Do you feel stuck with the old 4-20 mA-based control system that was installed when the process plant was built?

The original system, which was state-of-the-art when originally put in, wasn't designed to supply all information necessary to streamline the process and breathe profitability into the now old facility. What is the control engineer to do? With the size of many process plants, revamping a complete control system to provide additional information is a major undertaking. Bus systems are often unfamiliar territory to control engineers. Replacement of all that old discrete copper wire and field devices seems inviting but the planning, expense, and work involved in changes at that level are often prohibitive. Is there another viable solution?

HART communication is a master/slave system. The handheld communicator,
available from dozens of Foundation members, is a secondary master used by a field
technician to 'talk' to devices for commissioning or maintenance-related data
gathering. Graphic courtesy of The HART Foundation.

Highway Addressable Remote Transducer or HART was originally a product of Fisher-Rosemount (F-R, Eden Prairie, Minn.). A 'child' of the 1980s, HART was developed as a proprietary communication system for process control systems that need extra information handling ability. The idea was simple-even if deceptively so. Layering a digital signal over the existing analog one would provide that ability. The line of 'smart' instruments that F-R was developing at its Minnesota location needed more communication ability in order to handle the addition of on-line commissioning and diagnostic functions.

Once the system was developed and proven with their own new instrumentation, F-R saw that the technology would greatly benefit 4-20 mA users by allowing them to adapt smart instrumentation to their processes. In a move reminiscent of the VHS technology giveaway, F-R released the technology free to the industry as an 'open' standard on which future device designs could be based.

Other instrumentation companies were quick to see the advantages of the protocol and adapted it to their instrumentation offerings. As the list grew, users' groups formed. In the early 90s with HART successful and out in the field, demands on its developer increased. In order to shift these support and monetary responsibilities, the HART Communication Foundation (HCF) was formed as a nonprofit trade organization in June 1993. It became the custodian of the protocol's standard and provides worldwide support for application of the technology.

The HART protocol uses the Bell 202 Frequency Shift Keying (FSK) standards to superimpose digital communication signals at a low level on top of an analog signal. The HART protocol communicates at 1,200 bps without interrupting the analog signal and allows a host application to get two or more digital updates per second from a field device. The FSK signal, which varies between ±0.5 mA, is either 1,200 or 2,200 Hz. 1,200 Hz represents a logical '1' and 2,200 Hz, a logical '0.' As the FSK signal is phase continuous, there is no interference with the 4-20 mA signal.

HART is a master/slave protocol, which means that a field (slave) device only speaks when spoken to by a master. The protocol can be used in various modes for communicating information to or from field instruments and the control or monitoring center. HART provides for up to two masters (primary and secondary). This allows a handheld or secondary communicator to be used without interfering with communications from the control or monitoring system or primary master.

The most commonly used communication mode is communication of digital information simultaneous with the analog transmission. Digital communication with field devices is either point-to-point or multidrop network configuration. Within the protocol there are other communication modes and installation options available that provide the necessary flexibility to make HART adaptable to a wide range of process control applications.

The digital-over-analog HART signal uses 1,200 Hz frequency for a logical '1' and 2,200 Hz for
a logical '0.' Its range is ±0.5 mA. Graphic courtesy of The HART Foundation.

There is no dearth of control communication systems available to control engineers interested in adding smart field devices to their process application. Any number of proprietary and open hybrid and bus systems are available.

According to Ron Helson, executive director of the HCF, there are definite advantages to using the foundation's open hybrid protocol.

'HART upgrades can be done on a limited basis and only involve the part of the system that needs smart devices. Since the communication wiring remains the same and only the HART interface need be added to the existing device's transmitter, changeovers require a smaller investment due to limited hardware replacement,' Mr. Helson says. 'This allows control personnel unfamiliar with digital communication to 'see' its benefits and learn the technology on an incremental basis, without disturbing the rest of the system,' he adds.'

HART integrates digitally with other existing plant networks and works with PC-based control using its own OPC-compatible software. Because its technology improvements are developed by the HCF, the protocol remains compatible in both directions with existing systems.

Mr. Helson admits that HART technology is not for everyone. 'Compared to emerging fieldbuses, HART's digital communication is much slower. It is intended for process control and, as such, can handle 80% of them. It is not intended for the much faster device-level buses found in many discrete manufacturing systems. HART also has networking ability as a multidrop system in an analog system. While good for variables like level and temperature, it is slower than a similar bus-based system,' he says.

Common misconceptions about HART are that it is a 'temporary' communications solution and is only good for commissioning and periodic maintenance of instruments. In fact, explains Mr. Nelson, HART gives an existing 4-20 mA system the ability to use tools for commissioning (reranging, units changing, etc.) and diagnostics built into retrofitted smart field devices. Its use can bring substantial savings to a maintenance effort and can facilitate the move to a preventive or predictive maintenance system. Additionally, HART facilitates the use of multivariable devices that allow collection of both direct and inferred process variable values.

With 80 million devices installed and between one and two million being sold per year worldwide, HART is hardly a flash-in-the-pan. With a base of 132 HCF member companies worldwide, the control engineer can find both devices and support for most any application. Among respondents to a Control Engineering networking survey in December 1999, 18% said they use HART.

For how long? According to Ron Helson, 'HART will be around for the next 10 to15 years.'