Redundant vs. fault tolerant fieldbus wiring

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Dear Control Engineering: I was watching one of the videos from Emerson Global Users Exchange, an interview with Scott Saunders from MooreHawke. We’ve been looking at installing some Foundation fieldbus segments, and I was interested in his comments about redundant and fault tolerant segments. What is the difference between those two approaches?

One of the things that users find scary about fieldbus architecture in general is that all the devices on a segment send their data over one cable, and if anything happens to that cable or supporting components, communication is lost to that whole group of devices. Users have tried to mitigate this by figuring out ways to add redundant components but still have all the signals transmitted by a single trunk line.

There are other approaches that create a fault tolerant situation where there are actually two trunk lines going to the segment. For all practical purposes, the segment is connected from both ends, and if a trunk line or power supply is damaged, the devices can still communicate in the other direction. This approach works equally well with Foundation fieldbus H1 and Profibus PA.

Redundant and fault tolerant wiring schemes add cost, but if you analyze the degree of criticality of the devices on a given segment, the cost of lost production from a failed segment can be much higher. Read Economics of Fault-Tolerant Fieldbus Wiring for a more in-depth discussion of these wiring strategies, with their costs and relative effectiveness.

--Peter Welander, pwelander(at)cfemedia.com