Allen-Bradley 1756-L6 Series, Reference Manual, Page: 49 / 134

Rockwell Automation Publication 1756-RM001I-EN-P - May 2012 49
ControlLogix I/O Modules Chapter 5
In Figure 21 , a user-defined percentage of acceptable deviation (that is, tolerance)
is applied to the configured input range of the analog inputs (that is, range) and
the result is stored (that is, delta). This delta value is then added to and subtracted
from one of the input channels; the results define an acceptable High and Low
limit of deviation. The second input channel is then compared to these limits to
determine if the inputs are working properly.
The input’s OK bit preconditions a Timer run that is preset to accommodate an
acceptable fault response time and any communication filtering lags in the
system. If the inputs miscompare for longer than the preset value, a fault is
registered with a corresponding alarm.
Figure 21 - Comparison Logic for Two Analog Inputs
The control, diagnostics and alarming functions must be performed in sequence.
For more information on faults, see
Chapter 8 , Faults in the ControlLogix
System on page 87 .
Configure Modules
When using identical modules, configure the modules identically, that is, by using
the same RPI, filter values, and so on.
When using different modules for improved diversity, make sure the module’s
scaling of data does not introduce error or fault conditions.
Timer Done
Timer
Analog Inputs
Faulted
Alarm to Operator
Inputs OK
SUB
Delta
Input 1
Low Limit ADD
Delta
Input 1
High Limit MULT
Range
Tolerance%
Delta
Analog Inputs Faulted
LIM
Low Limit
Input 2
High Limit
Inputs OK
Allen-Bradley Motors