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HART® Communication Foundation Document Number: HCF_LIT-24Document Title: Remote Monitoring Using the HART® Protocol Document Revision: 2.0

Version: A 09/14/96 Page: 3 of 11

Table of Contents

1. INTRODUCTION........................................................................................................................................4

2. REFERENCES...........................................................................................................................................4

3. HART COMMUNICATION PROTOCOL.......................................................................................................4

3.1 HART TECHNICAL OVERVIEW .................................................................................................................5

4. TELEPHONY.............................................................................................................................................7

5. REMOTE MONITORING AND DATA ACQUISITION....................................................................................7

6. A HART-BASED MONITORING SYSTEM...................................................................................................8

6.1 HOST SYSTEM..........................................................................................................................................86.2 MODEM SELECTION .................................................................................................................................96.3 TELEPHONE NETWORK ............................................................................................................................9

6.3.1 Multiple Remote Locations..............................................................................................................106.4 REMOTE MONITORING SITES .................................................................................................................10



1. Introduction

Remote data acquisition and monitoring plays an increasing role in business due to both competitive andregulatory demands. Traditionally, Supervisory Control and Data Acquisition (SCADA) Systems havecarried the burden for the remote collection and communication of data back to centralized monitoringfacilities. The remote collection of data was necessitated by the nature of the process measurements beingmade. At the time the field measurement and control devices were relatively simple analog devices. Theelectronics in the field devices basically conditioned and linearized the sensor signal. As a result, theremote terminal unit (RTU) was responsible for analog to digital and digital to analog conversion, simplecontrol activities and telemetry to and from the central monitoring facilities.

Today's field devices have been revolutionized by the microprocessor. Most field devices developed todayinclude an onboard microprocessor that performs the measurement and signal conditioning tasks. Thereare many reasons for the advent of "smart" field devices including lower manufacturing costs and, formost applications, higher quality data.

Since smart instruments have become accepted and wide spread in industry, innovative architectures andsolutions enabled by smart field device technology are beginning to appear. This paper discusses anarchitecture utilizing the HART Communication Protocol to implement remote monitoring and dataacquisition.

2. References

For additional information concerning this subject refer to the following HART CommunicationFoundation document:

HCF_SPEC-11 HART® - SMART Communicaitons Protocol, Protocol Specifications

3. HART Communication Protocol

The HART Communication Protocol is widely accepted in the industry and has become the de-factostandard for smart field devices. HART-based field devices use the same wiring for simultaneous analog4-20mA and digital communications. The protocol also supports multidrop networking of several fielddevices on a single twisted-wire pair.

HART was developed by Rosemount in 1986 for their smart transmitters. After continued developmentand expansion to support other devices it was made an open protocol in late 1989 followed by theformation of the HART Users Group in 1990. The HART Users Group grew from 18 companies in1990 to 79 in 1993. In 1993 the HART Communication Foundation (HCF) was formed and Fisher-Rosemount transferred ownership of the HART Protocol to the HCF. The HART CommunicationFoundation is an independent, non-profit corporation, organized to serve growing industry interest in theHART Protocol and the needs of HART users. Its sole mission is to coordinate, promote, and support theapplication of HART Technology worldwide. Operating costs are offset by membership andtraining/support service fees. Membership is open to all suppliers, end users, and others interested in useof the HART Protocol.



A wide range of products from an increasing number of suppliers are currently available, and many moreare in development. HART technology provides many of the same "advanced capabilities" and benefitstoday, that fieldbus technologies promise for the future, while maintaining the compatibility and familiarityof existing 4-20mA systems. The enhanced communication capability of HART-based field devices cansignificantly improve plant information management, provide solutions to today's business challenges,and yield substantial cost savings.

3. 1 HART Technical Overview

HART is an acronym for "Highway Addressable Remote Transducer". Unlike Fieldbus technologieswhich are all digital, the HART protocol leverages traditional analog signaling technology to permit 2-waydigital communication simultaneously with transmission of the analog signal. The HART protocol wasdesigned specifically to enhance communication with smart field instruments and maintain compatibilitywith existing 4-20mA systems. The HART protocol enables a primary variable to be transmitted by the4-20mA analog signal and information pertaining to other variables, parameters, device configuration,calibration, or device status to be communicated digitally at the same time.

The HART Protocol uses the Open Systems Interconnection (OSI) reference model as a guide. HARTimplements a subset of the OSI model--layers 1,2 and 7 (see Figure 1). The HART Application Layer(OSI Layer 7) consists of three classes of commands or messages:

• Universal Commands that are required to be implemented by all HART devices;

• Common Practice Commands generally applicable to a wide cross section of field devices;and

• Device Specific Commands unique to the individual field device.

