From the perspective of 2013, it seems unlikely that back in the 1980s the architects of the Highway Addressable Remote Transducer (HART) protocol would have envisioned that three decades later HART technology would still dominate the process instrumentation communication landscape, much less that the majority of process automation devices and systems sold today would still speak HART.
The original wired HART specification, which superimposed a frequency shift-keyed (FSK) communication scheme atop the industry standard 4-20mA electronic transmitter signal, was to all appearances a bridge to the fieldbus standard that was soon to debut. But a funny thing happened on the way to an all-digital field.
For users, HART technology offered a compelling, "out-of-the-box" value proposition for streamlining instrument configuration and commissioning tasks via a handheld communicator or PC while maintaining the familiar and reliable 4-20mA signal. Many users of the time didn't see much farther than this. And for instrument suppliers, it was relatively easy -- and economical -- to meet this relatively limited use case by bundling the HART protocol into an analog transmitter that already packed a board's worth of integrated circuits. As a result, many instrument-makers settled on HART as a de facto base level offering. Meanwhile, by the time industry eventually settled on Foundation and Profibus-PA as its primary pure digital approaches to field instrument networking, millions of HART-capable devices already were installed, bolstering familiarity and acceptance of the HART standard among users.
This virtuous cycle of low cost, low complexity, familiarity, backward compatibility and reliability -- combined with the responsive support of a broad supplier base united through the HART Communication Foundation that turns 20 (and counting) this year -- add up to an enormous installed base and vigorous ecosystem that to this day continue to find new ways to leverage HART technology for the business benefit of the process industries.
The Case for Simplicity
A big part of the HART standard's continued appeal is that end users really don't want to think about communication protocols, says Bob Karschnia, vice president, wireless, Emerson Process Management. "Users want to keep their plant up and running, maximize productivity, ensure safety and environmental protection. They're looking for technology that is extremely reliable, proven and easy to use."
"With HART, it's simple," adds Frank Fengler, head of device integration for ABB, noting that some 95% of the company's current transmitter shipments are HART-capable. Only 5% are full digital Foundation fieldbus or Profibus-PA, and most of those are associated with large grassroots projects, Fengler says. "With Profibus-PA or Foundation fieldbus you need a separate mechanism to get the process variable into the host. Systems people have to interact with device people. But with HART, no configuration file is needed for simple applications," Fengler says.
All other network technologies since HART have added complexities, Emerson's Karshnia agrees. For example, while Foundation fieldbus is a powerful tool, its potential to facilitate control-in-the-field blurred the functional lines between instrumentation and control departments within user organizations. "HART stayed true to its calling. The supplier members of the HART Communication Foundation effectively said, ‘We're going to take communications off the table in terms of competitiveness.' They've also continued to invest in and evolve the protocol, which is why they were first with wireless," Karschnia says.
Digital networks also bring with them the need for a new set of skills and training regimens. "But any maintenance tech that can do 4-20mA can deal with HART," says John Yingst of Honeywell Process Solutions, product manager for the company's Field Device Manager (FDM) plant asset management solution. "You can get diagnostics right away and have very low risk."
Further, the need for additional training presented by digital networks will persist, even with the next generation of a "digital native" workforce, notes Yuri Zelenkov, product manager, process networks, for Rockwell Automation. Millennials may relate more intuitively to handheld apps and collaboration platforms, Zelenkov contends, "but not to the physics of what happens on a bus. The concept of point-to-point wiring is easy to understand, and mistakes are seldom made."
"HART is a relatively inexpensive protocol, and it's more than adequate for most process measurements," adds Thorsten Szczepanski, CEO of ifak system, a developer of communication interfaces for a range of industrial protocols. "HART does what's needed for device communications, and with industrial Ethernet as a backbone, it's a powerful combination," Szczepanski says.
Continued backward compatibility with the same set of tools with which users have three decades of familiarity contributes mightily to the staying power and low risk of HART communication, according to Jim Shields, Fluke product marketing manager for process calibration tools. "Our industry is necessarily conservative. It's safety first -- for technicians and for the public," Shields notes. "HART speaks to all that. Meanwhile it's evolving without leaving its legacy behind. WirelessHART is new, for example, but it doesn't change any of the existing infrastructure."