Subsequent Alarms Are Not Registered in SystemLink

Issue Details

If I set an alarm in SystemLink for a tag parameter - say, alarm should be registered if a > 50.

I have noticed that once a > 50 is satisfied, unless the alarm generated is acknowledged and cleared, new alarms are not generated for subsequent situations where a > 50

Solution

This is expected behavior. The life-cycle of an instance of an alarm begins the first time it is triggered (a > 50), and ends when it has been both acknowledged and cleared. If the condition is triggered again after that point, a new alarm instance will be created.

This behavior is laid out in ISA 18.2, which describes an industry standard state machine for alarms. Please find the state transition diagram below


Alarm instances also have a transition list which records the history of SET and CLEAR events for a given instance of an alarm. For example, if 'a' passes below 50 and then rises above 50 again, the alarm instance will have two new transitions: a CLEAR transition for when it passed below 50, and a SET transition for when it rose above 50 again.

Note that the alarm service only persists the most recent 100 transitions for each instance of an alarm.