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5/40
Selecting the communication architecture
When selecting an architecture, performance should be taken into account at the
earliest possible stage. To do this, the developer must:
b
Know exactly what he needs:
v
quantity and type of devices to be interconnected
v
volume and type of exchanges
v
expected response times
v
environment
b
Compare his needs with the characteristics of the offers available and be aware
that the actual performance level between any 2 points in an architecture depends
on the weakest link in the chain, which can be:
v
dependent on the hardware
v
but also dependent on the applications (size, architecture, operating system,
machine power rating, etc) which are often only vaguely defined at this stage of the
project.
b
Work out from these which is the most suitable architecture.
The purpose of the next few pages is to provide the main information and
instructions needed to answer the second point. Given that the performance of an
Ethernet architecture is linked to many parameters, these pages do not provide all
the information needed to calculate the network performance. Their aim is to focus
on the following main aspects:
b
Guidelines for evaluating the network load so as to design an Ethernet network
that meets the application requirements
b
Application response time to be obtained according to the configuration used
(
see pages 5/41 to 5/43)
b
Processing capacities of Modicon M340, Modicon Premium and Modicon
Quantum platforms so as to be able to select the processor and define the number of
Ethernet connections required on the PLC according to the application (see pages
5/44
and 5/45)
Evaluating the network load
Introduction
When evaluating the load on an Ethernet network, all the communication services of
all the peripheral devices connected to the network need to be calculated.
Because of the outstanding performance of the Ethernet network, the load is often
less than the Ethernet network limits and does not significantly affect the application
response time. This is due to the high speed of the Ethernet network: the network
transaction time is less than 10% of the total application response time. In order to
ensure a low network load and avoid large theoretical calculations, it is highly
advisable to separate the collision domain so as to limit the network load, by using
only the switched network (tree, star or daisy-chain topology).
Performance
Modicon Premium automation
platform
Ethernet Modbus/TCP network
Performance
2
1
3
4
5
6
7
8
9
10