Siemens Simatic S7-300 Technical Data Manual page 80

Memory concept
4.2 Memory functions
The data written to load memory are portable and retentive on CPU memory reset.
Evaluation of measured values:
• Measured value DBs saved to load memory can be uploaded and evaluated by other
4-18
communication partners (PG, PC, for example).
Note
The active system functions SFC 82 to 84 (current access to the MMC) have a distinct
influence on PG functions (block status, variable status, load block, upload, open, for
example). This typically reduces performance (compared to passive system functions) by
the factor 10.
Note
For CPUs with firmware V2.1.0 or higher, you can also generate non-retentive DBs using
SFC 82 (parameter ATTRIB -> NON_RETAIN bit.)
Note
As a precaution against loss of data, always make sure that you do not exceed the
maximum number of delete/write operations. For further information, refer to the
Technical Data of the Micro Memory Card (MMC) in the General Technical Data of your
CPU.
Caution
Data on a SIMATIC Micro Memory Card can be corrupted if you remove the card while it
is being accessed by a write operation. In this case, you may have to delete the MMC on
your PG, or format the card in the CPU. Never remove an MMC in RUN mode. Always
remove it when power is off, or when the CPU is in STOP state, and when the PG is not
a writing to the card. When the CPU is in STOP mode and you cannot not determine
whether or not a PG is writing to the card (e.g. load/delete block), disconnect the
communication lines.
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05