Siemens S7-300 appendix Physical cycle time depending on communication load, Tips

Cycle and reaction times 5.2 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. In our sample we have chosen a cycle time of 10 ms.

Cycle time 30 ms

The communication load can be defined in this area.

25ms

20ms

15ms

10ms

5 ms

0 % 5 % 10 % 20 % 30 % 40 % 50 % 60 %

Communication load

Influence on the physical cycle time

From the statistical viewpoint, asynchronous events—such as interrupts—occur more frequently within the OB1 cycle when the cycle time is extended as a result of communication load. 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

•Use the default setting wherever possible.

•Increase this value only if the CPU is used primarily for communications and if the user program is not time critical.

•In all other situations you should only reduce this value.

5-10	CPU 31xC and CPU 31x, Technical data
	Manual, Edition 08/2004, A5E00105475-05