Siemens Memory Service Manual download pdf (Page 22)

Memory usage and application examples

3.1 Memory usage for recipes

Structure and Use of the CPU Memory

22 Function Manual, 01/2013, A5E03461664-01

Processing sequence

● Saving a recipe in load memory

The various data records of a recipe are filled in a non-runtime-relevant DB in STEP 7

and then loaded to the CPU. In order to configure a non-runtime-relevant DB, you must

activate the Only store in load memory" block attribute. Thus the recipes only use storage

space in load memory, and not in work memory.

● Working with recipe data in the user program

The "READ_DBL" instruction reads a data record belonging to 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

You can use the "WRIT_DBL" instruction to write new or changed data records of a

recipe from the user program back to the load memory. The data written to load memory

are portable and not affected by a memory reset. If you back up changed data records

(recipes) on the PG/PC, data blocks must be uploaded and saved on the PG/PC.

You can find information on uploading data blocks in the online help for STEP 7 under

"PLC programming > Compile and download blocks > Download blocks > Download

blocks from a memory card".

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Figure 3-1 Processing sequence with "READ_DBL" and "WRIT_DBL"

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.

Note

Service life of the SIMATIC memory card

Only a limited number of delete and write operations are possible on the SIMATIC

memory card. Avoid repeated (cyclic) write operations by the user program

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