The HART Protocol - A Solution Enabling Technology

Abstract

HART Field Communications Protocol is widelyaccepted in the industry as the standard for digitally en-hanced 4-20mA communication with smart field instru-ments. A wide range of products from an increasingnumber of suppliers are available today, and many moreare in development. The enhanced two-way communica-tion capability of instruments using the HART protocolcan significantly improve plant information management,provide solutions to today's business challenges, andyield substantial cost savings. Initial installa-tion/commissioning savings of $400 to $500 per instru-ment and annual maintenance/operations savings of $100to $200 per instrument are commonly reported.

Introduction

For many years, the field communication standard forprocess automation equipment has been a milliamp (mA)analog current signal. The milliamp current signal varieswithin a range of 4-20mA in proportion to the processvariable being represented. In typical applications a sig-nal of 4mA will correspond to the lower limit (0%) of thecalibrated range and 20mA will correspond to the upperlimit (100%) of the calibrated range. If the system iscalibrated for 0-100 PSI, then an analog current signal of12mA (50% of range) will correspond to a pressure of 50PSI. Virtually all installed systems use this interna-tional standard for communicating process variable infor-mation between process automation equipment.

HART Field Communications Protocol extends this 4-20mA standard to enhance communication with smartfield instruments. The HART protocol was designedspecifically for use with intelligent measurement and con-trol instruments which traditionally communicate using4-20mA analog signals. HART preserves the 4-20mAsignal and enables two-way digital communications tooccur without disturbing the integrity of the 4-20mA sig-nal. Unlike other digital communication technologies,

the HART protocol maintains compatibility with existing4-20mA systems, and in doing so, provides users with auniquely backward compatible solution. HART Com-munication Protocol is well established as the "de facto"industry standard for digitally enhanced 4-20mA fieldcommunication.

HART Field Communications Protocol is an extremelyimportant technology for both end users and suppliers.HART technology is a major step in the evolution ofprocess control, and as an enabling technology is bringingsignificant innovation in the capabilities of field instru-ment systems. The enhanced communications capabilityof intelligent field instruments employing the HART pro-tocol, offers significantly greater functionality and im-proved performance over traditional 4-20mA analog de-vices. The HART protocol permits the process variableto continue to be transmitted by the 4-20mA analog sig-nal, and additional information pertaining to other vari-ables, parameters, device configuration, calibration, anddevice diagnostics to be transmitted digitally at the sametime. Thus, a wealth of additional information related toplant operation is available to central control or monitor-ing systems through HART communications.

A wide breadth of products supporting the HART proto-col are available from major instrumentation suppliers,and the number of products and suppliers incorporatingthe technology continues to grow. The HART protocolprovides many benefits today that are promised by field-bus technologies in the future, and yet retains the com-patibility and familiarity of existing 4-20mA systems.The HART protocol is field proven, simple to imple-ment, use and maintain. HART technology is beingused in a wide variety of applications worldwide to gainsignificant improvements in plant performance, providesolutions to regulatory compliance issues (ISO 9000,OSHA, EPA, DOT, etc.) and realize substantial cost sav-ings in initial installation/commissioning and ongoingmaintenance/operations. Current estimates exceed 1.4million installations worldwide.

The HART Protocol - An Overview

HART is an acronym for "Highway Addressable RemoteTransducer". The HART protocol makes use of the Bell202 Frequency Shift Keying (FSK) standard to superim-pose digital communication signals at a low level on topof the 4-20mA as shown in Figures 1 and 2. This en-ables two-way field communication to take place andmakes it possible for additional information beyond justthe normal process variable to be communicated to/from asmart field instrument. The HART protocol communi-cates at 1200 bps without interrupting the 4-20mA signaland allows a host application (master) to get two or more

Analog Signal

+0.5mA

-0.5mA

Simultaneous Analog + Digital Communication

2200 Hz

0

1200 Hz

1

FSK freq:

Logical:

