Technical data

Table Of Contents
Copy and Block Functions
System Software for S7-300/400 System and Standard Functions - Volume 1/2
A5E00709327-01
3-21
3.11 Writing a Data Block in Load Memory with SFC 84
"WRIT_DBL"
Description
With the SFC 84 "WRIT_DBL" (write data block in load memory) you can transfer
the contents of a DB or a DB area from the work memory to a DB or a DB area in
the Micro Memory Card. The source DB must be process relevant; that is, it must
not have been created with the attribute UNLINKED. However, it may also be
generated with SFC 22 "CREAT_DB".
To ensure consistency, you must not change the source area while the SFC 84 is
being processed (i.e. as long as the BUSY parameter has the value TRUE).
The following restrictions apply to the parameters SRCBLK (source block) and
DSTBLK (destination block):
For an ANY pointer of the type BOOLEAN the length must be divisible by 8.
For an ANY pointer of the type STRING the length must be equal to 1.
If required, you can determine the length of the target DB with SFC 24 "TEST_DB".
SFC 84 does not change the checksum of the user program if you describe a DB
that was generated via SFC. However, when writing a loaded DB the first entry in
this DB changes the checksum of the user program.
Note
SFC 84 is not suitable for frequent (or cyclical) writing of variables to the load
memory writing. This is because the technology of Micro Memory Cards means
that only a certain number of write accesses can be made to a Micro Memory
Card. For more information, see the reference manual "SIMATIC S7-300
Programmable Controller CPU Data: CPU 31xC and CPU 31x".