Clarifications on Process Fieldbuses

As 2010 was winding down, I did a post Digital Busses Team Up for Successful Results in which I referenced a book Catching the Process Fieldbus–An Introduction to Profibus for Process Automation.

Over the holidays, I received an email from Emerson’s Jonas Berge, whom you may recall from many digital bus and electronic device description language (EDDL) standard-related posts. After reading the book, he had few clarifications he wanted to offer.

First, it’s important to note that control systems support many digital buses and each bus has its inherent strengths. Also field device suppliers, including Emerson, support many digital protocols, as evidenced by news of Rosemount 3051 pressure transmitters and Rosemount 644 temperature transmitters supporting Foundation fieldbus, Profibus PA, and HART.

On the subject of devices per bus segment, the book offered anecdotes with 24 and even 30 devices per bus. Although not stated, Jonas notes that this could only be for monitoring and not control applications since the control loop response would be very long with such a loaded bus. Multiple bus cycles would be required to carry data from input to output since the Profibus PA, Profibus DP, and controller are not synchronized with each other. Also, for hazardous locations where intrinsic safety is used, the number of devices per bus must be lower due to power limitations.

To meet typical process licensor control response requirements, less than 12 devices per bus is more realistic, comparable with Foundation Fieldbus. When control-in-the-field (CIF) is used with a Foundation fieldbus digital bus, the control execution is synchronized with the communication such that the entire loop from sensor to valve is executed in a single bus cycle–thus Foundation fieldbus is very fast.

Jonas had some clarifications on the physical layer of the various bus technologies. The book indicated that other process fieldbuses only have one type of physical layer available. He notes that HART supports Bell 202 FSK, IEEE 802.15.4, RS-485, and Ethernet, while Foundation fieldbus supports H1 and HSE.

The book indicated that Profibus requires knowledge of one protocol and one set of tools. Jonas notes that Profibus DP is based on RS-485 and uses the BT200 wire-checking tool while PA is based on IEC 61158-2 and uses the FBT-6 tester. The signals are different and Profibus DP has no bus power while Profibus PA does, and that these busses have different topologies. Jonas observes that these differences are comparable to an installation using Foundation fieldbus and Profibus DP for the variable speed drives and motor control centers. He pointed to an earlier post, Digitally Connected Variable Speed Drives, on applying the appropriate digital bus for the application.

One final area from the book with which Jonas wanted to take exception is profiles. The book indicated that common core parameters (profiles) are unheard of with other protocols. HART and Foundation fieldbus do it with universal/common-practice commands and standard transducer blocks.

I also wanted to share that Jonas has just started a LinkedIn group on the EDDL standard to share news and developments on this standard for field device / application interoperability.

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