storing values in S7-400


Thread Starter


How it is possible to keep existing values in DB of some SIEMENS SIMATIC S7-400 controller in case of power outage without backup battery?

Bostjan Naralocnik


according to Siemens Help in Simatic Manager,
this is not possible (see copy of online help

See also Siemens FAQ
ID 15178940 and 1117849 (manual)

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.

Bostjan Naralocnik
[email protected]

With a FEPROM you can do something.

By coping RAM to ROM al the changed items in the datablocks are transfered to the FEPROM.

Entry ID:15389520 on the link WWW4.AD.SIEMENS.DE
have the following information:
What happens to the contents of the data blocks when implementing the function "Copy RAM to ROM"?

When you implement the function "Copy RAM to ROM", in CPUs with MMC the actual values of the data blocks are transferred to the load memory (MMC) as new initial values of the DBs.
In this way you backup your data as initial values if you have to do an overall restart and then the runtime-relevant blocks are transferred again from the load memory into the main memory. In this case, the data blocks are reinitialized, i.e. they receive their initial values again.

In the case of CPUs with MC card or integrated FEPROM load memory (CPU 312 IFM, 314 IFM), when the "RAM to ROM" function is implemented, the actual values of the data blocks are loaded from the main memory into the FEPROM load memory or the FEPROM memory card.

With SFC20 you can write back information to the Loadmemory. By power down the CPU will get his information back from the FEPROM (LOAD-memory).

Then you have to make datablocks unlinked on the LOAD-memory and work with the parameters like a recepy. Thats copying Unlinked DB's from the LOAD- to the WORK-memory when you start the CPU.

Entry ID: 7302549 on the link WWW4.AD.SIEMENS.DE have the information on the memory setup of a S7-400 and also the advantages for a FEPROM:
Extending memory with a memory card:
When using a RAM memory card, you must operate the system with a back-up battery to ensure that the data in the memory card and that in the internal RAM is buffered if there is a power failure.

When using a Flash EPROM (FEPROM) memory card, the user program is stored power-failure-safe in the memory card. The FEPROM is a non-volatile memory. The data in the internal RAM are buffered by the back-up battery.
When you slot a memory card, the operating system requests an overall reset (STOP LED blinks slowly). You start the reset procedure by holding the operating switch in the "MRES" position. Then the runtime-relevant parts of the program are transferred from the memory card into the main memory.

The memory card must remain slotted for as long as the program is being processed.

Behavior of the CPU after Power ON
The CPU determines whether the Power ON is buffered or non-buffered.
In the case of a buffered Power ON the program part contained in the main memory is used for further processing.
In the case of a non-buffered Power ON the runtime-relevant parts are transferred from the load memory into the main memory.

You can also place a 10A powersupply thats having 2 batterys, this batterys can be checked by OB81 so you can give a message when there is one empty.

Best regards,