As defined in OPC 10000-14, a WriterGroup defines the Publisher of a NetworkMessage. It is recommended that the PubSubConfiguration contains a WriterGroup for each instance of PubSubCommunicationFlowConfigurationType. For the example illustrated in Figure E.5, a WriterGroup in AC_A is generated based on the parameters in Flow1, and a WriterGroup is generated for each AC_B and AC_C based on the parameters in Flow2.
Figure E.20 illustrates the relation between the PubSubCommunicationFlowConfiguration and the PubSubConnection and WriterGroup within the PubSubConfiguration. If any changes are made to the PubSubCommunicationFlowConfiguration, the related PubSubConnection information, as illustrated in Figure E.20, will be updated.
Depending on the transport profile and on the type of Address (unicast or multicast), the Address will be in the PubSubConnection or in the WriterGroup’s TransportSettings; see OPC 10000-14 for details. As an example, if UDP-UADP is used as the transport profile, a unicast address is set in the WriterGroup, but a multicast address is set in the PubSubConnection.
TransportProfileUri is used to set up the information in the PubSubConnection, whereas HeaderLayoutUri, PublishingInterval, Qos, SecurityMode and SecurityGroupId are used to set up the information in the WriterGroup.
The PubSubConnection elements ConnectionProperties and TransportSettings are transport specific. For the TransportProfileUri “http://opcfoundation.org/UA-Profile/Transport/pubsub-udp-uadp” (UDP-UADP), both are set to null.
The WriterGroup elements GroupProperties and TransportSettings are transport specific. For UDP-UADP, GroupProperties is set to null. For UDP-UADP, the elements MessageDelay, MessageRepeatCount, Address, QosCategory, and DatagramQos are defined in TransportSettings. The value of MessageDelay and MessageRepeatCount depends on the header layout. For HeaderLayoutUri. For “http://opcfoundation.org/UA/PubSub-Layouts/UADP-Periodic-Fixed” (UADP-Periodic-Fixed), both are set to 0. QosCategory and DatagramQos are derived from Qos.
The WriterGroup element MessageSettings is transport-specific. For UDP-UADP, the GroupVersion is defined by the generator of the ConnectionConfigurationSet. To ensure a consistent definition of the GroupVersion, it is recommended that the WriterGroup is configured completely within one ConnectionConfigurationSet. NetworkMessageContentMask and DataSetOrdering are derived from the HeaderLayoutUri. PublishingOffset and SamplingOffset may be set to a negative value, meaning not used. They may be used when additional Quality of Services are specified.
For MaxNetworkMessageSize, follow the recommendations in OPC 10000-14. For UADP-PeriodicFixed, KeepAliveTime should be set to PublishingInterval. LocaleIds may be set to a value different from null if the translation of Strings will be done by the Publisher. Priority may be used to set the relative priority of the WriterGroup for processing it.