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54 Rockwell Automation Publication 1756-RM001I-EN-P - May 2012
Chapter 5 ControlLogix I/O Modules
Using Analog Output
Modules There are a number of general application considerations that you must make
when using analog output modules in a SIL 2 application.
A single analog output module, along with an analog input module for
monitoring is required to achieve SIL 2.
The following sections describe those considerations specific to the use analog
output modules.
Considerations for Using Analog Output Modules
Conduct Proof Tests
Periodically perform a system validation test. Manually or automatically test all
outputs to make sure that they are operational. Field signal levels should be varied
over the full operating range to make sure that the corresponding channel data
varies accordingly. For more information, see
Proof Tests on page 20 .
Calibrate Outputs
Analog output modules should be calibrated periodically, as their use and
application requires. ControlLogix 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 your ControlLogix I/O modules
are properly calibrated for your specific application.
You can employ tests in application program logic to determine when a module
requires recalibration. For example, to determine whether an output module
needs to be recalibrated, you can determine a tolerance band of accuracy for a
specific application. You can then measure output 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 output be calibrated at least every 3 years to verify
the accuracy of the signal and avoid nuisance application shutdowns.
IMPORTANT
It is strongly recommended that you do not use analog outputs to execute
the safety function that results in a safe state. Analog output modules are
slow to respond to an ESD command and are therefore not recommended
for use ESD output modules.
The use of digital output modules and actuators to achieve the ESD
de-energized state is recommended.