Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 83
FactoryTalk Alarms and Events Logix-based Instructions (ALMD, ALMA) Chapter 1
Use care when determining where to place alarm instructions in the application.
The accuracy of the timestamps are affected by how quickly the instruction is
scanned after the alarm condition changes state. MinDuration time accumulation
and Rate of Change calculations require repeated scanning, within time intervals
determined by the user application. Alarm instructions must continue to be
scanned after the alarm condition becomes false, so that the ReturnToNormal
transition may be detected. For example, if you desire 10 ms accuracy on
timestamps, you could place the alarm instructions that need that resolution in a
10 ms periodic task.
Controller Memory Use
As a guideline, use the following alarm sizes for a rough calculation of controller
memory usage:
· Typically 1 KB per digital alarm with no associated tags
Digital Alarm Example Approximate Size
Digital alarm with no associated tags and this configuration:
· Alarm message: Contactor Fault
· Alarm Class: Tank Farm A
1012 bytes
Digital alarm with two associated tags and this configuration:
· Alarm message: Contactor Fault
· Alarm Class: Tank Farm A
· Associated Tag 1 = DINT data type
· Associated Tag 2 = DINT data type
1100 bytes
Digital alarm with two associated tags and this configuration:
· Alarm message: Contactor Fault
· Alarm Class: Tank Farm A
· Associated Tag 1 = DINT data type
· Associated Tag 2 = STRING data type
1522 bytes
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