Siemens SITRANS TW 7NG3242 Betriebsanleitung Seite 110

Functions / Operation by HART
4.2.16.1 Message relay
The message relay can monitor different limit values and error states with simple limit
value components. The components are switched on or off in SIMATIC PDM or in
the HART communicator in the Limit value mode menu item. The following limit value
components can be parameterized in all combinations.
a) Sensor error detection (break and / or short-circuit)
b) Monitoring of hardware and firmware errors
c) Monitoring of lower limit value
d) Monitoring of upper limit value
e) Monitoring of analog output for saturation
A switching delay tv of the message relay can be parameterized for parameterization
of the limit value components. Exceeding / dropping below a limit value causes a
delay until the alarm is triggered. There is no switching delay if the limit value is
dropped below of / exceeded again. The switching delay helps to achieve suppres-
sion of brief exceeding / dropping below of limit values. A stable message behavior
can be achieved in the event of fluctuations around the limit value by a hysteresis
which is also parameterizable. Figure 15, page 111 shows examples of a limit value
monitoring.
The message relay can be parameterized in the idle current principle and open circuit
principle states (see also chapter 6.3, page 126). When operating with the HART
communicator, the "Relay closes" parameter must be set as follows:
• with idle current principle "ON"
• with open circuit principle "OFF"
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