Fisher DPR900 User Manual page 71

(FISHER'I
Configuration
5-17
56.3 Process Value Monitoring and Alarms
5.6.3.1
If the arithmetic function (FCODE 107) is applied
to the process value All, the remaining arithmetic
function (FCODE 108 - RSP calculation) cannot
be used at the same time.
If the square-root function is used on All in
conjunction with selecting PV monitoring, the
second analog input (A13) must have the same
square root measuring range and engineering unit
values. The square-root function cannot be used
with a calculation.
Two sensors can be used in applications where a sensor fault could
have serious consequences. The controller can be configured to
receive two transmitter input signals (All and A13) and monrtor the
status of these signals. The signals are scanned every 200 ms with
the averaged values used as the PV for the PID algorithm. If one of
the signals drifts and the deviation becomes excessive, or a sensor is
disconnected, the suspect sensor is not used.
It is important to note that the controller can be configured to utilize
DO3 and DO4 in one of several ways, including: process value
monitoring, process value alarm, setpoint deviation alarm, and
setpoint ramping indication or alarms.
Process Value Monitoring
Configure PV monitoring using FCODE 22, 23, and 116. The
controller uses the average value of the two signals connected to All
and Al3 in the monitoring-control algorithm. The control algorithm
compares the absolute difference between the previously calculated
mean sensor value and the current sensor value with the maximum
error signal as configured in FCODE 22. If the resulting value for All
is greater than the allowed deviation, an error is reported out on D03.
If the error is for the transmitter connected to Al3, the resulting alarm
is sent on D04. Note that DO3 and DO4 are mutually exclusive-only
one or the other is displayed, not both.
Change 2 -July 1992 UM6.l:DPR900'9147