Siemens SIMATIC S7-300 Manual For Migrating page 63

5 Software Conversion
Functions (FC)
For functions in STEP 7 (FCs), appropriate input and output signals can be
declared and transferred to the FC when they are called. In addition, the FC can
provide a direct return value of the function. Temporary tags and constants can be
declared in an FC.
Function blocks (FB)
For function blocks in STEP 7 (FBs), appropriate input and output signals can also
be declared and transferred to the FB when they are called. For each call of an FB,
an instance (in required, multiinstance) is assigned as memory, in which the values
of its tags are saved for the processing in the next program cycle. Static and
temporary local tags as well as constants can be declared in a STEP 7 FB.
Data blocks (DB)
Data blocks are used to save relevant data. In STEP 7 V5.x as well as in STEP 7
TIA Portal there are global data blocks and also instance data blocks. However,
there is a big difference in the use and the handling of the data blocks. Whilst in
STEP7 V5.x only DBs with up to 64kbyte can be used, in TIA Portal blocks up to a
size of 16Mbyte can be created and used. This depends on the properties of the
blocks (optimized/non-optimized), the CPU and the available memory. In addition,
optimized data blocks can be reloaded in running operation and their interface can
be changed ("Loading without reinitialization"). This is made possible by the
memory reserve in the data blocks. Another function that improves and facilitates
working with data blocks, is the creation of snapshots. This is where the user can
accept the values that have existed at a certain time, as start values or as actual
value.
PLC data types
PLC data types in STEP 7 TIA Portal are similar to the UDTs in STEP7 5.x.
To facilitate a reusability of structures in the program, PLC data types can be
created. In contrast to "Structs", PLC data types are globally valid and not only in
the block in which they have been defined. They are used within data blocks (for
example, the interface definition) and also in other locations in the program. If a
detail has to be changed in a PLC data type, all places used in the program will be
corrected automatically.
Migration S7-300/S7-400 nach S7-1500
Entry ID: 109478811, V1.1, 05/2017
63