4 SEND/RECEIVE Interface in the User Program
A-98
S7-CPs for Industrial Ethernet Configuring and Commissioning
Release 01/2007
C79000-G8976-C182-07
4.4.2 Recommendations for Use with a High Communications Load
Overview
The points below will help you to avoid overload situations on your CPU when
using the Ethernet CPs.
In particular when you replace an older CP with a newer CP and are then
confronted with overload problems, you should check your application for the
pitfalls outlined below.
Known Problems
The functions for sending and receiving (FC5/FC6 or FC50/60) 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 generally and there may be a lack of resources when
SEND/RECEIVE FCs are called cyclically in OB1.
Remedy
The recommendations below will help to avoid these situations:
Do not call communication blocks cyclically in OB1!
Communication should be called time-controlled in a suitable cyclic-interrupt
OB. The call interval of this OB should be significantly higher than the average
cycle time of your program.
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 remedy 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 parameters set for “cyclic load due to communication” in the
properties dialog of the CPU.
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