Crossing the Chasm

Digital Fieldbus Networks Bridge the Gap from Field Devices to the Control Room

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For more on fieldbuses, go to www.controlglobal.com/fieldbusguide.html

By Dan Hebert, P E, senior technical editor

Experts juggle swords, but the rest of us are better off just watching.  Put another way, just because someone somewhere has successfully installed a digital fieldbus network doesn’t mean it can by done by you or anyone at your company without proper training.

Those who don’t look before leaping across the fieldbus chasm are destined to end up like these necessarily anonymous respondents from a major U.S.-based engineering firm. “We have been reluctant to specify Foundation Fieldbus, HART, Profibus and other digital fieldbus networks,” says the first engineer. “We have seen the presentations, attended the brown bags, said thank you, and I hope we will continue to wait on this.”

He has good reason to be hesitant. “I have a friend working for a systems integrator who has been tearing out what is left of his hair for about two years trying to get a fieldbus network to work properly. Maybe there is a place for magic wires and magic devices in industrial applications where advanced data acquisition is justified and where the owner’s on-site technicians can understand and maintain it. But in the industry we serve, we do not want it or need it yet,” concludes our first respondent.

Another engineer summarizes his firm’s general opinion of digital fieldbus technology. “Fieldbus is great in theory, but for whatever the reason, in practice it doesn’t yet seem to live up to the promise. Some difficulties seem to be related to training and to understanding of the technology by designers and installers. Other problems seem to be related to the complexity of configuring the systems. Overall it sounds like the benefits are offset by the associated headaches.”

More Complex Than PC Networks

Consider the analogy to PC networks. You have at least one PC at work and likely a few more at home. You may even know a little bit about networking if you have a home wired or wireless LAN. But that doesn’t make you a computer networking expert. Likewise, you probably also know quite a bit about instrumentation and control systems. You may have even designed and installed one or two small-scale digital fieldbus instrument networks. But that doesn’t mean you are a digital fieldbus expert.

You know that to manage a large scale office network, your company needs at least a small group of experienced and highly qualified computer networking professionals.

If you want to make the transition from 4-20mA to a digital fieldbus, you will also need a small group of experienced and highly qualified digital fieldbus professionals. “There is a big learning curve for someone to become fully digital,” says Richard McCormick, principal at Mick Automation, Levis, Quebec. “This is especially true for instrument techs that may have limited experience working with PCs.”

Don’t make the mistake of thinking that a digital fieldbus network is simpler than an office network just because it has fewer nodes. You may have only 50 instruments, but other factors can make even a small digital fieldbus network as complex as a PC network with hundreds of users.

These factors include multiple incompatible protocols, harsh installation conditions and multiple vendors who would often rather compete than cooperate. “Not only is there a learning curve associated with first-time evolution to a digital fieldbus network, but one must also deal with incompatible device drivers and with multiple vendors when you need problem resolution,” observes Thomas Toms, lead engineer at Progress Energy, headquartered in Raleigh, N.C.

Profinet Integrates Fieldbus Systems

End users must often deal with multiple fieldbus networks in their plants. This can complicate network design, installation, and maintenance. Courtesy of Profibus International.

Courtesy of Profibus International

Dealing with multiple vendors requires care. “The master unit of one vendor’s network may not be fully compatible with another vendor’s spur blocks,” relates Philip Daniels, an engineer with the Wastewater Treatment Division of King County in Washington.

“One vendor may have the intrinsic barrier located at the master unit, while another may have it at the spur block. If the wrong combination of vendor products is selected, there will be no intrinsic barrier in the circuits,” adds Daniels.

“Another problem is addressing of the units. The Foundation Fieldbus network is just like any other network, and the addresses of the various components must be unique. We ended up addressing field devices just like our office network intranet addressing. Another issue that caused us problems was individual component programming. Each device has unique programming needs, and vendors do not always provide proper documentation,” concludes Daniels.

So how do you get up to speed? One way is to hire a digital fieldbus expert who has prior experience with the design, installation and maintenance of your desired network and your existing control system. Good luck finding such a person.

This is not like the PC world with its true and few interoperable standards, its long track record and its throngs of qualified and certified network engineers. You instead have to deal with multiple incompatible technologies, relatively new products and a paucity of experienced professionals.

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