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The Benefits of HART Protocol Communication inSmart Instrumentation Systems

Introduction

HART® Field Communications Protocol is widely recognizedas the industry standard for digitally enhanced 4-20 mA smartinstrument communication. Use of the technology is growingrapidly, and today virtually all major global instrumentationsuppliers offer products with HART communication.

The HART protocol provides a uniquely backward compat-ible solution for smart instrument communication as both 4-20 mA analog and digital communication signals are trans-mitted simultaneously on the same wiring. HART providesmany benefits promised by fieldbus, while retaining the com-patibility and familiarity of existing 4-20 mA systems.

This paper provides a brief overview of the HART protocoland the benefits achievable with this important technology.Per instrument cost savings benefits of $300-500 in initial in-stallation/commissioning and $100-200 per year in ongoingmaintenance/operations are commonly reported.

Analog + Digital Communication

For many years, the field communication standard used byprocess automation equipment has been a milliampere (mA)

analog current signal. In most applications the milliamperesignal varies within a range of 4-20 mA in proportion to theprocess variable being represented. Virtually all installed plantinstrumentation systems use this international standard tocommunicate process variable information.

HART Field Communications Protocol extends the 4-20 mAstandard to enhance communication with intelligent measure-ment and control instrumentation. A major step in the evolu-tion of process control, the HART protocol is fostering signifi-cant innovation in process instrumentation capabilities. Theenhanced communication characteristics of this importanttechnology are reflected in the protocol name, HART whichstands for �Highway Addressable Remote Transducer�.

The HART protocol enables two-way digital communicationwith smart instruments without disturbing the 4-20 mA analogsignal. Both the 4-20 mA analog and HART digital communi-cation signals can be transmitted simultaneously over thesame wiring. Primary variable and control signal informationis carried by the 4-20 mA (if desired), while additional mea-surements, process parameters, device configuration, cali-bration, and diagnostics information is accessible throughthe HART protocol over the same wires at the same time. Unlikeother �open� digital communication technologies for processinstrumentation, HART is compatible with existing systems.

The HART Technology

The HART protocol makes use of the Bell 202 Frequency ShiftKeying (FSK) standard to superimpose digital communica-tion signals at a low level on top of the 4-20 mA. Since thedigital FSK signal is phase continuous, it does not interferewith the 4-20 mA signal. A logical �1� is represented by afrequency of 1200 Hz and a logical �0� is represented by afrequency of 2200 Hz as shown in Figures 1 and 2.

The HART FSK signaling enables two-way digital communi-cation and makes it possible for additional information be-yond just the normal process variable to be communicated toor from a smart field instrument. The HART protocol commu-nicates at 1200 bits per second without interrupting the 4-20mA signal and allows a host application (master) to get twoor more digital updates per second from a field device.

Simultaneous Analog + Digital Communication

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

Figure 2 - HART digital communication signal superimposed on the4-20 mA analog signal

Digital Capability• Access to all instrument parameters & diagnostics• Supports multivariable instruments• On-line device status

Analog Compatibility• Simultaneous analog & digital communication• Compatible with existing 4-20 mA equipment & wiring

Interoperability• Fully open de facto standard• Common Command and data structure• Enhanced by Device Description Language

Availability• Field proven technology with more than 1,400,000

installations• Large and growing selection of products• Used by more smart instruments than any other in the

industry

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Flexible Application

HART is principally a master/slave protocol which means thata field (slave) device speaks only when spoken to by a mas-ter. Two masters (primary and secondary) can communicatewith slave devices in a HART network. Secondary masters,such as handheld communicators, can be connected almostanywhere on the network and communicate with field deviceswithout disturbing communication with the primary master. Aprimary master is typically a DCS, PLC, or computer basedcentral control or monitoring system. A typical installation withtwo masters is shown in Figure 3.

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

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

Analog + Digital or Digital Only CommunicationAnalog signal is not interrupted

�Slave� responds to Commands/Requests from �Master�Typical 500 ms response (2 values per second)

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

Figure 5 - Some devices support HART Burst CommunicationMode (Optional)

Figure 6 - The HART field devices can be connected in a MultidropNetwork for some applications

Burst or Broadcast mode

Master / Slave or Poll / Response

From an installation perspective, the same wiring used forconventional 4-20 mA analog instruments carries the HARTcommunication signals. Allowable cable run lengths will varywith the type of cable and the devices connected, but in gen-eral up to 3,000 meters for a single twisted pair cable withshield and 1,500 meters for multiple twisted pair cables witha common shield. Unshielded cables can be used for shortdistances. Intrinsic safety barriers and isolators which passthe HART signals are readily available for use in hazardousareas.

