Recommendations For Use With A High Communications Load - Siemens CP 443-1 Manual

10 Further Notes on Operation
10.4 Recommendations for Use with a High Communications
Load
Background
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
S The functions for sending and receiving (FC 5/FC6 or FC 50/60) 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.
S HMI systems access data of the CPU too often using S7 functions. This slows
down communication generally and can lead to lack of resources on the CPU if
SEND/RECEIVE FCs are called in OB1 cyclically (effect: reduced data
throughput or increased reaction time).
Remedy
The recommendations below will help to avoid these situations:
S 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.
S 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.
S 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.
B4A−66
CP 443-1 Advanced for Industrial Ethernet / Manual Part B4A
Release 01/2007
C79000-G8976-C193-06