Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 64)

64 Publication 1756-UM0009C-EN-P - December 2010
Chapter 4 Non-isolated Analog Voltage/Current Input Modules (1756-IF16, 1756-IF8)
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.
For example, if you set the 1756-IF16 module (with normal scaling in volts) to
a rate alarm of 1.0 V/S, the rate alarm will only trigger if the difference
between measured input samples changes at a rate > 1.0 V/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 volts and at time 100 ms measures
5.08 V, the rate of change is (5.08V - 5.0V) / (100 mS) = 0.8 V/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 will set. Absolute value is applied because rate alarm checks for the
magnitude of the rate of change being beyond the trigger point, whether a
positive or negative excursion.
Wire Off Detection
The 1756-IF16 and 1756-IF8 modules will alert you when a signal wire only
has been disconnected from one of its channels or the RTB has been removed
from the module. When a wire-off condition occurs for this module, two
events occur:
• Input data for that channel changes to a specific scaled value.
• A fault bit is set in the owner-controller that may indicate the presence
of a wire-off condition.
IMPORTANT
Rate alarms are not available in integer mode or in applications
using 1756-IF16 module in the single-ended, floating point
mode. The values for each limit are entered in scaled
engineering units.