Recommendation For Use With A High Communications Load - Siemens S7-400 Equipment Manual

Configuration and operation
6.9 Interface in the user program
Note: The TSAPs can be 2-16 bytes long. The first two bytes must be occupied as described,
you can use the other bytes to suit your task.
Note
Note that the number of dynamically established connections also depends on the number of
configured, statically established connections.
You will receive corresponding condition codes on the call interface of the FBs.
Refer to the documentation of the program blocks in the online help and in the
documentation for STEP 7. You will also find examples of parameter assignment there.
6.9.6

Recommendation for use with a high communications load

Reason
When using the CP described here, the points below will help you to avoid overload situations
on your CPU.
In particular when you replace an older CP with the CP described here and are then
confronted with overload problems, you should check your application for the pitfalls
outlined below.
Known problems
• The program blocks for sending and receiving AG_SEND / AG_RECV (FC5/FC6, FC50/60 or
FC53/63) are often called cyclically in OB1. This leads to constant communication between
the CPU and CP. As a result, other types of communication such as PG functions cannot be
executed or only very slowly.
• HMI systems access data of the CPU too often using S7 functions. This slows down
communication in general and resource bottlenecks on the CPU can occur if SEND/RECEIVE
FCs are called cyclically in OB1 (effect: reduced data throughput or increased reaction
time).
Remedy
Note the following recommendations:
• Do not call communication program blocks cyclically in OB1!
Communication should be called timecontrolled in a suitable cyclicinterrupt OB. The call
interval of this OB should be significantly higher than the average cycle time of OB1.
• You should 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" in the properties of the CPU.
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Equipment Manual, 03/2023, C79000-G8976-C256-07
CP 443-1 Advanced