Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 585
27.2.3.8 Retentive Memory Areas on S7-300 CPUs
If a power outage occurs or the CPU memory is reset (MRES), the memory of the S7-300 CPU
(dynamic load memory (RAM), work memory, and system memory) is reset and all the data
previously contained in these areas is lost. With S7-300 CPUs, you can protect your program and
its data in the following ways:
• You can protect all the data in the load memory, work memory, and in parts of the system
memory with battery backup.
• You can store your program in the EPROM (either memory card or integrated on the CPU,
refer to the "S7-300 Programmable Controller, Hardware and Installation" Manual).
• You can store a certain amount of data depending on the CPU in an area of the nonvolatile
NVRAM.
Using the NVRAM
Your S7-300 CPU provides an area in the NVRAM (non-volatile RAM) (see figure below). If you
have stored your program in the EPROM of the load memory, you can save certain data (if there is
a power outage or when the CPU changes from STOP to RUN) by configuring your CPU
accordingly.
To do this set the CPU so that the following data are saved in the nonvolatile RAM:
• Data contained in a DB (this is only useful if you have also stored your program in an EPROM
of the load memory)
• Values of timers and counters
• Data saved in bit memory.
On every CPU, you can save a certain number of timers, counters, and memory bits. A specific
number of bytes is also available in which the data contained in DBs can be saved.
The MPI address of your CPU is stored in the NVRAM. This makes sure that your CPU is capable
of communication following a power outage or memory reset.