Understanding speed when it comes to fieldbus networks

Fieldbus networks have a slow data transfer rate. Why can't they be faster?

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Dear Control Engineering: I was reading the article about recent improvements to the Foundation fieldbus physical layer. It says that the data transfer rate is 31.25 kbit/sec. Isn’t that really slow? Why don’t they use a faster protocol?

You’re correct, 31.25 kbit/sec. is slow, and there are indeed much faster protocols. For starters, consider plain old USB that you see everywhere. USB 2 typically runs at 480 Mbit/sec., and USB 3 can hit 5 Gbit/sec. However, you have to look at it in context.

First, a device-level fieldbus does not have to carry large amounts of data. We’re talking about a process variable and a block of diagnostic information, so the file size is really small.

Second, the protocol is optimized to carry this kind of information. Ethernet is much faster, but it is very inefficient at carrying small packets of data because it has too much overhead. A fieldbus is typically better at this kind of application in spite of being slower.

Third, both Foundation fieldbus and Profibus PA (which use the same physical layer) are designed to run in hazardous environments. A slow data rate helps keep the power level very low. This is key for working in environments such as oil refineries.

Ultimately, it all comes down to the idea that a fieldbus protocol is purpose designed to perform very specific tasks in very specific conditions very efficiently. It does not have to serve as a mechanism to download photos from your phone one minute and communicate with an external disk drive the next. It may be slow, but it gets the job done.

Peter Welander, pwelander(at)cfemedia.com