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What Is Fieldbus?

Asked on Nov 28 2007 4:14:41:000PM

Q

I would like to know exactly what is fieldbus (is this cables or devices) and why we need them. Also, what is the difference between modbus and profibus?

Noton Notone, Kuwait
A"Fieldbus" is a technology that provides 2-way communications between a "smart" field instrument or other device in the field (process) or shop floor (discrete manufacturing) and a host controller. Examples of fieldbuses are Foundation Fieldbus, Profibus-PA, and Modbus. Foundation Fieldbus additionally allows most of the control logic to be solved by the intelligence in the field instrument or control valve positioner. Profibus-PA is not designed for control loops to be solved in the field instruments, but the signal conditioning computations can be done in Profibus-PA instruments. Modbus is really designed for communications between two computing devices in which the data is mapped to "registers" in a well-defined way.

If your choice is to perform all computations including signal conditioning and closed-loop control in centralized controllers, and your instruments have no computation capability, then you do not need a fieldbus.

Meet the expert

Richard H. Caro

CEO, CMC Associates

In addition to running CMC Associates, Dick is the Chairman of ISA SP50 and formerly of IEC (International Electrotechnical Committee) Fieldbus Standards Committees. Before joining ARC Advisory Group, Dick held the position of Senior Manager with Arthur D. Little, Inc. in Cambridge, MA, and was a founder of Autech Data Systems, and director of marketing at ModComp. In the 1970s, The Foxboro Company employed Dick in both development and marketing positions. He holds a BS and MS in Chemical Engineering, and an MBA. He holds the rank of Fellow of the ISA, and is a Certified Automation Professional. In 2005 Dick was named to the Process Automation Hall of Fame. He has published three books on automation networks.
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