The MonitoredItem Service Set is used by the Client to create and maintain MonitoredItems. MonitoredItems monitor Variables, Attributes and EventNotifiers. They generate Notifications when they detect certain conditions. They monitor Variables for a change in value or status; Attributes for a change in value; and EventNotifiers for newly generated Alarm and Event reports.
Each MonitoredItem identifies the item to monitor and the Subscription to use to periodically publish Notifications to the Client (see 7.11). Each MonitoredItem also specifies the rate at which the item is to be monitored (sampled) and, for Variables and EventNotifiers, the filter criteria used to determine when a Notification is to be generated. Filter criteria for Attributes are specified by their Attribute definitions in OPC 10000-4.
The sample rate defined for a MonitoredItem may be faster than the publishing rate of the Subscription. For this reason, the MonitoredItem may be configured to either queue all Notifications or to queue only the latest Notification for transfer by the Subscription. In this latter case, the queue size is one.
MonitoredItem Services also define a monitoring mode. The monitoring mode is configured to disable sampling and reporting, to enable sampling only, or to enable both sampling and reporting. When sampling is enabled, the Server samples the item. In addition, each sample is evaluated to determine if a Notification should be generated. If so, the Notification is queued. If reporting is enabled, the queue is made available to the Subscription for transfer.
Finally, MonitoredItems can be configured to trigger the reporting of other MonitoredItems. In this case, the monitoring mode of the items to report is typically set to sampling only, and when the triggering item generates a Notification, any queued Notifications of the items to report are made available to the Subscription for transfer.