Time-stamped digital and time-stamped analog alarms differ to other alarm types, as they do not rely on the polling of variables to determine alarm conditions. They operate via a process where the Alarm Server is notified of any value changes to a specified variable using the Cicode function AlarmNotifyVarChange.
The Alarm Server will then use this information to update the alarms that monitor the variable. This process allows for an accurate timestamp to be associated with an alarm condition.
This process is used to update the Variable Tag and Setpoint properties for time-stamped analog alarms.
Events trends can be used in conjunction with time-stamped analog alarms to provide millisecond accuracy for both trend and alarm data. See TrnEventSetTableand TrnEventSetTableMS.
To configure an analog time-stamped alarm:
Note: For time-stamped analog alarms to function correctly, you need to configure the Cicode function AlarmNotifyVarChange, as it notifies the alarm server of any value changes for the associated variable. It also passes the timestamp with the notification message. The AlarmServer maintains a queue for notification messages and runs a separate task to check this queue.