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16
Logical Combination of Alarms
Two alarms may be combined logically to create an AND/OR situation. Any suitable output
may be assigned as a Logical Alarm Output, configured for Reverse-acting or Direct action.
Also refer to Alarm Operation
Direct Acting
OFF
ON
OFF
ON
Direct Acting
OFF
ON
OFF
ON
Loop Alarm Enable
Enables or disables a loop alarm. A loop alarm is a special alarm, which detects faults in the
control feedback loop, by continuously monitoring process variable response to the control
output(s). The loop alarm can be tied to any suitable output. When enabled, the loop alarm
repeatedly checks if the control output(s) are at the maximum or minimum limit. If an output
is at the limit, an internal timer is started: thereafter, if the high output has not caused the
process variable to be corrected by a predetermined amount 'V' after time 'T' has elapsed,
the loop alarm becomes active. Subsequently, the loop alarm mode repeatedly checks the
process variable and the control output(s). When the process variable starts to change value
in the correct sense or when the output is no longer at the limit, the loop alarm is deactivated.
For PID control, the loop alarm time 'T' is always twice the Automatic Reset parameter value.
For On-Off control, a user defined value for the Loop Alarm Time parameter is used.
The value of 'V' is dependent upon the input type. For Temperature inputs, V = 2°C or 3°F.
For Linear inputs, V = 10 least significant display units
Control output limits are 0% for Single output (Primary only) controllers and -100% for Dual
output (Primary and Secondary) controllers.
Correct operation of the loop alarm depends upon reasonably accurate PID tuning. The loop
alarm is automatically disabled during manual control mode and during execution of the Pre-
Tune mode. Upon exit from manual mode or after completion of the Pre-Tune routine, the
loop alarm is automatically re-enabled.
Display code = +5.% ,default value = D<:5 ,
Also refer to Loop Alarm Time, Manual Mode, On-Off Control, Pre-Tune, and Process
Variable.
59305, Issue 5 – March 2005
- DIN Controllers & Indicators - Product Manual
Table 37. Logical Alarm Outputs
Logical OR: Alarm 1 OR Alarm 2
OFF
OFF
ON
ON
Logical AND: Alarm 1 AND Alarm 2
OFF
OFF
ON
ON
OFF
OFF
ON
ON
ON
OFF
ON
ON
OFF
OFF
OFF
ON
OFF
OFF
ON
ON
Glossary
Type: General Definition
Reverse-Acting
OFF
OFF
ON
ON
Reverse-Acting
OFF
OFF
ON
ON
Type: Controller Parameter
ON
OFF
OFF
OFF
ON
ON
ON
OFF
Page 115
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