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By John Rezabek
Since beginning to commission fieldbus segments in early 2000, our site has used MTL-Relcom’s FBT-3 and FBT-6 segment testers, the Fluke “Scopemeter,” and the Pepperl + Fuchs Advanced Diagnostics Module (ADM).
My plant’s main issue has been failing power conditioners—back in 1999, our systems integrator mounted all of the originals right next to one another in rows of eight or more. The consequence is overheating, causing degradation which manifests itself as low-frequency noise. Often, the first clue we get is communication errors from devices.
The Fluke Scope Meter shows the issue when it’s more advanced. In at least one case, the problem wasn’t obvious on the scope, but was detected on the FBT-6. The Relcom and P+F offerings “listen” to all the signals on the network and accumulate data (such as peak noise in the frequency bands) the whole time they are attached. This is better than staring at a screen and trying to catch something suspicious.
I haven’t tried the P+F on any of these because I have the “backplane” version, and all our P+F conditioners have been solid. P+F does have a portable model with identical functionality.
The Relcom offerings have a two-button interface for scrolling through the readings, a segment-powered LCD display (or USB-powered when one is uploading files to Excel on the FBT-6 only). Most everything you need to ensure adequate signal strength, signal-to-noise, shield termination errors, etc., is there. The jazzy coolness factor is low, but you get a wealth of information unavailable with a scope alone.
For extra jazzy coolness, it’s tough to beat the P+F ADM. You do need a Windows PC, but in theory you won’t need a scope, since it already has this function built in. The interface is not “real time.” It samples for a few seconds, then displays what the module captured, which can then be zoomed in or out by many powers of 10. This is useful; even with a scope we “zoom way out” to look for the low-frequency variation we see when power conditioners start to die. You can set it to capture a certain transaction from a node (e.g. “token pass from address 33”). I think this would be invaluable if you had a suspect device—a potential “bad actor” that wasn’t playing well. This function could be used to “clear” or “indict” a given device post-haste. The P+F also has preset (but adjustable) alarm levels for numerous parameters (noise, jitter, voltage levels, etc.) and lights a red LED if it determines something’s amiss.
An interface for the ADM’s FDT (“field device tool”) is in the works for Yokogawa’s PRM and Emerson’s AMS. Presently we have to use the “Pactware” DTM. The process of getting it going with the P+F ADM is reminiscent of SAP, or maybe like finding the “Easter egg” in Excel 2000. I’m not sure where PACTware ands and the P+F DTM begins, but overall, it couls be more streamline and user-friendly. Once you get the P+F DTM open and pointed to the ADM, it is fairly easy and intuitive to access the diagnostics. To be fair, my interface may be more complex, since it’s intended for 16 or more ADMs. The single-unit interface may be easier.
The portable ADM connects to a PC via USB. Since it’s already certified for use in Division 2/ Zone 2 areas, you could obtain an IS laptop (GD-Itronix’s “GoBook XR-1” is one that offers this option) and connect to segments in most classified areas. The Relcom FBT-6 is approved for use in Division 1/ Zone 0.
So what do I grab when troubleshooting a suspect segment? More often than not, it’s the FBT-6. For me, it’s quicker, easier and detects perhaps 99% or more of likely issues. If I were putting 20 or 200 segments out in Siberia or the North Sea, I would give strong consideration to the P+F “fixed” modules or their competitors. I get the impression the P+F engineers set out to design a slick multi-segment “fixed” solution and adapted it to the portable. The way we work—with no “dedicated” PC for the DTM—it takes more time and effort to get the interface going. If you were doing a large project and had a PC “parked” in the rack room for this purpose, it would be less of an issue.
There’s another portable tool: Emerson’s 375 Communicator. We haven’t tried one for testing fieldbus segments, but it has some of the same capabilities as the Relcom FBT series, according to its data sheets. If you’re using it as an aid in commissioning fieldbus segments, I’d be interested to hear about your experiences.
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