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Entry type: FAQ, Entry ID: 53843369, Entry date: 01/24/2012

How can you change the size of the available free main memory of a SIMATIC S7-400 CPU?

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The size of the main memory of a SIMATIC CPU depends on the following parameter settings:

  • Number of messages in the diagnostics buffer of the CPU
  • Maximum number of communication jobs
  • Size of the process-image input and output areas
  • Amount of local data for all priority classes

It is particularly effective to change these parameters with CPU types that have small main memories.

A significant part of the main memory used by the CPU for programs is taken up by the diagnostics buffer of the CPU. Here you have to decide how many messages are required by your application in the diagnostics buffer.

Fig. 1: Setting options for the number of messages in the diagnostics buffer of the CPU

You can also change the number of communication jobs. You can read the requirement in your application in the "Communication jobs of which currently loaded" line (Fig. 4). Change the maximum number as necessary. Here, you must take into account a reserve of approx. 30% is achieved.
You can also change the message texts in this dialog.

Fig. 2: Communication job and local data setting options

If you pay attention to assign the IO addresses of the IO modules as far as possible without gaps, you can reduce the process image for inputs/outputs.

Fig. 3: Setting options for the process image

You can see how the main memory is occupied online in the Properties of the CPU. In the "Memory" tab you select the field with the "main memory code" values (click in the field) and then click the "Main memory details" button.

Fig. 4: Occupation of the main memory of CPU 412

More information about the structure and calculation of the main memory is available in the device manual entitled "Automation System S7-400 CPU Specifications", in the section "Overview of the Memory Concept of the S7-400-CPUs".

Storage capacity, Compression, Safety matrix, Program memory, Code memory

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