Rockwell Automation FactoryTalk Alarms and Events Quick Start - Manual (Page 20)

FactoryTalk Alarms and Events System Configuration Guide
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Tag-based alarm monitoring works like this (similar to HMI Tag Alarm
Monitoring):
This approach has several disadvantages:
„ Programming is required in both the controller and the HMI software or Tag
Alarm and Event Server.
„ Tags must be duplicated in the HMI server and mapped to the controller. For Tag
Alarm and Event Servers, controller tags must be mapped to alarms — in either
case, a tedious, error-prone process.
„ Alarms are detected and processed twice, first in the controller logic and then
again in the HMI software or Tag Alarm and Event Server.
„ Polling between the HMI server or Tag Alarm and Event Server and controller
tags increases network overhead.
„ Alarm time stamps are delayed because they are applied by the HMI server or Tag
Alarm and Event Server after polling and processing, rather than immediately
when they occur. Time stamps are not synchronized among multiple alarm
servers.
„ Alarm acknowledge and enable states are held in the computer, and not in the
controller. If the computer goes down, alarm state information is lost.
About device-based alarm monitoring
With device-based alarm monitoring, alarm detection also occurs in the controller, but
unlike software-based alarm monitoring, monitoring for alarm conditions is done in
the controller as well.