Communication Load - Siemens SIMATIC S7-400H System Manual

S7-400 cycle and reaction times

16.4 Communication load

16.4 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). If this processing
capacity is not required for communication, it is made available to the other processes.
You can set a communication load between 5 % and 50 % in your hardware configuration.
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 then only takes a few or 0 %
in the next time slice.
The formula below describes the influence of communication load on the cycle time:
Figure 16-4
Data consistency
The user program is interrupted to process communications. This interruption can be
triggered after any statement. These communication requests 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.
Figure 16-5
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 15-3.
Example: 20 % communication load
In the hardware configuration, you have set a communication load of 20 %.
The calculated cycle time is 10 ms.
This means that a setting of 20 % communication load allocates an average of 200 µs to
communication and 800 µs to the user program in each time slice. So the CPU requires
10 ms / 800 µs = 13 time slices to execute one cycle. This means the physical cycle time is
equivalent to 13 times 1-ms time slice = 13 ms, if the CPU fully utilizes the configured
communication load.
That is to say, 20 % communication does not extend the cycle by a linear amount of 2 ms,
but by 3 ms.
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Formula: Influence of communication load
Distribution of a time slice
System Manual, 09/2007, A5E00267695-03
S7-400H