Siemens SIMATIC S7-400H - Communication Load

S7-400 cycle and response times
19.4 Communication load
S7-400H
366 System Manual, 03/2012, A5E00267695-11
19.4 Communication load
The operating system provides the CPU continuously with the configured time slices as a
percentage of the overall CPU processing resources (time slice technique). Processing
performance not required for communication is made available to other processes.
In the hardware configuration you can specify a communication load value between 5% and
50%. The default value is 20%.
This percentage is to be interpreted as mean value, i.e. communication resources may take
significantly more than 20% of a time slice. The communication portion is then only a few %
or 0% in the next time slice.
The formula below describes the influence of communication load on the cycle time:
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Figure 19-4 Formula: Influence of communication load
Data consistency
The user program is interrupted to process communications. This interruption can be
triggered after any statement. These communication jobs may lead to a change in user data.
As a result, data consistency cannot be ensured over several accesses.
How to ensure data consistency in operations comprising more than one command is
described in the "Consistent data" section.
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Figure 19-5 Distribution of a time slice
The operating system takes a certain portion of the remaining time slice for internal tasks.
This portion is included in the factor defined in the tables starting at 16-3.