Subscription Management

Plant SCADA's OPCUA driver uses the subscription model described in 5.13.1 of the OPC UA standard to receive value updates from an OPC UA server.

The driver can create up to three OPC UA subscriptions per I/O device with different publishing intervals to provide granular handling of Plant SCADA subscription update rates.

The publishing interval of OPC UA subscriptions can be set by using the ini parameters Subscription1PublishingInterval, Subscription2PublishingInterval, Subscription3PublishingInterval (see Subscription Parameters).

The default value of Subscription1PublishingInterval parameter is set to 400 msec. The default values of Subscription2PublishingInterval and Subscription3PublishingInterval are set to 0, which means that by default the driver will only create one OPC UA subscription with a publishing interval specified by Subscription1PublishingInterval parameter.

The user can change Subscription2PublishingInterval and Subscription3PublishingInterval parameters to enable OPC UA subscriptions with these publishing intervals.

By default, the driver sets the OPC UA subscription sampling interval to half the length of the associated publishing interval. You can also set the sampling interval to a specific value using the INI parameters Subscription1SampleRate, Subscription2SampleRate and Subscription3SampleRate.

The driver adds monitored items for all I/O Device tags to each of the enabled OPC UA subscriptions. The monitoring mode for all items is set to DISABLED. When the driver gets a subscribe request, it will define the OPC UA subscription in which the item should be enabled based on the Plant SCADA subscription update rate and then it will change the item monitoring mode in that subscription to REPORTING.

If the Plant SCADA subscription update rate changes, the driver will re-evaluate the corresponding OPC UA subscription for the item and change it if required.