Figure 1 - HART uses Frequency Shift Keying to encode digital information on top of the 4-20mA analog signal

digital updates per second from a field device. As thedigital FSK signal is phase continuous, there is no inter-ference 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 protocol can be used in various modes forcommunicating information to/from smart field instru-ments and central control or monitoring systems. HARTprovides for up to two masters (primary and secondary) asshown in Figure 3. This allows secondary masters suchas handheld communicators to be used without interfering

with communications to/from the primary master, i.e.control/monitoring system. The most commonly em-ployed HART communication mode is master/slavecommunication of digital information simultaneous withtransmission of the 4-20mA signal as shown in Figure 4.The HART protocol permits all digital communicationwith field devices in either point-to-point or multidropnetwork configurations. Figure 5 highlights the optional"burst" communication mode where a single slave devicecan continuously broadcast a standard HART reply mes-sage. Higher update rates are possible with this optionaldigital communication mode and use is normally re-stricted to point-to-point topologies.

Considerable installation savings are possible with themultidrop networking capability of HART which allowsmultiple field devices to be connected to the same pair ofwires. In multidrop applications, communication withfield devices is restricted to digital only as the loop cur-rent is fixed at a minimum value and loses any meaningrelative to the process. From an installation perspective,HART signals are carried over the same wiring as is typi-cally used for conventional 4-20mA analog instrumentstoday. As one might expect, cable run lengths can belonger for individually shielded twisted pairs, than formultiple twisted pairs with an overall shield as reflectedin Figure 6. For short distances, unshielded cables maybe used. HART compatible intrinsic safety barriers andisolators are also available which pass the HART signalsfor use in hazardous areas.

The HART Command Set is organized into three groupsand provides read/write access to the wealth of additionalinformation available in smart field instruments employ-ing this technology. Universal Commands must be im-plemented by all HART devices and provides interoper-ability across the large and growing base of products from

Figure 2 - HART digital communication signal superimposed on the 4-20mA analog signal

Note: Not to Scale

Ti me

Digital Signal

Analog Signal

20 mA

4 mA

11 1

11

0 00

0

Figure 3- The HART protocol enables two Master devices to access information in Slave (field) devices

4 20mA

HARTInterface

2 digital updates/sec (typical)

Multiple Masters

Analog

Digital

Up to 3000 m(Same cable as today)

HART

+ 4-20mA

MASTER / SLAVE or POLL / RESPONSE

REQUEST MESSAGEREPLY MESSAGE

Analog + Digital or Digital Only Communication

Analog signal is not interrupted

Slave responds to Commands/Requests from Master

Typical 500 ms response (2 values per second)

Master Slave

Figure 4 - Master/Slave Communication(Normal HART Mode)

BURST OR BROADCAST MODE

All Digital Communication Mode.

Continuous transmission of a Selected Standard Reply Message such as PV.

Gaps between Messages allow Master to change Command or Mode

3 to 4 updates per second typical

Master Slave

Figure 5- HART Burst Communication(Optional Mode)

different suppliers supporting the HART technology.Universal Commands provide access to information thatis useful in normal plant operation such as the instrumentmanufacturer, model, tag, serial number, descriptor, rangelimits, and process variables. Common Practice Com-

mands provide access to functions which can be carriedout by many devices though not all, and Device SpecificCommands provide access to functions which may beunique to a particular device. Figure 7 highlights thetype of information that can be obtained from thesedevices. The integrity of HART communication is verysecure as status information is included with every replymessage and extensive error checking occurs with eachtransaction. Up to four process variables can be commu-nicated in one HART message and each device may haveup to 256 variables.

Device Description Language (DDL), a recent enhance-ment to the HART technology extends interoperability toa higher level than provided through the Universal andCommon Practice Commands. As reflected in Figure 8,DDL provides a field device (slave) product developerwith the means to create a complete description of theirinstrument and all relevant characteristics, such that it cantalk to any host device using the language. This isanalogous to a printer driver in the personal computerworld which enables an application to talk with a printer

such that what gets printed on the page is what was ex-pected by the application. Universal hand-held communi-cators capable of configuring any HART-based instrumentthrough DDL are available today. Broader application inother types of host systems is expected. The HARTCommunication Foundation manages the centralizedlibrary of all registered Device Descriptions and DDL isbeing supported by all members of the Foundation.

