Cycle and Response Times of the S7-400
9.4 Communication Load
S7-400 Automation System, CPU Specifications
9-10 Manual, 10/2006, 6ES7498-8AA04-8BA0
Dependency of the Actual Cycle Time on the Communication Load
The following figure describes the non-linear dependency of the actual cycle time on the
communication load. This example uses a cycle time of 10 ms.
0 % 10 % 20 % 30 % 40 % 50 % 60 %
10 ms
20 ms
25 ms
15 ms
5 ms
30 ms
5 %
Cycle time
You can set the communication load in this range
Communication load
Figure 9-6 Dependency of the Cycle Time on the Communication Load
Further Effect on the Actual Cycle Time
Due to the increase in the cycle time as a result of the communications component, even
more asynchronous events occur, from a statistical point of view, within an OB 1 cycle than,
say, interrupts. This also increases the OB 1 cycle. This extension depends on the number
of events that occur per OB 1 cycle and the time required to process these events.
Notes
● Check the effects of a change of the value for the parameter "Cycle load due to
communications" in system operation.
● The communication load must be taken into account when you set the maximum cycle
time, since time errors will occur if it is not.
Recommendations
● If possible, apply the default value.
● Use a larger value only if the CPU is being used primarily for communication purposes
and the user program is non-time-critical. In all other cases select a smaller value.
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