Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 126)

126 Publication 1756-UM009C-EN-P - December 2010
Chapter 6 Temperature-measuring Analog Modules (1756-IR6I, 1756-IT6I, and 1756-IT6I2)
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. This feature is
available only in applications using floating point.
To see how to set the rate alarm, see page 210 .
10 Ohm Offset
With this feature, you can compensate for a small offset error in a 10
Ω
copper RTD. Values can range from -0.99…0.99
Ω
in units of
0.01
Ω
. For example, if the resistance of a copper RTD used with a channel is
9.74
Ω
at 25
o
C, you would enter -0.26 in this field.
To see how to set the 10 Ohm Offset, see page 215 .
IMPORTANT
You must use RSLogix 5000 software, version 12 or later, and
module firmware revision 1.10 or later, to use the rate alarm for
a non-ohm input on the 1756-IR6I module and a non-millivolt
input on the 1756-IT6I and 1756-IT6I2 modules.
EXAMPLE
If you set a 1756-IT6I2 module (with normal scaling in Celsius)
to a rate alarm of 100.1 °C/s, the rate alarm only triggers if the
difference between measured input samples changes at a
rate > 100.1 °C/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 355 °C and
at time 100 ms measures 363 °C, the rate of change is
(363…355 °C) / (100 ms) = 80 °C/s. The rate alarm would not
set as the change is less than the trigger point of 100.1 °C/s.
If the next sample taken is 350.3 °C, the rate of change is
(350.3…363 °C)/(100 ms)=-127 °C/s. The absolute value of this
result is > 100.1 °C/s, so the rate alarm will set. 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.