Siemens SIMATIC S7 Configuration And Programming Manual page 27

Use of Local Data in an F-FB or F-FC
Note
F-blocks are automatically added when the safety program is compiled to create
an executable safety program from your safety program. If you use the local data
memory area in an F-FB/F-FC, remember the following limit (irrelevant for F-CPUs
from the S7-400 range):
Local data requirement< max. size of local data per block
(see Technical Specification about the F-CPU used in the Product Information
Mean local data requirement in bytes =
2 x amount of local data of F-FB/F-FC of data type BOOL
+ 4 x amount of local data of F-FB/F-FC of data type INT
+ 6 x amount of local data of F-FB/F-FC of data type TIME
+ 12
+ 14 (if a fixed-point function is programmed)
+ 6 (if an F-FB, F-FC, or F-application block is called)
+ 24 (if a called F-FB, F-FC or F-application block contains a parameter of
data type TIME)
If the amount of local data required is greater, you cannot download your safety
program to the F-CPU. Reduce the local data requirement of your programmed
F-FB or F-FC.
Product Information for the S7 Distributed Safety, Configuring and Programming Manual
A5E00169432-02
27