Siemens SIMATIC S7 F Manual page 97

Programming
F_TESTM: Automatic placement of the F_TESTM block and associated chart in
the slowest OB that contains a piece of the failsafe program.
F_CYC_CO: Automatic placement of a F_CYC_CO block and associated chart in
each OB that contains a piece of the failsafe program. The user will be requested
to enter the maximum cycle time (MAX_CYC) at the first compile.
F_TEST/F_TESTC: Automatic placement of the F_TEST and F_TESTC blocks and
associated chart in each OB that contains a piece of the failsafe program.
Shutdown Logic: Automatic placement of the Shutdown Logic for the failsafe
program. This would include all necessary blocks and charts and any connections
to the failsafe RTG's.
Note
Please note that although the CFC Editor automatically creates the necessary
logic for the user's Safety Program, it may not delete it once the user deletes the
Safety Program. If the user wishes to delete the Safety Program, the user may
have to manually delete the Safety Program's system level run-time groups.
You may arrange your fail-safe user logic in any run-time order (following the
above guidelines). You may mix standard and fail-safe run-time groups, as shown
in the graphic below. In the example below, there are three user standard run-time
groups, which are S1, S2, and S3. There are two fails-afe user run-time groups
that are placed and the CFC Editor automatically places the '@' run-time groups.
You should place the fail-safe run-time groups before the standard run-time groups
in the run sequence if possible. This will avoid any variable amounts of delay
encountered when executing the standard run-time groups before the execution of
the fail-safe diagnostics.
Fail-Safe Systems
A5E00085588-03
5-15