The same HART reply message (PV or other) is continuouslybroadcast by the slave until the master instructs the slave todo otherwise. Data update rates of 3-4 per second are typi-cal with �burst� mode communication and will vary with thechosen command. Burst mode should be used only in singleslave device networks.

The HART protocol also has the capability to connect mul-tiple field devices on the same pair of wires in a multidropnetwork configuration as shown in Figure 6. In multidrop ap-plications, communication is limited to master/slave digitalonly. The current through each slave device is fixed at a mini-mum value to power the device (typically 4 mA) and no longerhas any meaning relative to the process.

�Burst� is an optional communication mode (Figure 5) whichallows a single slave device to continuously broadcast a stan-dard HART reply message. This mode frees the master fromhaving to send repeated command requests to get updatedprocess variable information.

The HART protocol can be used in various modes for com-municating information to/from smart field instruments andcentral control or monitoring equipment. Digital master/slavecommunication simultaneous with the 4-20 mA analog signalis the most common. This mode, depicted in Figure 4, allowsdigital information from the slave device to be updated twiceper second in the master. The 4-20 mA analog signal is con-tinuous and can still carry the primary variable for control.

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Powerful Command Set

HART communication is command based, i.e., a master is-sues a command and the slave responds. Three types ofHART commands provide read/write access to informationavailable in HART-compatible field instruments (see Figure7). Universal and Common Practice Commands are definedin the HART protocol specifications. The third type, DeviceSpecific Commands, provide freedom for product specificparameters or functions unique to a particular device.

ModelTag ID

Description

Common Practice Commands

Read VariablesChange Upper & Lower Range

Adjust (zero, span)Initiate Self TestSerial Number

Time Constant Values

Device Specific Commands

Model Specific FunctionsSpecial Calibration Options

Start, Stop, or Clear TotalizerSelect Primary VariableEnable PID, Change SP

Adjust Tuning Parameters

Universal Commands

Figure 7 - HART Universal & Common Practice Commandsensure interoperability across products of different manufacture

Common Practice Commands provide access to functionswhich are implemented in many devices though not all. Thesecommands are optional, but if implemented, must be as speci-fied. Device Specific Commands allows access to uniqueproduct features. Mostly used for device configuration pa-rameters, these commands can send a new set point to a PIDalgorithm in the device.

Device status information included in every HART commandresponse provides increased system integrity for critical loops.The device status bits in each reply message indicate devicemalfunctions or other problems such as analog output satu-rated, variable out of limits, or communication errors. SomeHART-compatible devices can monitor the device status bitscontinuously and provide early warning alarms or shutdownsif problems are detected.

Device Description Language

HART Device Description Language (DDL), extends interop-erability beyond the Universal and Common Practice com-mands. A field device (slave) manufacturer uses DDL to cre-ate a software file with all relevant device characteristics, suchthat a DDL capable host can fully communicate with the de-vice. A Device Description (DD) for a HART device is analo-gous to a printer driver in the personal computer world wherethe printer driver links an application to the printer such that itprints properly on the page.

Universal hand-held communicators capable of configuringany HART-based instrument through DDL are available fromseveral manufacturers. Other host applications that under-stand DDL are beginning to emerge. A central library of allHART-compatible Device Descriptions is managed by theHART Communication Foundation.

Universal Commands ensure interoperability across the largeand growing base of products from different suppliers, andprovide access to information useful in normal plant opera-tion, i.e., reading process measurements/variables, upper andlower range values, and other information such as manufac-turer, model, tag, and description. A basic rule of the HARTprotocol is that HART-compatible slave devices must respondto all Universal Commands. These commands are powerfulas for example, Universal Command 3 allows up to four dy-namic variables to be read in a single command response.

MessageDate

Range Values

Instrument LimitsProcess Measurements

Device Status

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Innovative Application Example

The power of the HART protocol is evident in the control dia-gram of Figure 8. This innovative application uses the inher-ent feature of the HART protocol that both 4-20 mA analogand digital communication signals are transmitted simulta-neously over the same wiring.

