Siemens S7-300 appendix Retentivity of the load memory, system memory and RAM, System memory, Ram

Memory concept

4.1 Memory areas and retentivity

System memory

The RAM system memory is integrated in the CPU and cannot be expanded.

It contains

•the address areas for address area memory bits, timers and counters

•the process image of the I/Os

•local data

RAM

The RAM is integrated in the CPU and cannot be extended. It is used to run the code and process user program data. Programs only run in RAM and system memory.

Table 4-1	Retentivity of the RAM
All CPUs except CPU 317		CPU 317
RAM is always retentive.		256 KB of RAM can be used for retentive data
		modules. The remainder of the RAM can only be
		used for code blocks and non-retentive data blocks.

4.1.2Retentivity of the load memory, system memory and RAM

Your CPU is equipped with a service-free retentive memory.i.e. its operation does not require a buffer battery. Data is kept in retentive memory across POWER OFF and restart (warm start).

Retentive data in load memory

Your program in load memory is always retentive: It is stored on the MMC, where it is protected against power failure or CPU memory reset.

Retentive data in system memory

In your configuration (Properties of CPU, Retentivity tab), specify which part of memory bits, timers and counters should be kept retentive and which of them are to be initialized with "0" on restart (warm restart).

The diagnostic buffer, MPI address (and transmission rate) and operating hour counter data are generally written to the retentive memory area on the CPU. Retentivity of the MPI address and baud rate ensures that your CPU can continue to communicate, even after a power loss, memory reset or loss of communication parameters (e.g. due to removal of the MMC or deletion of communication parameters).

4-2	CPU 31xC and CPU 31x, Technical data
	Manual, Edition 08/2004, A5E00105475-05