Micromod MOD: 1800P - MOD 30ML Identity Module (Version 2) Algorithms, Tables and Sequential Logic Functions - Manual (Page 52)

Logic Functions - Book 2
PROCESS ALARM BLOCK
8-44
Figure 8-23. Process Alarm Block (PA), General Menu
8.6.1 Process Alarm Block Capabilities
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 8-24, 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.
The following table shows how the alarm calculation is done for the six 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