Siemens SIMATIC S7 F Manual page 283

Fail-Safe Blocks
Note
If, when you create the program, you preset the QL output in CFC with the initial
value 1, it will remain set after startup (cold restart or warm restart) if U_LL < U
<= (U_LL + HYS).
It is only reset if U > (U_LL + HYS).
Note that the initial values of output parameters do not appear in the printout of the
CFC chart. They must be checked in the printout of the safety program.
Error Handling
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
Fail-Safe Systems
A5E00085588-03
Description
Invalid REAL number at the inputs U, U_LL, HYS (DATA component) or,
generated during the calculations involving U, U_LL, HYS and SUBS_IN
Error in the safety data format of the inputs U, U_LL, HYS
(Error due to online modification of the Safety Program or internal CPU
fault)
8-95