Siemens SITRANS DA400 Operating Instructions Manual page 61

Step 2: Formation of the alarm messages
In accordance with the "Block diagram for the formation of alarm messages" figure, the
measured value is then monitored with a limit monitor with hysteresis for exceeding of a pre-
alarm "PrAL*" (parameters P.5, P.7, P.9, P.11) and a main alarm "AL*" (parameters P.6, P.8,
P.10, P.12).
The resulting signals "PrAL'*" and "AL'*" are used for evaluation by various trace timers. The
final alarms "PrAL*" and "AL*" are generated only when the signals "PrAL'*" and "AL'*" are
present for the configurable time "P.13/Time". The same response time also applies if the
value is below the alarm values. When the main alarm responds, the pre-alarm is also
activated.
Depending on parameter P.14/AcK, the alarms "PrAL*" and "AL*" are saved and can be
reset with the "ACK" button.
In the event of a sensor fault (cable failure), the alarm for the damaged channel will be
generated in addition to the fault summation message depending on configuration (P.1, P.2,
P.3, P.4).
Figure 7-2 Block diagram for formation of the alarm messages
SITRANS DA400 for material flow monitoring with protection against explosion
Operating Instructions, 09/2007, A5E00841152-01
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Signal processing
7.1 Evaluating acoustic channels
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