Fail-Safe Blocks
Fail-Safe Systems
8-84 A5E00085588-03
8.6.17 SFC F_CTRL
SFC F_CTRL is a System Function Call in the CPU that is called in the event an
internal diagnostic determines there is a failure of the hardware or a diagnostic
used to determine timeouts is tripped. SFC F_CTRL is called from function blocks
that have diagnostics for such conditions. These include, but are not limited to, the
function blocks F_M_DO10, F_M_DO8, F_M_DI8, F_M_DI24, F_M_AI6, F_PLK,
F_PLK_O, etc. SFC F_CTRL has two purposes.
1. To report a diagnostic failure to the diagnostic buffer for users to observe as
the cause of failure
2. In an S7 F/H system, to force a switchover if the fault is detected in the master
only
As you can see from the two purposes above, SFC F_CTRL is used for diagnostic
purposes and for availability by forcing the CPU with the detected failure to
become the reserve CPU.
SFC F_CTRL is not responsible for any switchover actions in an S7 F (single
CPU), in a redundant S7 F/H in which the fault occurs on both CPUs (common
cause), or in the case of the detected failure in the reserve CPU in a redundant S7
F/H system.
The shutdown logic located in the @F_Shutdn chart is responsible for disabling the
F-run-time group with the detected failure.
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