Siemens SIMATIC S7 F Manual page 314

Fail-Safe Blocks
Error Handling
If the operation generates an invalid REAL number the event will be recorded in
the Diagnostic Buffer.
In the event of an error that is critical to safety, the system function SFC F_CTRL
is called. This records the event in the Diagnostic Buffer and requests a switch to
the reserve CPU if the error occurred only on the master CPU. For non-redundant
systems or a common-cause error occurring in both CPUs, the shutdown logic can
be configured to either disable the erred F-run-time group or the entire Safety
Program.
Error Information in Diagnostic Buffer
Error Code (W#16#...)
75D9H
75DAH
8-126
Description
Invalid REAL number generated by the operation.
Error in the safety data format of the input MIN or from
VALIDIN1 to VALIDIN 9 (error due to online modification of the
Safety Program or internal CPU fault)
Fail-Safe Systems
A5E00085588-03