Siemens SIMATIC S7-400 Reference Manual page 87

Important Note for CPUs with Configurable Division of the Working Memory
If you use parameter assignment to change the division of the working memory,
the working memory is reorganized when the system data are downloaded to the
CPU. The result of this is that data blocks that were created with SFC are deleted,
and the remaining data blocks are assigned initial values from the load memory.
The usable size of the working memory for code or data blocks is changed if you
change the following parameters for loading the system data:
• Size of the process image (byte by byte; "Cycle/Clock Memory" tab)
• Communication resources (S7-400 only; "Memory" tab)
• Size of the diagnostic buffer ("Diagnostics/Clock" tab)
• Number of local data for all priority classes ("Memory" tab)
Basis for Calculating the Required Working Memory
To ensure that you do not exceed the available amount of working memory in the
CPU, you must take into consideration the following memory requirements when
assigning parameters:
Table 2-1
Parameter
Size of the process image
(inputs)
Size of the process image
(outputs)
Communication resources
(communication jobs)
Size of diagnostic buffer
Volume of local data
Automation System S7-400 CPU Specifications
A5E00165965-01
Memory Requirements
Required Working Memory
12 bytes per byte in the process
input image
12 bytes per byte in the process
output image
72 bytes per communication job
20 bytes per entry in the
diagnostic buffer
1 byte per byte of local data
Memory Concept and Startup Scenarios
In Code/Data Memory
Code memory
Code memory
Code memory
Code memory
Data memory
2-3