Allen-Bradley 1756-L6 Series, Reference Manual, Page: 42 / 134

42 Rockwell Automation Publication 1756-RM001I-EN-P - May 2012
Chapter 5 ControlLogix I/O Modules
Application logic is used to compare input values for concurrence.
Figure 13 - Logic Comparing Input Values or States
The user program must also contain rungs to annunciate a fault in the event of a
sustained miscompare between two points.
Figure 14 - Rungs Annunciating a Fault
The control, diagnostics and alarming functions must be performed in sequence.
For more information on faults, see
Chapter 8 , Faults in the ControlLogix
System .
Using 1756 Digital Output
Modules ControlLogix digital output modules are divided into two categories:
• Diagnostic output modules
• Standard output modules
These modules share many of the same inherent architectural characteristics.
However, the diagnostic output modules incorporate features that allow
diagnosing of field-side failures, including:
• No-Load (loss of load) reporting.
• Blown Fuse reporting.
• Output verify.
• Output pulse test.
To achieve SIL 2, the output module must be wired back to an input module for
monitoring. An exception is to use a diagnostic digital input module.
Input B Input A
Actuator
No Faults
Input B Input A
Input B Input A
Timer
Timer preset in milliseconds to
compensate for filter time and
hardware delay differences.
Fault
Timer Done
Alarm to Operator
Fault