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26 Rockwell Automation Publication 1756-RM001I-EN-P - May 2012
Chapter 2 Features of the ControlLogix SIL 2 System
This ability of the controller to monitor the health of I/O modules in the system
and take appropriate action based on the severity of a fault condition gives the
user complete control of the application’s behavior. It is your responsibility to
establish the course of action appropriate to your safety application.
For more information on Fault Handling, see Chapter 8 , Faults in the
ControlLogix System on page 87 .
Data Echo Communication
Check Output data echo allows the user to verify that an ON/OFF command from the
controller was received by the correct output module, and that the module will
attempt to execute the command to the field device.
During normal operation, when a controller sends an output command, the
output module receiving that command will ‘echo’ the output command back to
the controller upon its receipt. This verifies that the module has received the
command and will try to execute it. By comparing the requested state from the
controller to the data echo received from the module, you can validate that the
signal has reached the correct module and that the module will attempt to
activate the appropriate field-side device. The echo data is technically input data
from the output module and is located with the other output module data. For
example, an output module at local slot 3 will have Local:3O and Local:3I, where
3O are outputs and 3I are inputs. Again, it is your responsibility to establish the
course of action appropriate for your safety application.
When used with standard ControlLogix output modules, the data echo validates
the integrity of communication up to the system-side of the module, but not to
the field-side. When you use this feature with diagnostic output modules, you can
verify the integrity from the controller to the output terminal on the module.
Diagnostic output modules contain circuitry that performs field-side output
verification. Field-side output verification informs you that commands received
by the module are accurately represented on the power side of the module’s
switching devices. In other words, for each output point, this feature confirms
that the output is ON when it is commanded to be ON or OFF when
commanded to be OFF.