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Which Fieldbus Is For You?

By Dick Caro

E-Zine October 2005

Fieldbus is a generic term applied to an open or generic network of automation devices used for factory automation or process control. However, in this article, the term "fieldbus" is limited to the specialized fieldbuses for process control.

HART was the first fieldbus and was initially designed with networking capabilities. However, it has been the relatively large turndown, wide rangeability, and the ability change the range of HART transmitters after installation that has enabled its strong acceptance. These features reduce the number of models for suppliers, and the cost of operational units in spare parts inventory for users. Only one unit of a type usually needs to be stocked to take the place of many similar units operating at different ranges. This economy has helped make HART widely accepted.

FOUNDATIONT Fieldbus H1 was designed to reduce the cost of wiring field instrumentation by using the international fieldbus standard developed by ISA for this purpose. Similarly, the use of the same type of wide ranging field transmitters as HART enables a similar inventory reduction. PROFIBUS-PA was developed to accomplish these same objectives using a different higher-level bus protocol with identical bus wiring and signaling protocol. FOUNDATIONT Fieldbus H1 (but not PROFIBUS-PA or HART) allows Field Control, where control loops can be executed in the field instrumentation. This can also reduce the cost of the control system by reducing the number of multifunction controllers used.

With the introduction of FOUNDATIONT Fieldbus HSE, the cost of process controllers is further reduced by performing more process control in field devices. The H1 version of FOUNDATIONT Fieldbus practically requires the elements of the control loop to be located on the same bus segment. HSE relaxes this restriction when configuring controls in field devices. As a result, the number of multifunction process controllers in the distributed control system (DCS) can be reduced, thereby lowering the cost of the control system. Nonetheless, there is considerable discussion among users and suppliers regarding the trade-offs associated with locating control in field devices.

FOUNDATIONT Fieldbus HSE also allows several ways to reduce the cost of the long cables running from the field to the control room. Since high speed Ethernet is utilized, commercially available equipment to extend Ethernet can be used. This includes the use of fiber optic cable and Ethernet repeaters as well as the wireless solutions now available for Ethernet.

Because there are many details involved in the selection, installation, and maintenance of network-based control systems, my recent book The Consumer Guide to Fieldbus Network Equipment for Process Control focuses on the transition in thought between conventional analog field instruments in a classical control system and Field Control. A control system with Field Control can be expected to deliver the inventory cost reduction of HART, the wiring savings of FOUNDATIONT Fieldbus H1, and the controller savings of Field Control. Fieldbus and control system vendor selection is a tradeoff because fieldbus selection limits instrument availability, whereas control system selection limits fieldbus availability.

While PROFIBUS-PA can be expected to achieve the inventory reduction of HART and the wiring savings of a fully capable fieldbus, it does not support Field Control and therefore cannot achieve the controller savings of FOUNDATIONT Fieldbus. However, there are those companies who prefer a more traditional approach to control systems, for which PROFIBUS-PA will be an excellent solution. There are considerable savings that can be obtained by using the intelligence embedded in PROFIBUS-PA field transmitters.

ISSN 1538-5280

Spitzer and Boyes, LLC
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