Installation guide

Estimating Network Performance
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3.1 Overview
This chapter describes the major factors you should consider as you
plan the layout of your Modbus Plus network. It explains how you can
use MSTR and Peer Cop methods for communicating in your
application, and shows how your use of these methods affects network
performance. It gives examples of message handling between nodes,
and presents guidelines for predicting the performance of single and
multiple networks.
3.1.1 Your Network Performance Goal and Options
Your Goal The goal of your planning is to achieve a network design that meets
your needs for information transfer among the devices in your
application. The network’s message handling capacity must be
sufficient to assure that each device has the data it needs, within the
timing requirements of your application and with margins for safety.
The network design must also be able to support future modifications
and expansion to your application.
Each device presents unique requirements for obtaining data from the
other devices. Data requirements can range from occasional updates of
statistics to nearly continuous exchanges of large blocks of information.
Control applications that interact between nodes require a network
design that provides fast responses to requests for data.
When multiple devices share the same network, each device’s data
requirements should not be viewed as being isolated from the
requirements of the other devices. Each device has a need for access to
the network token. The length of time each device needs to transmit its
application messages affects the network’s token rotation time, and
therefore also affects the network’s access for the other devices.
Responses to data requests are also determined by the processing
speeds of the devices, such as the scantimes of
programmablecontrollers and the efficiency with which their
application programs have been designed. The network should be
viewed as an interactive array of devices, in which overall performance
is affected by the physical count of nodes, their combined data
requirements, and their data handling capabilities.