Physical

Presentation

1

7

6

Network3establishes network

connections

connects the equipment

converts data

Session5handles the dialogue

Transport4 secures the

transport connection

Application provides formatted data

FUNCTION

Bell 202

2 Link establishes the data

link connection HART protocol

Message Structure

HART Commands

HARTLAYER

Figure 1 HART and the OSI Model.



The Data Link Layer (OSI Layer 2) is responsible for the reliable transmission of data packets across theHART network. HART is a Master/Slave, Character-Oriented Protocol. The HART Data Link Layerconsists of:

• Asynchronous 8-bit data bytes;

• The data bytes are organized into frames;

• Transactions consisting of a Master command and Slave response frame pair;

• Bus timing and arbitration rules governing access to the network.

• Error checking consists of:

-- Parity on each character (“vertical parity”); and

-- A check byte at the end of the frame (“longitudinal parity”).

• Addressing:

-- Short Frames with 4-bit addresses (polling addresses);

-- Long Frames with 38-bit addresses.

The Physical Layer (layer 1) of the HART protocol uses the Bell 202 Frequency Shift Keying (FSK)standard to communicate digital information at 1200 bits/second by superimposing the FSK signals at alow level on top of the 4-20mA analog signal. This enables additional information beyond just the normalprocess variable (PV) to be communicated to/from a field instrument using standard HART commands.Since the FSK signal has an average value of zero, the value of the analog signal being transmitted is notimpacted.

Furthermore, by utilizing a modulated signal HART uses has a narrow frequency bandwidth. It is thechoice of Bell 202 signaling characteristics for use in the HART Physical layer that will be leveraged inthis remote monitoring and data acquisition architecture.

2200 Hz“0”

1200 Hz“1”

Analog Signal

+0.5mA

-0.5mA

Figure 2 HART Signaling Characteristics



4. Telephony

Telephone networks are designed to carry the human voice have a limited bandwidth. The telephone voicecircuits and lines operate from 300Hz to 30000Hz as shown in Figure 3. As can be seen the HARTdigital signals fall within in the telephone voice band. As a result, HART digital signals can be transmittedvia any voice grade circuits.

300Hz 3000HzFrequency

1200Hz 2200Hz

Telephone Frequency Band

HART FSK Tones

Figure 3 Telephone Frequency Band

This is very different from Fieldbus and many network protocols. These protocols use higher frequencysquare wave signals. Square waves have many frequency components starting from the square wavefrequency and upwards. In most cases, the frequency components are much higher than 3000Hz andcannot be directly transmitted via voice circuits at all.

5. Remote Monitoring and Data Acquisition

Remote monitoring and data acquisition requirements vary greatly. Typical considerations include:

• The volume of data to be collected;

• The rate at which the data must be updated;

• The distance / geographic area to be covered;

• Control actions that must be accommodated;

These attributes, in turn, dictate choices of communication medium, system architecture and processingand storage capacities. As a results, there are a large number of SCADA solutions available to fit thedifferent applications each with its own combinations of these attributes.

However there are a substantial number of monitoring applications that require relatively slow updates butcover a wide geographic area. One such example is gas pipelines. In these kinds of systems HART canplay a key role implementing the monitoring solution.



6. A HART-based Monitoring System.

Figure 4 shows a simplified diagram of a monitoring system utilizing the HART Protocol. The systemconsists of:

• A host system;

• A modem connection to the Public Switched Telephone Network (PSTN);

• The PSTN network supplying a Bell 3002 leased line service; and

• The remote monitoring site.

6. 1 Host System

The host system monitors, records and displays data from the network. This system acts as a HARTHost issuing HART Commands and processing data contained in the field device responses. This HARThost is typically a MS-DOS PC although the HART host could be simply a gateway in a largerMonitoring system. For example, there are HART multiplexors that can reside on Modbus and other datahighway. Furthermore, there are MS-Windows Dynamic Link Libraries available the can allow access toHART from many popular software packages.

