Siemens SIMATIC S7-400H System Manual page 118

Memory concept
10.1 Overview of the memory concept of S7-400H CPUs
Memory types of the S7-400H CPUs
● Load memory for the project data, e.g. blocks, configuration, and parameter settings.
● Work memory for the runtime-relevant blocks (logic blocks and data blocks).
● System memory (RAM) contains the memory elements that each CPU makes available to
● System memory of the CPU also provides a temporary memory area (local data stack,
Important note for CPUs after the parameter settings for the allocation of RAM have been changed
If you change the work memory allocation by modifying parameters, this work memory is
reorganized when you load system data into the CPU. As a consequence, the data blocks
generated by means of SFC are deleted and the remaining data blocks are initialized with
values from load memory.
The amount of work memory that is made available for logic or data blocks during the
download of system data is adapted if you modify the following parameters:
● Size of the process image (byte-oriented; in the "Cycle/Clock Memory" tab)
● Communication resources (in the "Memory" tab)
● Size of the diagnostics buffer ("Diagnostics/Clock" tab)
● Number of local data for all priority classes ("Memory" tab)
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the user program, such as bit memories, timers and counters. System memory also
contains the block stack and interrupt stack.
diagnostics buffer, and communication resources) that is assigned to the program for the
temporary data of a called block. This data is only valid as long as the block is active.
By changing the default values for the process image, local data, diagnostics buffer, and
communication resources (see object properties of the CPU in HW Config), you can
influence the work memory available to the runtime-relevant blocks.
NOTICE
Please note the following if you expand the process image of a CPU. Reconfigure the
modules whose addresses have to be above the highest address of the process image
so that the new addresses are still above the highest address of the expanded process
image.
System Manual, 03/2012, A5E00267695-11
S7-400H