Siemens SIMATIC S7-400 Reference Manual page 88

Memory Concept and Startup Scenarios
Memory Types in S7-400 CPUs
• Load memory for project data, such as blocks, configuration and parameter
assignment data, including symbols and comments as of version 5.1.
• Working memory for the runtime-relevant blocks (code blocks and data blocks).
• System memory (RAM) contains the memory elements that each CPU makes
available to the user program, such as memory markers, timers, and counters.
The system memory also receives the block stack and the interrupt stack.
• System memory of the CPU also makes temporary memory available (local
data stack, diagnostic buffer and communication resources) that is assigned to
the program when a block is called for its temporary data. These data are only
valid as long as the block is active.
By changing the default values for the process image, local data, diagnostic
buffer and communication resources (see the object properties of the CPU in
HWCONFIG), you can control the working memory available to the
runtime-relevant blocks.
Notice
Please note the following if you enlarge the process image of a CPU. Make sure
that you configure the modules that can only be operated above the process
image in such a way that they are also positioned above the enlarged process
image. This particularly applies to IP and WF modules that you operate in the S5
adapter casing in a S7 400.
Flexible Memory Capacity
• Working memory:
The capacity of the working memory is determined by selecting the appropriate
CPU from the finely graded range of CPUs.
In the case of the CPU 417, working memory can be expanded.
• Load memory:
The integrated load memory is sufficient for small and medium-sized programs.
The load memory can be increased for larger programs by inserting the RAM
memory card.
Flash memory cards are also available to ensure that programs are retained in
the event of a power failure even if there isn't a backup battery. Flash memory
cards can also be used (as of 2 MB for standard CPUs, as of 4 MB for
fault-tolerant CPUs) to send and execute operating system updates.
Backup
• The backup battery provides backup power for the integrated and external part
of the load memory, the data section of the working memory and the code
section.
2-4
Automation System S7-400 CPU Specifications
A5E00165965-01