CODESYS OPC UA PubSub SL Article no. 2311000001
Communication via the OPC UA Pub/Sub protocol provides, in addition to the Client/Server communication, also a possibility to exchange data between the subscribers of a network in compliance with the rules of the OPC UA Foundation. The structure of the data (DataSet) can be freely defined and is agreed in advance between the sender and the receiver. This eliminates the need to transport additional (meta)data. The data transfer takes place with the help of the OPC UA PubSub SL library via UDP/IP according to the rules defined for UADP. A Publisher publishes its data to an unknown number of Subscribers. So the sender and receiver do not know each other. That is why the number of receivers also does not have any repercussions for the sender. A reply to the sender as to whether its messages have reached the receivers cannot be sent via the protocol due to the nature of the protocol. If necessary, such a reply has to be managed in an application-specific way. The data is transferred in a binary format according to the rules of the OPC UA Foundation. The OPC UA PubSub SL library takes over the conversion of IEC data types into the corresponding OPC UA data types and back. The implementation thereby follows the profiles below:

Publisher: PubSub Publisher UADP Periodic Fixed Settings

Subscriber: PubSub Subscriber UADP Periodic Fixed Settings

The message length is limited to 1500 bytes (MTU) (Chunked NetworkMessages not supported). As long as the rules for a Time Sensitive Network is still not available, hard real-time conditions cannot be complied with. However, the implementation of the OPC UA PubSub SL library attempts to keep the jitter as low as possible.