Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 79)

Memory concept
4.2 Memory functions
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
4-17
4.2.6 Measured value log files
Introduction
Measured values are generated when the CPU executes the user program. These values
are to be logged and analyzed.
Processing sequence
Acquisition of measured values:
•
The CPU writes all measured values to a DB (for alternating backup mode in several
DBs) which is located in RAM.
Measured value logging:
•
Before the data volume can exceed work memory capacity, you should call
SFC 84 "WRIT_DBL" in the user program to swap measured values from the DB to load
memory. The figure below shows how to handle measured value log files:
:
Measured values 1
Measured values 2
Measured values n
Current measured
values
Working memory
(CPU)
Loading memory
(MMC)
SFC 82 CREA_DBL
SFC 84 WRIT_DBL
• You can call SFC 82 "CREA_DBL" in the user program to generate new (additional) static
DBs in load memory which do not require RAM space.
Reference
For detailed information on SFC 82, refer to the System Software for S7-300/400, System
and Standard Functions Reference Manual, or directly to the STEP 7 Online Help.
Note
SFC 82 is terminated and an error message is generated if a DB already exists under the
same number in load memory and/or RAM.