The HART Protocol
- A Solution Enabling Technology -

ĒšThe HART Protocol - An Overview
ĒšThe Benefits of Fieldbus Technologies Today!
ĒšHART Technology Support

By Ron Helson, Director of the HART Communication Foundation

The HART® Field Communications Protocol is widely accepted in the industry as the standard for digitally enhanced 4-20mA communication with smart field instruments. A wide range of products from an increasing number of suppliers are available today, and many more are in development. The enhanced two-way communication capability of instruments using the HART protocol can significantly improve plant information management, provide solutions to today's business challenges, and yield substantial cost savings. Initial installation/commissioning savings of $400 to $500 per instrument and annual maintenance/operations savings of $100 to $200 per instrument are commonly reported.


For many years, the field communication standard for process automation equipment has been a milliamp (mA) analog current signal. The milliamp current signal varies within a range of 4-20mA in proportion to the process variable being represented. In typical applications a signal of 4mA will correspond to the lower limit (0%) of the calibrated range and 20mA will correspond to the upper limit (100%) of the calibrated range. If the system is calibrated for 0-100 PSI, then an analog current signal of 12mA (50% of range) will correspond to a pressure of 50PSI. Virtually all installed systems use this international standard for communicating process variable information between process automation equipment.

HART Field Communications Protocol extends this 4-20mA standard to enhance communication with smart field instruments. The HART protocol was designed specifically for use with intelligent measurement and control instruments which traditionally communicate using 4-20mA analog signals. HART preserves the 4-20mA signal and enables two-way digital communications to occur without disturbing the integrity of the 4-20mA signal. Unlike other digital communication technologies, the HART protocol maintains compatibility with existing 4-20mA systems, and in doing so, provides users with a uniquely backward compatible solution. HART Communication Protocol is well-established as the "de facto" industry standard for digitally enhanced 4-20mA field communication.

Fig 1

HART Field Communications Protocol is an extremely important technology for both end users and suppliers. HART technology is a major step in the evolution of process control, and as an enabling technology is bringing significant innovation in the capabilities of field instrument systems. The enhanced communications capability of intelligent field instruments employing the HART protocol, offers significantly greater functionality and improved performance over traditional 4-20mA analog devices. The HART protocol permits the process variable to continue to be transmitted by the 4-20mA analog signal and additional information pertaining to other variable, parameters, device configuration, calibration, and device diagnostics to be transmitted digitally at the same time. Thus, a wealth of additional information related to plant operation is available to central control or monitoring systems through HART communications.

A wide breadth of products supporting the HART protocol are available from major instrumentation suppliers, and the number of products and suppliers incorporating the technology continues to grow. The HART protocol provides many benefits today that are promised by fieldbus technologies in the future, and yet retains the compatibility and familiarity of existing 4-20mA systems. The HART protocol is field proven, simple to implement, use and maintain. HART technology is being used in a wide variety of applications worldwide to gain significant improvements in plant performance, provide solutions to regulatory compliance issues (ISO 9000, OSHA, EPA, DOT, etc.) and realise substantial cost savings in initial installation/commissioning and ongoing maintenance/operations. Current estimates exceed 4 million installations worldwide.

The HART Protocol - An Overview

Fig 2HART is an acronym for "Highway Addressable Remote Transducer". The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA as show in Figures 1 and 2. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. The HART protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.

HART is a master/slave protocol which means that a field (slave) device only speaks when spoken to by a master. The HART protocolfig 3 can be used in various modes for communicating information to/from smart field instruments and central control or monitoring systems. HART provides for up to two masters (primary and secondary) as show in Figure 3. This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system. The most commonly employed HART communication mode is master/slave communication of digital information simultaneous with transmission of the 4-20mA signal as shown in Figure 4. The HART protocol permits all digital communication with field devices in either point-to-point or multidrop network configuration. Figure 5 highlights the optional "burst" communication mode where a single slave device can continuously broadcast a standard HART reply message. Higher update rates are possible with this optional digital communication mode and use is normally restricted to point-to-point topologies.

Fig 4

Considerable installation savings are possible with the multidrop networking capability of HART which allows multiple field devices to be connected to the same pair of wires. In multidrop applications, communication with field devices is restricted to digital only as the loop current is fixed at a minimum value and loses any meaning relative to the process. From an installation perspective, HART signals are carried over the same wiring as is typically used for conventional 4-20mA analog instruments today. As one might expect, cable run lengths can be longer for individually shielded twisted pairs, than for multiple twisted pairs with an overall shield as reflected in Figure 6. For short distance, unshield cables may be used. HART compatible intrinsic safety barriers and isolators are also available which pass the HART signals for use in hazardous areas.

