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Why HART and Foundation Fieldbus Don't Just Get Married

Dec. 10, 2014
The Disparate Origins of These Protocols Mean a One-Size-Fits-All Heir Is Unlikely
About John Rezabek
John Rezabek is a process control specialist for ISP Corp., Lima, Ohio. Email him at [email protected]How do you tell a HART device from a Foundation fieldbus (FF) device? There have been times when we've puzzled over it. Did the supplier make a mistake and send us a HART device when we ordered FF? On the surface, devices of the same heritage and generation have little beyond a subtle difference in the model number to distinguish themselves. With the merging of the HART Communication Foundation and the Fieldbus Foundation, some have wondered if there will be some new offspring, or if devices from one protocol will become conversant in the other. Foundation fieldbus and HART technologies have a lot of things in common. Why wouldn't they play nicely together some day?

We can begin by stating, the Foundation fieldbus/HART merger has no goal aiming to make one protocol out of the two. The two technologies are intended to progress on their individual paths, and neither has "convergence" on the roadmap. That doesn't mean that there won't be some common territories along those paths, as there have been already. The two have covered a lot of common ground with enhanced electronic device description language (EDDL) and NE 107 (NAMUR role-based diagnostics), and will be progressing through the specifications for field device integration (FDI) concurrently. But while moving along similar paths, the two technologies aren't likely to mate or bring forth an heir to whom they'll pass all their most valued capabilities.

To understand why this duo won't be doing a rumba, one has to revisit the origins of these two similar, yet different technologies. Why do we have HART? HART came about as an answer to the first (proprietary) protocol to feature complete digital integration of mainstream field devices with a leading process-industry host: Honeywell "digitally enhanced" (DE). Such devices communicated digitally with the Honeywell LCN-based controllers of the late 1980s. Honeywell was winning projects by bundling the DE transmitters with its DCS, and its competitors needed a response.

See Also: Fieldbus Protocol Foundations Merge

[pullquote]In those days, a 300-baud modem for a dial-up connection was considered pretty geeky, so when HART adopted a 1,200-baud communication rate, no one thought it terribly slow. Superimposing this relatively high-frequency signal on the low-frequency (e.g., 10- Hz) signal of analog 4-20 mA transmitters enabled HART to preserve the backward compatibility and interoperability that makes it so popular and ubiquitous today. HART communicates 1s and 0s by "frequency shift keying," that is, modulating the frequency of the signal. It would be like me singing you two notes, say "Do" and "So", where "Do" was a zero and "So" was a one. Those little HART devices are humming their signal back to the host or handheld communicator. But why don't Foundation fieldbus devices have an ear for the HART music?

FF grew out of the ISA SP50 committee, the same one that created the 4-20 mA standard for analog communications. The committee had high participation from end users who desired a single, interoperable, vendor-independent bus for control and digital integration of field devices. FF wasn't just concerned with handy tools for calibration, range change and fetching useful diagnostics; it sought to create a complete digital infrastructure where devices could communicate with each other, executing a repertoire of function blocks that included closed-loop control, i.e., "control in the field." The physical layer for FF is twisted-pair copper—the same as HART—but instead of humming discretely on top of 4-20 mA, the devices blast a square wave of +/- 10 mA into an impedance of about 100 ohms, at a fixed frequency more than 25 times the characteristic frequency of HART devices—31.25 kHz. They aren't listening for a tune. 1s and 0s are determined by "zero crossings." That's the "direction" of crossing high-to-low (a "1") or low-to-high (a "0") of the square wave, in a window of about a millisecond. The frequency, which is slow compared to the Ethernet transmission speeds that are commonplace today, was chosen for backward compatibility to existing analog twisted-pair wiring, noise immunity and a network maximum length of almost two kilometers with no switches or repeaters.

The chipware and boards needed to transmit and receive these two protocol cousins are therefore very different, and the effort to adapt a device to contain both probably exceeds the cost of just stocking one of each. There are some skunkworks where new chips are being brewed that may reduce the cost, so while HART and FF may never interoperate, maybe someday they'll just be a board-swap apart.

About the Author

John Rezabek | Contributing Editor

John Rezabek is a contributing editor to Control

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