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Tech Tip: Designing for Available Bandwidth

A frequently expressed concern of PROFINET users is bandwidth. Is there enough? How can I be sure?

This is a legitimate concern, prompting a reminder that there’s never a substitute for doing the engineering. Architect the system with bandwidth usage in mind. PROFINET messages tend to be small. But topology should be designed to minimize the chance of using too much bandwidth.

PROFINET supports many topologies: star, tree, line, ring, wired, and wireless. When using line topology remember that each device includes a switch which will introduce a small amount of latency. Placing a bandwidth-intensive device at the end of the line could result in excess bandwidth issues on the line.

PI offers a design guideline that includes a bandwidth calculation tool. This document will help the user through good design practices. It’s an excellent starting point in the design process.

So, you used the design guideline. You designed your network correctly, but stuff happens -unexpected stuff. For example: network floods. A device starts pumping out Ethernet frames. Maybe an unexpected non-PROFINET device is added to the network. Here’s how to anticipate and prevent the problem. Ethernet switches (even those in PROFINET devices) offer standard IT features that you can use. One feature is Simple Network Management Protocol (SNMP). SNMP can be used to read switch information like disconnected cable or excessive retries. Therefore you can include some screens in the HMI to track and alarm these errors.

And if you positively have to make sure that there’s bandwidth for your I/O messages, PROFINET does that too. Some PROFINET controllers and devices support bandwidth reservation. It’s part of PROFINET Isochronous Real Time (IRT). To give you an idea of how that works, check out this video of IRT in a drive application:

There’s never a substitute for doing the engineering. Use the design guideline to set your network topology. Use the diagnostic power of SNMP to alert on network problems.

For further study on PROFINET diagnostics like using SNMP:

Webinar: Diagnostics for PROFINET and Industrial Ethernet

InTech magazine article: Profibus and Profinet troubleshooting

For an explanation of SNMP and other IT protocols as they relate to PROFINET: PROFINET and IT Protocols

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