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Technical Fundamentals

Technical fundamentals of binary data encoding and header layout

Machine-to-machine communication is the main area of application of the CODESYS library OPC UA PubSub SL. The OPC Foundation provides the "UADP-Periodic-Fixed" profile in [A6] C.2 for the use case of real-time-oriented cyclic data transmission.

. Publisher

  • PubSub Publisher UADP Periodic Fixed Settings

  • The publisher supports the encoding of NetworkMessages and DataSetMessages messages with UADP Message headers for periodic data with a fixed layout.

. Subscriber

  • PubSub Subscriber UADP Periodic Fixed Settings

  • The subscriber supports the decoding of NetworkMessages and DataSetMessages messages with UADP Message headers for periodic data with a fixed layout.

Because space and transmission time are limited, data encoding and transmission via UADP must be as efficient as possible. The aim is to create a configuration of the pub/sub system which meets the demands for efficient transmission. This means as little metadata as possible and as much user data per network message as possible. These aims can be achieved very well when the specifications from [A6] C.2 are implemented.

The OPC Foundation requires the following properties for a pub/sub implementation, which follow the UADP-Periodic-Fixed profile:

  • OPC UA Binary Data Encoding

  • Fixed message layout

  • UADP-Periodic-Fixed Header Layout [A6] C.2

  • Publishers and subscribers are expected to know in advance the structures which they support.

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