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Scott Saunders, sales and marketing vice president at Moore Industries, adds, “Interoperability between vendors has been and always will be a bit of an art. Many vendors implement versions of standard fieldbuses, but users often verify that just because a transmitter supports a fieldbus doesn’t mean it will just start communicating out of the box. Organizations like the Fieldbus Foundation, Profibus Trade Organization, and OPC Foundation will continue to play a key role in vendor compliance with fieldbus standards.”
Saunders also thinks fieldbus is here to stay. “Foundation fieldbus and Profibus still have their places on the plant-floor, where wireless is less popular,” he says. “While many users are implementing wireless technology, many seasoned process control engineers still prefer wires to wireless. Supervisory control and data acquisition are often accomplished by wireless networks, but critical control loops are still hardwired. This is where fieldbus technology offers peace of mind to the process control engineer.”
Charles Piper, fieldbus product manager for Invensys Process Systems, also believes in fieldbus’ staying power. “You can expect Foundation fieldbus, HART, Profibus, DeviceNet, and other fieldbuses to be with us for some time to come,” he says. “They meet today’s needs quite well, and gain considerable inertia from large investments made by vendors and end users alike. You’re more likely to see gradual physical layer evolution than significant change in the upper communication and user layer portions of the technology behind these standards.”
But Which Fieldbus?
When it comes to choosing a fieldbus for process control, the big five appear to be Profibus (See Figure 4), Ethernet, Foundation fieldbus, DeviceNet, and HART. Some people claim that HART isn’t a fieldbus, but we beg to differ. Though HART doesn’t have all the bells and whistles of the others, it can perform many of the same functions.
|FIGURE 4: PROFINET POWERS PLANT|
The Bewag Power Station in Germany uses Foxboro controls and Profibus communications. Source: Foxboro
Sasol, a large chemical company in Sasolburg, South Africa, uses HART as a fieldbus. “We have excellent experience with HART technology, and have standardized on HART for our field devices,” says Johan Claassen, engineering manager at Sasol. At its plant in Midland, South Africa, Sasol has 3,500 HART instruments tied to a Honeywell PlantScape system. “We believe one of the improvement areas for HART is the speed of communication. Up to now this has not been problematic, but for larger installations this can become troublesome.”
Similarly, some may claim that DeviceNet is strictly for discrete automation, but Rockwell sees it as a critical part of a networking strategy for discrete and process control applications. “The networks we support share a common protocol enabling seamless flow of information between all components of Rockwell’s Integrated Architecture from the smallest device up through the enterprise business system,” explains McEldowney.
Likewise, others claim that Ethernet, by itself, isn’t a fieldbus. However, Ethernet is incorporated in several of the major fieldbuses in one form or another. “Ethernet is not threatening, but rather speeding up the installation and commissioning of Foundation fieldbus’ HSE, Profibus’ Profinet, DeviceNet’s EtherNet/IP, and some other fieldbuses,” notes Offner of Phoenix Contact.
Rockwell’s Appleby agrees, explaining, “It's important to remember the fieldbuses provide a standard for the data transmitted, while Ethernet provides a standard for transmitting the data. Networks define the physical link for moving the data, and fieldbuses define what the data is and means. Each technology brings something to the equation—it’s about the protocol and the data that's in the device.”
Some end users have used them all. For example, Dayton & Knight are consultants in North Vancouver, B.C., who work on water/wastewater systems, pump stations and treatment facilities. “We have used DeviceNet, Modbus, Profibus, and Ethernet on various projects,” says Doug Rhodes, manager of Dayton & Knight’s electrical power and automation group. “For example, we used DeviceNet to link drives and starters within treatment facilities. It’s robust, but a bit costly and tricky to set up. We used Modbus to link drives and starters within pump station MCCs. It’s less robust, but it’s economical and easy to set up. We used Profibus for instrumentation several years ago with no major issues, but would at least review Foundation fieldbus if a similar application arose today.”
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