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Friends or Foes?

As the International Electrotechnical Commission ( Technical Committee 65’s Working Group 7 polished its definitions of electronic device descriptions and the languages (EDDLs) to support them, the need arose to define something relating to field-instrument repair technicians’ ability to maintain such instruments, recalls network consultant Dick Caro, principal and founder of CMC Associates (, Acton, Mass.

But because of Foundation Fieldbus (FF) and Profibus International (PI) discord during that standards-setting process, ABB ( introduced field device tool (FDT) in lieu of EDDLs, he recalls. When IEC published IEC 61804-2 (“Function blocks [FB] for process control–Part 2: Specification of FB concept and Electronic Device Description Language”), though, FF and PI ended their spat, he remembers. Then, together with the Hart Foundation, they agreed to accept WG-7’s definitions of EDDLs, Caro recollects.

Yet, EDDL still competes with FDT-DTM (for Device-Type Manager). It finds use almost exclusively for device configuration, says Tom Wallace, PlantWeb global marketing manager with Emerson Process Management’s Rosemount Division (, Chanhassen, Minn. “That pretty much could be implemented using EDDL.”

Their main differentiator is operating platforms. FDT is an environment like a Web browser, Wallace explains, adding that DTM, used with FDT, is an application like a Web page. A concern he voices is FDT’s Windows-specific—and, sometimes, even Windows-service-pack-specific—dependence. While that may be an important consideration, it’s a non-issue, Caro believes. Why? Noting the OPC Foundation supports the IEC standard, he adds, “OPC (a connectivity standard) is the dominant universal language of standardizing systems—and it’s strongly dependent on Microsoft.”

Calling EDDL and FDT complementary, Invensys Process System’s ( Charlie Piper offers his view on their difference. “FDT standardizes interfaces between software modules contained within the PC (personal computer) used for asset management,” says Piper, who is Invensys’ Foxboro, Mass.-based manager of fieldbus programs. It is well suited for run-time operations, he asserts. And by enabling specialized user interfaces for advanced device performance and diagnostic analyses, FDT moves the user interface to the next level, Piper explains. “This targets improved device uptime and maintenance staff productivity.”

EDDL standardizes a language device vendors use to describe their devices, Piper says. And for most field devices, EDDL covers all functionalities, adds Wallace. But how are exceptions handled? In a Windows programming environment with a snap-on application in an asset-management application, Wallace says. However, if vendors had consistency in FDT, Wallace believes the suppliers could write just one DTM for exceptions that EDDL doesn’t handle. He doesn’t believe this capability exists yet, though.

Acknowledging that neither EDDL nor FDT-DTM is a complete solution unto itself, Wallace also notes another FDT limitation. “For existing field devices, device descriptions (DDs) are forward compatible. What that means is that any installed device with a DD can interface and work with an EDDL host, but not with an FDT-DTM host,” he says. Still, FDT-DTM can create elegant graphical interfaces, something that is user-friendly, which is not necessarily true for EDDL, Caro says.

Clearly, opinions differ on which technology is preferred and why. Caro calls EDDL the wave of the future with field instruments. “It is important because it adds the maintenance capability, including calibration,” he adds. However, Piper asserts EDDL and FDT compatibility. “They’re an example of how technologies can work together for improving plant performance across the entire asset lifecycle.”

C. Kenna Amos,, is an Automation World Contributing Editor.

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