Event-based alarm model

For event-oriented alarms (as in OPC), an acknowledgment is accepted only if it refers to the most recent activation event.

An alarm instance begins waiting for an acknowledgement when it first enters the alarmed state. If the instance is acknowledged and subsequently transitions to a new alarmed sub-state, it begins waiting for another acknowledgement. Each subsequent transition is assigned a sequence number, and the acknowledgement must have attached to it the sequence number of the transition to which it is responding.

The acknowledgement is accepted only if it is for the most recent transition. If it is accepted, it applies to all transactions of the alarm that have occurred so far. The alarm is considered acknowledged when the most recent instance is acknowledged.

A rejected acknowledgement may be logged for diagnostic purposes, but is not otherwise tracked in the system.

The event-oriented model ensures that if an alarm is changing between different states, the acknowledgement corresponds to current information. In systems with small latency times, this may look just like condition-oriented alarms. In other environments, such as the Internet, the features of this model may become important.