OPC Unified Architecture Data Access Client communication protocol

Protocol specification
Basic concepts
Initiation of communication
Communication line configuration
Protocol configuration on the communication line
Protocol configuration on the communication station
I/O tag configuration
Browser dialog window
Tell commands
Literature
Changes and modifications
Document revisions

...

The monitored items should be then inserted to these subscriptions. They correspond with the instances of I/O tags, which contain the parameters defined in the address dialog window.

From this moment, the server informs the client about the changes on the monitored objects by

Kotva
publish_message publish_message

"Publish message" in the periodic intervals (that have been set in the parameters of subscription). If the monitored objects have not been changed, the server will send "Publish message" once in (Max KeepAlive Count * Publishing Interval) seconds. The message informs the client that the subscription is still active. A similar check mechanism is also on the client side - it will send "Publish message" once every (Max KeepAlive Count * Publishing Interval). If the client does not send the acknowledgement "Publish message" within (LifeTime Count * Publishing Interval), the subscription will expire on the server side.

...