Reviewed resource: Profibus PA and Foundation Fieldbus—a cost comparison

-- Control Engineering, 2/6/2008

We recently received a new whitepaper from the Profibus Trade Organization (PTO) written by James Powell, a Siemens engineer, on the topic of fieldbus costs. The conclusion of the paper is that Profibus is less expensive than Foundation Fieldbus. You can take that claim as you will, but the larger issues of how the two platforms differ make for a more interesting discussion and a reason to read the whitepaper. Download the paper at the Control Engineering resource center.

This functionality has been a topic of much thought recently because of an article on fieldbus applications coming in the March issue of Control Engineering. In doing that research, I have heard users discuss the pros and cons of the two main instrumentation fieldbus protocols: Foundation Fieldbus (FF) and Profibus PA. So far, price comparisons have not been an issue, but differences of functionality and availability of devices have.

While the two platforms have many similarities, there are two key functional differences:

FF supports control in the field (defined as devices that can run their own control loops) which Profibus doesn’t. FF achieves this by placing control functionality in individual devices. Profibus users regulate loops from the DCS through the network.

Profibus is more adaptable to supporting hybrid networks (defined as networks that can operate drives and other discrete equipment, in addition to instrumentation and valves, using combined PA and DP) which FF doesn’t do, or at least doesn’t do well enough to be practical. Plants that use FF for instrumentation and valves typically deploy Profibus DP, DeviceNet, or something else for other types of equipment. While this approach works and is commonly used, a plant has to support multiple platforms from different vendors.

One user that I interviewed at a paper mill said his company had researched the topic extensively and decided to use Foundation Fieldbus for instrumentation to take advantage of the control capability. He went on to say that they also run Profibus DP as a second system to support drives, which is one of the points made in the whitepaper. Those FF-enabled instruments require additional circuitry in the device to support those control capabilities (whether you use them or not), which accounts for higher costs per device.

Cost, in general, is not one of the major concerns when deciding whether to use fieldbus networking or not, or which platform. The ultimate value of more sophisticated architectures over traditional hard-wiring comes from shortened commissioning times, simpler troubleshooting techniques, instrumentation diagnostics, and more effective maintenance to name a few. In some cases there are actual savings on wiring, but this is usually a relatively minor issue. The benefit of the whitepaper is that it will help you consider some of these functional issues. That in itself has value.

—Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Process & Advanced Control Monthly
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