Dependency Of The Cycle Time On The Communication Load - Siemens SIMATIC S7-400 Reference Manual

Cycle and Reaction Times of the S7-400
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. As an example, we have chosen a cycle time of
10 ms.
Cycle
30 ms
time
25 ms
20 ms
15 ms
10 ms
5 ms
Figure 3-6
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
increase depends on how many events occur per OB 1 cycle and how long event
processing lasts.
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.
3-12
You can set the communication load in this
range
0% 5% 10% 20%
Communication load

Dependency of the Cycle Time on the Communication Load

30% 40% 50%
Automation System S7-400 CPU Specifications
A5E00165965-01
60%