Siemens SIMATIC S7-400H System Manual page 269

Example: 50 % communication load
In the hardware configuration, you have set a communication load of 50 %.
The calculated cycle time is 10 ms.
This means that 500 µs remain in each time slice for the cycle. So 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. So 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 16-6
S7-400H
System Manual, 09/2007, A5E00267695-03
Dependency of the cycle time on the communication load
S7-400 cycle and reaction times
16.4 Communication load
269