The MonitoredItem Service Setis used by the Clientto create and maintain MonitoredItems. MonitoredItemsmonitor Variables, Attributesand EventNotifiers. They generate Notificationswhen they detect certain conditions. They monitor Variablesfor a change in value or status; Attributesfor a change in value; and EventNotifiersfor newly generated Alarmand Eventreports.

Each MonitoredItemidentifies the item to monitor and the Subscriptionto use to periodically publish Notificationsto the Client(see 7.11). Each MonitoredItemalso specifies the rate at which the item is to be monitored (sampled) and, for Variablesand EventNotifiers, the filter criteria used to determine when a Notificationis to be generated. Filter criteria for Attributesare specified by their Attributedefinitions in OPC 10000-4.

The sample rate defined for a MonitoredItemmay be faster than the publishing rate of the Subscription. For this reason, the MonitoredItemmay be configured to either queue all Notifications or to queue only the latest Notificationfor transfer by the Subscription. In this latter case, the queue size is one.

MonitoredItem Servicesalso 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 Serversamples the item. In addition, each sample is evaluated to determine if a Notificationshould be generated. If so, the Notificationis queued. If reporting is enabled, the queue is made available to the Subscriptionfor transfer.

Finally, MonitoredItemscan 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 Notificationsof the items to report are made available to the Subscriptionfor transfer.