Allen-Bradley 1756-L6 Series, Reference Manual, Page: 66 / 134

66 Rockwell Automation Publication 1756-RM001I-EN-P - May 2012
Chapter 6 FLEX I/O Modules
• Calibrate inputs periodically, as necessary . FLEX I/O modules ship
from the factory with a highly accurate level of calibration. However,
because each application is different, you are responsible for making sure
their FLEX I/O modules are properly calibrated for their specific
application.
You can employ tests in application program logic to determine when a
module requires recalibration. For example, to determine whether an
input module needs to be recalibrated, a user can determine a tolerance
band of accuracy for a specific application. You can then measure input
values on multiple channels and compare those values to acceptable values
within the tolerance band. Based on the differences in the comparison, you
could then determine whether recalibration is necessary.
Calibration (and subsequent recalibration) is not a safety issue . However,
we recommend that each analog input be calibrated at least every 3 years to
verify the accuracy of the input signal and avoid nuisance application
shutdowns.
• Compare analog input data and annunciate miscompares . When wiring
sensors to two inputs channels, the values from those channels must be
compared to each other for concurrence within an acceptable range for the
application before actuating an output. Any miscompare between the two
inputs outside the programmed acceptable range must be annunciated as a
fault.
In Figure 37 on page 67 , 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 input
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.