UA Pub/Sub configuration and will be linked to a network interface card based on the TwinCAT Realtime-Ethernet Adapter driver. The device may contain one or more publishers and/or subscribers as well as (global) data sets.
Publisher/Subscriber
A Publisher or Subscriber node defines whether the connected data sets are either send (published) or received (subscribed). Each Publisher/Subscriber node contains address information for the transport protocol that should be used.
Data set
Data sets can be specified in two different scopes. You can either define a data set locally on a Publisher or Subscriber or globally on the device. Global data sets can be shared by multiple Publishers or Subscribers. There are two different types of data sets which define the way how variables are created and handled:
- Data set with variables: this data set type can be configured with variables that appear in the process image of the data set. The variables can then be linked to other process image variables.
- Data set without variables: this data set type can be configured with variables via the TwinCAT Target Browser. The variables are then linked internally automatically and do not appear on the process image.
For more information, please visit our documentation article about data sets.
Variables
Variables are added to the data set as so-called “data set fields”. They can either be linked to other process image variables or are connected directly to other variables via the TwinCAT Target Browser. See our documentation article about data sets for more information.