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From the very beginning, HART Communication filled a crucial need for digital communications from instrument to control and maintenance systems. As the needs of the process industries have grown and changed, HART Communication has evolved to meet those needs, and is clearly the choice of the process industries for the communications protocol of the future.
The Highway Addressable Remote Transducer (HART) Protocol became an open protocol in 1990; HART Communication Foundation (HCF) was established in 1993, multiple vendors began offering HART communication as an option, and eventually it became the standard I/O solution.
"As it relates to digital field communications for instrumentation," Michael Robinson, solutions business manager at Endress+Hauser notes, "HART has been in the marketplace as a reasonable solution for a substantial amount of time. The market has embraced the technology, and the implementation costs are relatively low for either a brownfield or greenfield installation."
Shannon Foos, process segment manager, fieldbus and asset management, at Rockwell Automation says, "Coming from the world of electrical control based on 4-20 mADC, HART provides added value by superimposing digital information on the analog loop-- an incremental step above the traditional 4-20 mA device. This incremental step was easy for both users and automation vendors to implement."
Robinson continues, "If you also consider that the technology is integrated into 'de facto' PV/CV field signal technology, and that most 4-20 mA instrumentation outputs are supplied with HART communication as a standard option with no additional cost to the user, it does not surprise me that its adoption has been significantly more successful than the other bus technologies or protocols."
Gary Prentice, national sales manager for Moore Industries International, puts it bluntly. "Even the earliest versions of HART filled the need to manually perform configuration and diagnostics remotely. Many (HART) users have not outgrown this basic need. More sophisticated users take advantage of the multiple dynamic variables with a single process penetration and the online diagnostics. Most users' needs continue to be met with HART, and they are comfortable with the technology."
In 2012, HART has more than 35 million installed instruments and devices, far surpassing any other process fieldbus. Garrett Schmidt, wireless product manager for Phoenix Contact explains why. "HART is the world leader of field device protocols because it is backward-compatible to standard 4-20 mA. You can use HART instruments without upgrading the control systems."
Mark Nixon, lead system architect at Emerson Process Management, says that the reason HART has more than twice as many installations as its nearest competitor is, "Simplicity. The industry has been using 4-20 mA and HART for over 20 years, so there is a wealth of experience to draw on and a very well-defined set of tools and processes that have been developed. WirelessHART simply built on the success already achieved with HART. The HCF and its member companies continue to drive HART and its related technologies for customer value. The standard continues to evolve so that real-world problems can be solved in a standards-driven manner. For example, the latest release of HART includes support for discrete-oriented devices such as on/off valves."
Jeff Dobos, ProComSol president, says HART is "inexpensive and easy to use, makes use of existing wiring in plants, and cost and installed base will keep it dominant for a long time."
Thomas Holmes, president of MACTek Corporation, sums it up. "HART is here to stay because no other field communication solution can cover the wide range of devices and applications—wired and wireless, analog and digital. Since HART includes the 4-20 mA signal, it is the only solution that works with the existing installed base of control, safety and monitoring systems and the installed base of measurement devices."
There are three basic modes of operation for the HART protocol.
In point-to-point mode the digital signals are superimposed on the 4-20 mADC loop current. One process variable (PV) is sent on the 4-20 mA signal, and other signals, such as secondary or calculated PVs (up to four or eight in newer devices), as well as device setup, calibration and diagnostic and maintenance status, can be transmitted digitally over the 4-20 mA signal.
Multi-drop signals are transmitted with the current loop set at 4 mA. Multi-drop mode allows up to 64 devices on a single loop. Multi-drop mode "has been very successful on RTUs in remote wellhead monitoring," says Jonas Berge, director of applied technology for Emerson Process Management, Singapore.
The third mode is WirelessHART. This mode is true HART, but the data is sent wirelessly using an IEEE 802.15.4 radio. "The goal with WirelessHART was always to extend the capability of the proven wired technology," says Ed Ladd, senior business development manager for process automation at Mitsubishi Electric Automation, Inc. "It was designed to provide a reliable sensor level network that could interface with current systems in the field."
WirelessHART was designed so it could be used to retrofit wired HART devices with a wireless adapter that is a native HART device and can be located either at the device or anywhere along the current loop. Thus, any HART device can be upgraded to act as a WirelessHART device, within the capabilities of its native HART version.