Siemens SIMATIC S7-300 Series Reference Manual page 156

Tips and Tricks
With Backup Battery
All DBs are retentive,
whatever configuration has
been performed. The DBs
generated using SFC 22
"CREAT_DB" are also
retentive.
Watchdog Interrupt: Scan period > 5 ms
For the watchdog interrupt, you should set a scan period > 5 ms. With lower values
the risk of frequent occurrence of watchdog interrupt errors increases, for example,
depending on
S The program cycle time of an OB35
S Frequency and program cycle time of higher priority classes
S Programming device functions.
Hardware Interrupts Generated in I/O Modules
When using applications critical to hardware interrupt, insert the
hardware-triggering modules as close to the CPU as possible.
Reason: Interrupts generated in module rack 0, slot 4, are read first. The other
slots follow in ascending order.
5-2
Without Backup Battery
CPU program on Memory
Card or in the integral
EPROM of the
312IFM/314IFM
All DBs (retentive,
non-retentive) are transferred
from the memory card or from
the integral EPROM into RAM
on restart.
S
Data blocks or data areas you have generated with
SFC22 "CREAT_DB" are not retentive.
S
After a power failure, the retentive data areas are retained.
Note:These data areas are stored in the CPU, not on the
memory card. The non-retentive data areas contain
whatever has been programmed on EPROM.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
Memory card not plugged
in
The DBs configured as
retentive retain their contents
A5E00111190-01