Rockwell Automation FactoryTalk View Site Edition Users Guide - Manual (Page 265)

12 • S ETTING UP F ACTORY T ALK ALARMS
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In the following illustration, an increasing limit of 4000 rpm, with a deadband value of
500 rpm, means that the rpm has to fall to 3500 and then rise above 4000, before the rpm
value will trigger an alarm again.
A deadband range can only be an absolute (constant) value. If a buffer is not required, the
deadband must be set up as zero.
With a deadband of zero, alarms will be triggered as soon as the tag value crosses any of
its limits.
For information about how deadbands work with deviation alarms, see the FactoryTalk
Alarms and Events Help.
Deviation alarms
A deviation alarm compares the value of an input tag to the value of a target for a
deviation value.
The target can be either a constant or a tag, but the deviation value can only be an absolute
value (a constant, not a tag value).
If the target differs from the input tag by greater or less than the deviation value, an alarm
occurs.
Use the deadband carefully in safety-critical applications. In the preceding illustration, for
example, the variable has to fluctuate by more than 500 rpm before an alarm is triggered again.
5000
4000
3500
3000
2000
1000
0
In alarm
Return to normal
In alarm
again
rpm
Low limit
Time
Deadband
n
High High limit
High limit
Low Low limit
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