Installation guide
58 Estimating Network Performance
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Your Options You can plan your network application as a single network, with a
linear arrangement of nodes. You can also plan it as multiple
networks that are joined in a layered or hierarchical configuration.
The choices you make will be determined by your data requirements
between the nodes. Nodes which must exchange significant amounts
of data that is critical to the timing of some process should be
positioned on a compact network, with a bridge serving to forward
lesscriticaldata to devices on other networks.
Consider also how much of the data should be handled through
read/write message transactions between the nodes, and how much
through global database transactions. If your application requires that
devices must maintain multiple concurrent transactions with other
devices, consider how many data paths can be opened within the
devices.
As you choose your design options, your estimation of total throughput
for your network application should be conservative. When you
estimate ‘average’ performance, be aware that events in your
application will occur asynchronously, and will place heavier loads on
the network at various times. Instantaneous throughput between
devices in a timecritical process must remain within safety margins,
even with worst case loading.
By understanding the factors that affect your network’s performance,
and your options for device selection and programming, you can achieve
a network design that meets the goals for your current application and
future needs.
3.1.2 Design Options for I/O Servicing
If you are designing your network primarily for servicing I/O field
devices through DIO Drop Adapters and TIO modules, you will have
several important factors to consider.
Your network must service the I/O processes at a rate of speed that is
sufficient to control them efficiently and without excessive delay. In
general, the network’s I/O servicing rate will depend upon the amount
of nodes you employ, and the average message size. In addition, the
determinism or repeatability of the I/O servicing rate will be affected by
the types of node devices you connect to the network.
To assist you in planning for both of these factors (the speed of timing
for data transfers, and the repeatability of that timing), a guidebook is
available for network applications that are intended for I/O servicing.
See the Modbus Plus Network I/O Servicing Guide for further details.