Siemens Simatic S7-300 Technical Data Manual page 66

Memory concept
4.1 Memory areas and retentivity
Retentive behavior of a DB for CPUs with firmware >= V2.1.0
For these CPUs you can specify in STEP 7 (beginning with version 5.2 + SP 1), or at SFC 82
CREA_DBL (parameter ATTRIB -> NON_RETAIN bit), whether a DB at POWER ON/OFF or
RUN-STOP
• keeps the actual values (retentive DB), or
• accepts the initial values from load memory (non-retentive DB)
Table 4-3
At POWER ON/OFF or restart (warm start) of the CPU, the DB should
receive the initial values
(non-retentive DB)
Background:
At POWER ON/OFF and restart (STOP-
RUN) of the CPU, the actual values of the
DB are non-retentive. The DB receives the
start values from load memory.
Requirement in STEP 7:
•
•
Note
Note that only 256 KB of RAM can be used for retentive data blocks on a CPU 317. The
remainder of the RAM is used by code blocks and non-retentive data blocks.
4-4
Retentive behavior of DBs for CPUs with firmware >= V2.1.0
The "Non-retain" check box must be set
in the block properties of the DB, or
a non-retentive DB was generated with
SFC 82 "CREA_DBL" and the
corresponding block attribute (ATTRIB -
> NON_RETAIN bit.)
retain the actual values (retentive DB)
Background:
At POWER OFF/ON and restart (STOP-RUN) of the
CPU, the actual values of the DB are retained.
Requirement in STEP 7:
The "Non-retain" check box must be reset in the
•
block properties of the DB or
a retentive DB was generated with SFC 82.
•
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05