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Mixing instrumentation protocols
January 26, 2012
Dear Control Engineering: Looking at the article about the HART Plant of the Year, it sounds like that Shell facility uses both HART and Foundation fieldbus. How much can you mix those technologies?
You’re probably referring to this statement: “All the functionalities, beginning with pre-commissioning to normal operation, were the same for HART and Foundation fieldbus devices. Shell uses both and has taken full advantage of both technologies; the biggest benefit being that we did not have to subject our HART devices to any separate interoperability testing.”
The simple answer is that you can’t mix the two on the device level, but you certainly can have both technologies in the same plant. I suspect there are few, if any plants with more than a handful of tags that have 100% of their devices on fieldbus networks.
FF devices only work on fieldbus segments with fieldbus I/O. FF is an all-digital protocol, and between the two, it can carry more information.
HART is a hybrid using a 4-20 mA analog signal to carry the main PV with a digital overlay that carries additional variables, diagnostics, and configuration information. It is possible to use HART devices as purely analog without ever accessing the digital overlay. The two protocols can only talk to their respective I/O, and the device transmitters are different.
The interoperability testing the author refers to is likely something of a pre-configuration process which normally has to be done to both kinds of devices, at least to some extent.
Here are some additional articles that can fill in the more of the blanks:
Peter Welander, pwelander(at)cfemedia.com