Siemens SIMATIC CPU 410 System Manual page 363

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.
Example: 50% communication load
You configured a communication load of 50% in the hardware configuration.
The calculated cycle time is 10 ms.
This means that 500 µs remain in each time slice for the cycle. Therefore, the CPU requires
10 ms / 500 µs = 20 time slices to execute one cycle. This means the physical cycle time is
20 ms if the CPU fully utilizes the configured communication load.
So a setting of 50% communication load allocates 500 µs to communication and 500 µs to
the user program in each time slice. Therefore, the CPU requires 10 ms / 500 µs = 20 time
slices to execute one cycle. This means the physical cycle time is equivalent to 20 times 1-
ms time slice = 20 ms, if the CPU fully utilizes the configured communication load.
This means that 50% communication does not extend the cycle by a linear amount of 5 ms,
but by 10 ms (= doubling the calculated cycle time).
Dependency of the actual cycle time on communication load
The figure below describes the non-linear dependency of the actual cycle time on
communication load. In our example we have chosen a cycle time of 10 ms.
Figure 18-30 Dependency of the cycle time on communication load
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
Supplementary information
18.16 CPU 410 cycle and reaction times
363