Micromod MOD: 30ML Functions Data Base Reference - Manual (Page 58)

MOD 30ML Functions
PROCESS ALARM DISPLAY BLOCK
2.6.1 Process Alarm Block Operation
Active alarms are determined by comparing the value of the alarm source with the trip value.
The order of comparison is: Alarm Input value, Trip Condition, Trip Value. If the result of the
comparison is true, the alarm is active. Otherwise, the alarm is clear. Using the example
shown in Figure 2-20, let the trip value be 750 and the trip condition be greater than or equal.
While the alarm source value is less than 750, the alarm is clear, but when its value is greater
than or equal to 750, the alarm becomes active. The alarm becomes clear again when it falls
below 750 minus any hysteresis value.
HYSTERESIS (a floating-point value) is used to determine when an active floating point
alarm becomes clear. A floating-point value often represents some process variable. Since
there is often noise associated with a process variable, hysteresis is used to suppress this
noise. In other words, the use of hysteresis prevents the alarm state from rapidly fluctuating
between active and clear.
Version 2 of this block adds the following features. When the TRIP CONDITION is
DEVIATION, the target value is always zero. The TRIP VALUE, HYSTERESIS and HI
RANGE values are symmetrical. For example, if the TRIP VALUE is 5.0 it is also –5.0; if the
HYSTERESIS is 1.0, it is also –1.0; and if the HIRANGE is 25.0 which indicates the TOP of
the bar graph, then the bottom of the bar graph will indicate –25.0. The following
configuration rules apply.
• Used with floating point alarms only
• TRIP VALUE must be a positive value
• HYSTERESIS must be a positive value and less than TRIP VALUE
• BAR FORMAT must be DEV or NONE
• HIRANGE must be a positive value and greater than TRIP VALUE
• LORANGE is not used
The following shows how the alarm calculation is done for the seven configured trip
conditions (when alarm becomes active). Note that hysteresis is not used when the trip
condition is EQUAL or NOT EQUAL. Thus the use of these two trip conditions with floating
point variables may lead to rapid fluctuations as mentioned above.
Condition
When alarm becomes active When active alarm becomes clear
LESS ALARMINP < TRIPVAL ............. ALARMINP >= TRIPVAL + HYST
LESS EQUAL ALARMINP <= TRIPVAL ........... ALARMINP > TRIPVAL + HYST
GREATER ALARMINP > TRIPVAL ............. ALARMINP <= TRIPVAL – HYST
GREATER EQUAL ALARMINP >= TRIPVAL ........... ALARMINP < TRIPVAL – HYST
EQUAL
ALARMINP == TRIPVAL ........... ALARMINP != TRIPVAL
NOT EQUAL ALARMINP != TRIPVAL ............ ALARMINP == TRIPVAL
DEVIATION ALARMINP > HITRIPVAL.......... ALARMINP <= HITRIPVAL – HYST
DEVIATION ALARMINP < LOTRIPVAL......... ALARMINP >= LOTRIPVAL + HYST
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