Cycle Load Due To Communication - Siemens SIMATIC PCS 7 CPU 410-5H System Manual

Supplementary information
15.15 Cycle and response times of the CPU 410-5H
15.15.3

Cycle load due to communication

The operating system of the CPU provides the configured percentage of the overall CPU
processing capacity to the communication on a continuous basis (time slice technique). If
this processing capacity is not required for communication, it is made available to the other
processing.
In the hardware configuration you can specify a communication load value between 5% and
50%. The default value is 20%.
The parameter represents the share of the cycle load in the internal copy jobs created at the
communication end. Communication at the interfaces is not affected.
This percentage is to be interpreted as a mean value, i.e., within one time slice, the
communication portion may be significantly greater than 20%. On the other hand,
communication load in the next time slice is very small or not present.
The formula below describes the influence of communication load on the cycle time:
Figure 15-28 Formula: Influence of communication load
Data consistency
The user program is interrupted to process communications. This interruption can be
triggered after any command. 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 Chapter "Consistent data".
Figure 15-29 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.
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CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB