Recommendations For Use With A High Communications Load - Siemens SIMATIC S7-CPs Manual

Coordinating forwarding of the time of day with several CPs
If there is more than one CP in a station connected to the same network, only one
of these CPs is allowed to pass on time-of-day messages.
You can therefore made the following settings in the configuration:
S Automatic
The CP receives the time-of-day message from the LAN or from the station and
forwards it to the station or to the LAN.
If several CPUs are being operated in the station, this automatic setting can
lead to collisions. To avoid this, you can specify the direction of forwarding with
the following options:
S From station to LAN
S From LAN to station
Notice
During configuration, there is no consistency check relating to this option when
configuring several CPs
8.6 Recommendations for use with a high communications
load
Overview
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 (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.
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).
CP 443-1 for Industrial Ethernet / Manual Part B
Release 07/2010
C79000-G8976-C223−06
8 Further Notes on Operation
B−41