Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 101)

Publication 1756-UM009C-EN-P - December 2010 101
Sourcing Current Loop Input Module (1756-IF6CIS) and Isolated Analog Voltage/Current Input Module (1756-IF6I) Chapter 5
Rate Alarm
The rate alarm triggers if the rate of change between input samples for
each
channel exceeds the specified trigger point for that channel.
To see how to set the rate alarm, see page 212 .
IMPORTANT
The rate alarm is available only for applications that use
floating point mode.
EXAMPLE
1756-IF6CIS
If you set an 1756-IF6I (with normal scaling in mA) to a rate
alarm of 1.0 mA/s, the rate alarm only triggers if the difference
between measured input samples changes at
a rate > 1.0 mA/s.
If the module’s RTS is 100 ms (that is, sampling new input data
every 100 ms) and at time 0, the module measures 5.0 mA and
at time 100 ms measures 5.08 mA, the rate of change is (5.08
mA - 5.0 mA) / (100 ms) = 0.8 mA/s. The rate alarm would not
set as the change is less than the trigger point of 1.0 mA/s.
If the next sample taken is 4.9 mA, the rate of change is
(4.9 mA-5.08V)/(100 ms)=-1.8 mA/s. The absolute value of this
result is > 1.0 mA/s, so the rate alarm sets. Absolute value is
used because rate alarm checks for the magnitude of the rate of
change being beyond the trigger point, whether a positive or
negative excursion.
1756-IF6I
If you set an 1756-IF6I (with normal scaling in volts) to a rate
alarm of 1.0V/s, the rate alarm only triggers if the difference
between measured input samples changes at a rate > 1.0V/s.
If the module’s RTS is 100 ms (that is, sampling new input data
every 100 ms) and at time 0, the module measures 5.0V and at
time 100 ms measures 5.08V, the rate of change is (5.08V -
5.0V) / (100 ms) = 0.8V/s. The rate alarm would not set as the
change is less than the trigger point of 1.0V/s.
If the next sample taken is 4.9V, the rate of change is
(4.9V-5.08V)/(100 ms)=-1.8V/s. The absolute value of this result
is > 1.0V/s, so the rate alarm sets. Absolute value is used
because rate alarm checks for the magnitude of the rate of
change being beyond the trigger point, whether a positive or
negative excursion.