Siemens SIMATIC S7-400 Reference Manual page 101

Example: 20 % Communication Load
You have configured a communication load of 20% in the hardware configuration.
The calculated cycle time is 10 ms.
A 20% communication load means that, on average, 200 ms and 800 ms of the time
slice remain for communications and the user program, respectively. The CPU
therefore requires 10 ms / 800 ms = 13 time slices to process one cycle. This
means that the actual cycle time is 13 times a 1 ms time slice = 13 ms, if the CPU
fully utilizes the configured communication load.
This means that 20% communications do not increase the cycle linearly by 2 ms
but by 3 ms.
Example: 50 % Communication Load
You have configured a communication load of 50% in the hardware configuration.
The calculated cycle time is 10 ms.
This means that 500 ms of each time slice remain for the cycle. The CPU therefore
requires 10 ms / 500 ms = 20 time slices to process one cycle. This means that
the actual cycle time is 20 ms if the CPU fully utilizes the configured
communication load.
A 50 % communication load means that, on average, 500 ms and 500 ms of the
time slice remain for communications and the user program, respectively. The CPU
therefore requires 10 ms / 500 ms = 20 time slices to process one cycle. This
means that the actual cycle time is 20 times a 1 ms time slice = 20 ms, if the CPU
fully utilizes the configured communication load.
This means that 50% communications do not increase the cycle linearly by 5 ms
but by 10 ms.
Automation System S7-400 CPU Specifications
A5E00165965-01
Cycle and Reaction Times of the S7-400
3-11