S7-400 cycle and response times
19.4 Communication load
S7-400H
System Manual, 03/2012, A5E00267695-11
367
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.
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).
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