The Benefits of Fieldbus TechnologiesToday!

The relative simplicity of the HART protocol makes iteasy for both end users and suppliers to gain experienceand benefit from the enhanced two-way communicationcapability of smart field instruments using this technol-ogy. Powerful multiparameter instruments, efficiencywith remote communication, field device diagnostics,cost effective control in field devices, installation savingswith multidrop networking, and flexible/accurate digitaldata transmission are all achievable today with instru-ments that use HART Field Communications Protocol. Users can achieve many of the benefits promised by field-bus technologies in the future with the HART protocoltoday. And, the compatibility with 4-20mA makes iteasy for users to incrementally add HART-speaking in-struments incrementally protecting their investment inexisting systems and gaining the benefits of enhanced fieldcommunication.

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

Same Wire As Today!Similar Length LimitationsCompatible with telephone system for longer distances

Distance(m) Cable Type AWG/(mm)

1,524

Instrumentation products equipped with the HART proto-col are being used in a wide range of applications andindustries to provide cost saving benefits and improvedplant performance worldwide. From chemical/refiningoperations, to gas/liquid distribution systems, andremote/off-shore monitoring stations current installationsare addressing virtually all aspects of control, data acquisi-tion, and maintenance. Installation estimates alreadyexceed 1,400,000 and independent projections forecastexplosive growth over the next decade. Some of themany reasons cited for the tremendous growth and accep-tance of HART technology include:

The wide variety and increasing number of prod-ucts available today from a growing list of major instru-mentation suppliers around the world. HART is the only"open" communications protocol of its type and the"defacto" industry standard. Users have the freedom tochoose the right product for their application and interop-erability is assured by the common command and datastructure.

Relatively easy to understand and use, the HARTprotocol provides access to the wealth of additional infor-mation (variables, diagnostics, calibration, etc.) availablein smart field devices employing this technology. HARTenables field instrument suppliers to incorporate powerfulfeatures into their products such as PID controlalgorithms, diagnostics, and additional process meas-urements. User access to these features is providedthrough the enhanced communication capability ofHART.

HART is a no risk solution for enhanced fieldcommunication. For maintenance and operations people,HART is a relatively easy transition especially in pointto point applications. Any fears about being able to keepthe plant running are small as the 4-20mA signal can stillcarry the process variable (as with traditional instru-ments), .and the enhanced two-way field communicationcapability of HART can provide real benefit for improvedplant performance.

And, in applications where appropriate the mul-tidrop capability of HART provides the opportunity toconnect several field instruments on the same pair ofwires, substantially reducing installation costs.

HART Technology Support

As owner of the technology, the independent HARTCommunication Foundation maintains the standard andensures ongoing support for the HART protocol. TheFoundation is a not-for-profit corporation, specificallyorganized to coordinate and support the application ofHART technology worldwide. Educating the industry onthe capabilities and value of this important technology isa key role. Operating costs are offset by membership andtraining/support service fees. Membership is open to allsuppliers, end users, and others interested in use of theHART protocol.

The HART Communication Foundation supports theindustry, and its members, as the official source of infor-mation regarding HART technology worldwide. Addi-tional functions include: providing training and supportfor application of the technology, directing quality assur-ance programs to ensure the interoperability of HARTdevices, managing the centralized library and controlsdistribution of all registered Device Descriptions, andcoordinating trade show exhibits and promotional activi-ties for educating industry on the technology. The Foun-dation maintains a forum section on CompuServe devotedto the HART protocol (GO HARTCF) and a WorldWide Web site located at http://www.ccsi.com/hart/.

HART is a registered trademark of the HART Commu-nication Foundation. Information regarding the HARTprotocol can be obtained from the HART CommunicationFoundation, 9390 Research Blvd., Suite II-250, Austin,Texas 78759, Tel: (512) 794-0369 or Fax: (512) 794-8893.

Ronald B. HelsonHART Communication Foundation, 9390 Research Blvd., Suite II-250, Austin, Texas 78759

Tel (512) 794-0369, Fax (512) 794-8893,ronh@hartcomm.org, 75672.1514@compuserve.com

AbstractIntroductionOverviewBenefitsSupport