Siemens SIMATIC S7-300 Manual page 74

Cycle and response times
7.2 Cycle time
Physical cycle time depending on communication load
The figure below describes the non-linear dependency of the physical cycle time on
communication load. As an example, we have chosen a cycle time of 10 ms.
Figure 7-4
Influence on the physical 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 further extends the OB1 cycle. This extension depends on the number
of events that occur per OB1 cycle and the time required to process these events.
Note
Change the value of the "communication load" parameter to check the effects on the cycle
time at system runtime. You must consider the communication load when you set the
maximum cycle time, otherwise timing errors may occur.
Tips
• If possible, apply the default value.
• Increase this value only if the CPU is used primarily for communications and if the user
• In all other situations you should only reduce this value.
7-8
s
Dependency of the Cycle Time on the Communication Load
program is not time critical.
S7-300 CPU Data: CPU 315T-2 DP
Manual, 12/2005, A5E00427933-02