Fig 5

The HART Command Set is organised into three groups and provides read/write access to the wealth of additional information available in smart field instruments employing this technology. Universal Commands must be implemented by all HART devices and provide interoperability across the large and growing base of products from different suppliers supporting the HART technology. Universal Commands provide access to information that is useful in normal plant operation such as the instrument manufacturer, model, tag, serial number, descriptor, range limits, and process variables. Common Practice Commands provide access to functions which can be carried out by many devices though not all, and Device Specific Commands provide access to functions which may be

Fig 6

unique to a particular device. Figure 7 highlights the type of information that can be obtained from these devices. The integrity of HART communication is very secure as status information is included with every reply message and extensive error checking occurs with each transaction. Up to four process variables can be communicated in one HART message and each device may have up to 256 variables.

fig 7

fig 8Device Description Language (DDL), a recent enhancement to the HART technology, extends interoperability to a higher level than provided through the Universal and common Practice Commands. As reflected in Figure 8, DDL provides a field device (slave) product developer with the means to create a complete description of their instrument and all relevant characteristics, such that it can talk to any host device using the language. This is analogous to a printer driver in the personal computer world which enables an application to talk with a printer such that what gets printed on the page is what was expected by the application. Universal hand-held communicators capable of configuring any HART-based instrument through DDL are available today. Broader application in other types of host systems is expected. The HART Communication Foundation manages the centralised library of all registered Device Descriptions and DDL is being supported by all members of the Foundation.

The Benefits of Fieldbus Technologies Today!

The relative simplicity of the HART protocol makes it easy for both end users and suppliers to gain experience and benefit from the enhanced two-way communication capability of smart field instruments using this technology. Powerful multiparameter instruments, efficiency with remote communication, field device diagnostics, cost effective control in field devices, installation savings with multidrop networking, and flexible/accurate digital data transmission are all achievable today with instruments that use HART Field Communications Protocol. Users can achieve many of the benefits promised by fieldbus technologies in the future with the HART protocol today. And, the compatibility with 4-20mA makes it easy for users to incrementally add HART speaking instruments incrementally protecting their investment in existing systems and gaining the benefits of enhanced field communication.

The advantages of enhanced field communication enabled by HART technology have been quantified in terms of maintenance, installation, and commissioning cost savings in a wide variety of applications in industries. Initial installation commissioning savings of $400-$500 per instrument and annual maintenance/operations savings of $100-200 per instrument are commonly reported. Tremendous operational benefits are also being achieved. And today, the large and growing base from major instrumentation suppliers supporting the HART protocol, provides the freedom to choose the right product for the job in integrating with existing plant systems.

Instrumentation products equipped with the HART protocol are being used in a wide range of applications and industries to provide cost saving benefits and improved plant performance worldwide. From chemical/refining operations, to gas/liquid distribution systems, and remote/off-shore monitoring stations current installations are addressing virtual all aspects of control, data acquisition, and maintenance. Installation estimates already exceed 1,400,000 and independent projections forecast explosive growth over the next decade. Some of the many reasons cited for the tremendous growth and acceptance of HART technology include:

- The wide variety and increasing number of products available today from a growing list of major instrumentation suppliers around the world, HART is the only "open" communications protocol of its type and in "defactor" industry standard. Users have the freedom to choose the right product for their application and interoperability is assured by the common command and data structure.

- Relatively easy to understand and use, the HART protocol provides access to the wealth of additional information (variables, diagnostics, calibration, etc.) available in smart field devices employing this technology. HART enables field instrument suppliers to incorporate powerful features into their products such as PID control algorithms, diagnostics, and additional process measurements. User access to these features is provided through the enhanced communication capability of HART.

- HART is a no risk solution for enhanced field communication. For maintenance and operations people, HART is a relatively easy transition especially in point to point applications. Any fears about being able to keep the plant running are small as the 4-20mA signal can still carry the process variable (as with traditional instruments), and the enhanced two-way field communication capability of HART can provide real benefit for improved plant performance.

- And, in applications where appropriate, the multidrop capability of HART provides the opportunity to connect several field instruments on the same pair of wires, substantially reducing installation costs.

HART Technology Support

As owner of the technology, the independent HART Communication Foundation maintains the standard and ensures ongoing support for the HART protocol. The Foundation is a not-for-profit corporation, specifically organised to coordinate and support the application of HART technology worldwide. Educating the industry on the capabilities and value of this important technology is a key role. Operating costs are offset by membership and training support service fees. Membership is open to all suppliers, end-users, and others interested in use of the HART protocol.

The HART Communication Foundation supports the industry, and its members, as the official source of information regarding HART technology worldwide. Additional functions include: providing training and support for application of the technology, directing quality assurance programs to ensure the interoperability of HART devices, managing the centralised library and controls distribution of all registered Device Descriptions, and coordinating trade show exhibits and promotional activities for educating industry on the technology. The Foundation maintains a forum section on CompuServe devoted to the HART protocol (GO HARTCF) and a World Wide Web site located at http://www.hartcomm.org

This article was written and supplied by kind permission by Ron Helson, director of the HCF.