Siemens S7-300 Configuration Manual page 90

SEND/RECEIVE interface in the user program
4.4 Additional information
Solution
Note the following recommendations:
● Do not call communication program blocks cyclically in OB1!
Call up communication time-controlled in a suitable time OB. The call interval of this OB
should be significantly higher than the average cycle time of OB1.
● Set a minimum cycle time that is higher than the average runtime of OB1. This frees
resources for communication on the CPU. This is, for example, a solution for existing
applications when communication already takes place cyclically in OB1.
● If necessary, reduce the time taken for communication processing on the CPU by
changing the parameter "Scan cycle load from communication" of the CPU.
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Configuring and commissioning S7 CPs for Industrial Ethernet
Configuration Manual, 09/2013, C79000-G8976-C182-13