Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
586 Manual, 05/2010, A5E02789666-01
Using Battery Backup to Protect Data
By using a backup battery, the load memory and work memory are retentive during a power
outage. If you configure your CPU so that timers, counters, and bit memory are saved in the
NVRAM, this information is also retained regardless of whether you use a backup battery or not.
Configuring the Data of the NVRAM
When you configure your CPU with STEP 7, you can decide which memory areas will be retentive.
The amount of memory that can be configured in the NVRAM depends on the CPU you are using.
You cannot back up more data than specified for your CPU.
27.2.3.9 Retentive Memory Areas on S7-400 CPUs
Operation Without Battery Backup
If you operate your system without battery backup, when a power outage occurs or when you reset
the CPU memory (MRES), the memory of the S7-400 CPU (dynamic load memory (RAM), work
memory, and system memory) is reset and all the data contained in these areas is lost.
Without battery backup, only a restart (warm restart) is possible and there are no retentive memory
areas. Following a power outage, only the MPI parameters (for example, the MPI address of the
CPU) are retained. This means that the CPU remains capable of communication following a power
outage or memory reset.
Operation With Battery Backup
If you use a battery to back up your memory:
• The entire content of all RAM areas is retained when the CPU restarts following a power
outage.
• During a restart (warm restart), the address areas for bit memory, timers, and counters is
cleared. The contents of data blocks are retained.
• The contents of the RAM work memory are also retained apart from bit memory, timers, and
counters that were designed as non-retentive.
Configuring Retentive Data Areas
You can declare a certain number of memory bits, timers, and counters as retentive (the number
depends on your CPU). During a restart (warm restart) when you are using a backup battery, this
data is also retained.
When you assign parameters with STEP 7, you define which memory bits, timers, and counters
should be retained during a restart (warm restart). You can only back up as much data as is
permitted by your CPU.
For more detailed information about defining retentive memory areas, refer to your "S7-400,
M7-400 Programmable Controllers, Module Specifications" Reference Manual.