In this application, the HART-compatible transmitter has aninternal PID control capability. The device is configured suchthat the 4-20 mA loop current is proportional to the controloutput of the PID algorithm executing in the device (not themeasured variable as in most transmitter applications). Sincethe 4-20 mA loop current is regulated by the PID control out-put, it is used to drive the valve position directly.

The control loop executes entirely in the field between thetransmitter (with PID) and the control valve. The control ac-tion is continuous as the traditional 4-20 mA analog signaldrives the valve. HART digital communication links the op-erator with the control loop to change set point, and read theprimary variable, or valve position output. Substantial sav-ings are possible in applications where this innovative con-trol architecture is appropriate.

Best Solution

The HART protocol provides users with the best solution andmigration path for capturing the benefits of enhanced com-munication with smart instrumentation. No other communica-tion technology can match the base of support or wide rangeof products that are available with HART today. The technol-ogy is easy to use and HART-compatible products are avail-able from major instrumentation suppliers to address virtuallyall process measurement and control applications.

The emergence of fieldbus will not displace HART in eitherexisting or new production facilities. HART provides users withmany of the same benefits while retaining the compatibilityand familiarity of existing 4-20 mA systems. HART allows thecost saving benefits of remote communication, flexible/accu-rate digital data transmission, field device diagnostics, andpowerful multiparameter instruments to be captured withoutreplacing entire systems.

Connection to current and future plant networks is assuredby the digital communication capability and large installedbase (more than 5,000,000 installations and growing rapidly).Support of the HART Communication Foundation ensures thatthe technology will continue to evolve for serving the needsof smart instrumentation today and tomorrow.

Written by Ron Helson

HART Communication Foundation9390 Research Blvd., Suite I-350

Austin, Texas 78759 USA

HART® is a registered trademark of the HART Communication Foundation.

Figure 8 - Some HART-bassed instruments include PID algorithmsfor implementing cost effective control

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ABBAdaptive Instruments Corp.

Allen-BradleyAnalog Devices

Anderson Instrument Co., Inc.Apparatebau Hundsbach GmbH

Applied System Technologies, Inc.Arcom Control Systems

Beamex Oy, AbBesta AG

Bopp & Reuther Heinrichs GmbHBrooks Instrument

BTGDelta Controls LtdDrägerwerk AG

Drexelbrook Engineering Co.Elcon Instruments

Elsag Bailey - Hartmann & BraunEndress + Hauser GmbH

Endress + Hauser LtdFieldbus International ASFisher Controls Int�l Inc.

Fisher-Rosemount Systems, IncFlowdata, Inc.

Foxboro-Eckardt AGFuji Electric Co., Ltd.

Harold Beck & Sons, Inc.Helios AG

Hersey Measurement CompanyHoneywell, Inc.

Instytut Automatyki i RobotykiInstrumentfirman INOR ABJohnson Yokogawa Corp.

Kamstrup A/SKay-Ray/Sensall Inc.

KDG Mobrey LtdKnick Electronische Meßgeräte GmbH

K-TEK Corp.Krohne Meßtechnik GmbH

MACTek CorporationMagnetrol International

MasoneilanMeasurement Technology Ltd.

Meridian Instruments B.V.

HART Foundation Members

MESCO Engineering GmbHMicro Motion, Inc.

Milltronics Ltd.MMG Automatika Muvek Rt

Moore Industries-InternationalMoore Products Co.MTS Systems Corp.Neles-Jamesbury

Ohmart CorporationPR electronics A/S

Peek Measurement LtdPepperl + Fuchs

Pondus Instruments ABPrinco Instruments, Inc.

Raytek Inc.Rittmeyer Ltd

Robertshaw TennesseeRosemount Inc.

Rosemount Analytical Inc.Rössel Messtechnik GmbH

Saab Tank ControlSamson AG

SchlumbergerSiemens AG

SMC CorporationSmar International Corp

Softing GmbHSolartron Transducers

SOR, Inc.Southwest Research Institute

Sparling InstrumentsThe Foxboro Company

Toshiba CorporationTwo Technologies, Inc.Valmet Automation Inc.

Valtek InternationalVEGA-Grieshaber KGViatran CorporationW. C. Groenhuijzen

Westlock Controls CorporationWhessoe-Varec, Inc.

Wireless Scientific, Inc.Yamatake-Honeywell Co., Ltd.