Fault Routine for Safety Applications
If a fault condition occurs that is severe enough for the controller to shut down, the controller generates a major fault and stops the execution of logic.
Some applications do not want all safety faults to shut down the entire system. In those situations, use a fault routine to clear a specific fault and let the standard control portion of your system continue to operate or configure some outputs to remain ON.
ATTENTION:
You must provide proof to your certifying agency that your system can continue to operate safely after an override of a safety fault.
The occurrence of recoverable faults is an indication that the application code is not protecting itself from invalid data values or conditions. Consider modifying the application to eliminate these faults, rather than handling them at runtime.
The controller supports two levels for handling major faults in a safety application:
- Safety Program Fault Routine
- Controller Fault Handler
Both routines can use the GSV and SSV instructions.
Each safety program can have its own fault routine. The controller executes the program’s fault routine when an instruction fault occurs. If the program’s fault routine does not clear the fault, or if a program fault routine does not exist, the safety task faults and shuts down.
When the safety task faults, a standard major recoverable fault is also logged, and the controller proceeds to execute the controller fault handler, if one exists. If the controller fault handler handles this fault, then the standard tasks continue to run, even though the safety task remains faulted.
The controller fault handler is an optional component that executes when the program fault routine cannot clear the fault or does not exist.
You can create one program for the controller fault handler. After you create that program, you must configure a routine as the main routine.
The Logix 5000 Controllers Major, Minor, and I/O Faults Programming Manual, publication 1756-PM014, provides details on creating and testing a fault routine.
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