Siemens S7-1500 Function Manual page 40

Processing sequence
● Storing the recipe in the load memory
STEP 7 fills the individual data records of a recipe into a non-runtime-relevant DB and
downloads the DB to the CPU. In order to configure a non-runtime-relevant DB, you must
enable the "Only store in load memory" block attribute. The recipes then only use
memory space in the load memory and not in the work memory.
● Working with recipe data in the user program
You use the "READ_DBL" instruction to copy a data record of the current recipe from the
DB in the load memory to a runtime-relevant DB in the work memory. As a result, the
work memory only has to accommodate the data for the currently required recipe data
record. The user program can now access the data of the current data record.
● Saving back changed recipe data records
The "WRIT_DBL" instruction writes new or changed data records of a recipe from the
user program back to the load memory. The data written to the load memory is portable
and not affected by a memory reset. To back up changed data records (recipes), you
must upload the data blocks and back them up on the PG/PC.
You can find information on uploading data blocks in the STEP 7 online help under
"Programming a PLC > Compiling and downloading blocks > Downloading blocks for
S7-1200/1500 > Uploading blocks from a memory card".
Figure 3-1
Also note the FAQ "How do you configure data blocks with the "Only store in load
memory" attribute for the S7-1200/S7-1500? " in the Internet
(https://support.industry.siemens.com/cs/ww/en/view/53034113).
Note
Instructions that access the SIMATIC memory card have a lower performance than
instructions that access the work memory. The associated blocks (e.g. READ_DBL and
WRIT_DBL) are therefore asynchronous. Their execution extends if necessary over
several cycles.
Structure and Use of the CPU Memory
Function Manual, 10/2018, A5E03461664-AC
Processing sequence with "READ_DBL" and "WRIT_DBL"
Memory usage and application examples
3.1 Memory usage for recipes
39