E-Zine January 2010
Analog process instrumentation required a dedicated wire pair running from the instrument in the field directly to the termination junctions on the controller. The pair can consist of several cable runs, including the instrument to a junction box, junction box to marshalling cabinet, marshalling cabinet to other marshalling cabinets, and the control room marshalling cabinet to the correct termination wiring on the multifunction or dedicated loop controller. Power for the analog instrument is typically supplied at the field marshalling cabinet. All of this is carefully designed to assure that there is electrical continuity between the instrument in the field and the controller terminations. Each termination should be field tested to assure this continuity. This checkout usually entails a technician at each end to "ring-out" the connection. In effect, the address of the field instrument is determined by the termination on the controller.
Fieldbuses were created to eliminate much of this wiring complexity by sharing wires among several field instruments, and by addressing the field devices using software. By doing this, it became possible to also reduce the cost of wiring and termination design and engineering, since a continuous wire pair was no longer required for each instrument since addressing no longer depended upon controller termination location, but was handled using software. Further, installation labor could be reduced since verification of the connections of one (long) wire pair for each device was no longer necessary.
The term “fieldbus” has become generic and is frequently applied, especially in Europe, to any network used in industrial automation: process control, factory automation, materials handling, or motion control. In North America, the term “fieldbus” is usually associated with the versions used exclusively for process control, especially FOUNDATION™ Fieldbus and PROFIBUS-PA. Adding to the confusion, the Fieldbus Foundation uses the term "FOUNDATION™ Fieldbus" to name the particular set of specifications representing their particular version of the process control-oriented fieldbus.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control