6. 2 Modem Selection

2-Wire to 4-Wire

Converter

24V- +

600 OHM

5 mfd

Data Station Data Station TerminationTermination

Tellabs 4416E

Racal-Vadic UDS

Multi-TechCodex

Leased LineLeased LineModemModem

Telephone Telephone CompanyCompanyNetworkNetwork

Loop Power Loop Power SupplySupply

Four Wire ‘3002’ Leased

Line

Figure 4 Simplified Monitoring System Diagram



The selection of a modem to interface to the PSTN is an important aspect of the system. When evaluatingBell 202 compatible modems the operation of the carrier detect is critical. The turn around delay of thecarrier detect circuit must be less than 30 milliseconds. The following modems should be considered:

• Racal-Vadic "VA1251"; and

• Universal Data Systems "UDS202T";

(This is not an exhaustive list of Bell 202 modems.)

6. 3 Telephone Network

Telecommunications play an important role in any remote monitoring scheme. Whether via radio,satellite, or a land-lines the communication link is a vital path for the telemetry. In many cases a land-lineprovided by the telephone company is the communication medium of choice.

As discussed above, the HART digital communications is a modulated signal that falls directly in thefrequency band used by telephone voice circuits and, being based on the Bell 202 standard, HART signalscan be easily transmitted over Bell 3002 leased lines in the United States. Using Bell 3002 leased lines hasthe advantage the telephone company is responsible for signal amplification and quality. This ensuresreliable transmission of HART digital communication signals over long distances.

Essentially the telephone network can be thought of as a cloud. The circuit enters the PSTN at onelocation is routed by the telephone company through its networks and then the circuit reappears at the otherlocation. How the circuit is routed/provided is not obvious nor should that be of any particular concern.

The other aspect of HART that enables this architecture is the Master/Slave organization of transactions.A HART master has very specific rules it must follow to access/share a HART network. This allows theslave implementations to be relatively simple. A Typical HART transaction occurs as follows:

• At the appropriate time a HART master issues a Command;

• The HART slave, monitoring the network, sees its 38 bit address and responds; and

• The HART master relinquishes the network in order to provide another master access.

Since HART uses a long frame 38 bit address, each slave field device is assured of having a uniqueaddress on the network. This eliminates the chance for collisions "crashing" the network.

Thus this master/slave aspect of HART allows field devices to be multidropped on the network withoutthe need for additional wiring or communication devices.

6.3.1 Multiple Remote Locations



Figure 5 builds on the simplified network diagrammed in Figure 4 to show multiple sites supported withone Host connection and one leased line circuit. Telephone companies have the capability to allow theconnection to the leased line at many widely separated geographic locations.

Another benefit of the building on a standard, like Bell 202, is that a wider infrastructure exists that offersother useful capabilities. Telephone companies can provide "bridges". These bridges, which exist at thetelephone company, allow multiple "drops" off the leased line. As a result, the number of remotelocations and the number of HART devices is limited by the update rate desired. For example, a networkof 20 HART devices could be updated about 6 times per minute.

Telephone CompanyTelephone CompanyNetworkNetwork

BridgesBridges

Multitple Sites Multitple Sites ((Possibly Hundreds of Possibly Hundreds of

Miles Apart)Miles Apart)

Figure 5 Supporting Multiple Remote Sites.

6. 4 Remote Monitoring Sites

Like the rest of the monitoring network, each remote monitoring site is relatively simple. Figure 6 showsa block diagram of a remote monitoring site. Each site consists of:

• A station coupler;

• Power supply;

• Sense resistor; and

• One or more HART field devices.

Since HART field devices are used there is no need for extra electronics (like modems) at the remotemonitoring sites. Digital communication occurs directly with the HART field devices.



The station coupler is supplied by the telephone company. This is, basically, a 1 to 1 isolating transformerfor the connection to the PSTN. This is required to isolate the monitoring site from the telephone line. Insome cases, the telephone company may be require a Data Station Termination (DST) module (e.g.,Tellabs 4416e). The DST is used to fine tune signal amplitudes and ensure that the signal signals placedon the telephone line are balanced.

24VDC

- +

600 Ω

Station Coupler

5µF

Transient Suppressor

Line Power

Bell 3002 Leased Line

Figure 6 Remote Monitoring Station

HART field devices transmit a current signal (±0.5mA). The sense resistor is used to convert the currentto the voltage needed by a host or, in this case, the leased line. The 5µF capacitor is used to block the DCsignal from the station coupler.



24VDC is used to power the HART field devices and the DST module (if it isrequired). Selection of the power supply is straight forward as well. WhenHART field devices are placed in multidrop mode current consumption of thefield device is fixed at some minimum. For most devices this is 4mA. As aresult, in many cases, a substantially smaller power supply can be used.

Since the monitoring stations will usually be at a remote site and thusexposed, a good transient suppression scheme should be employed. A singletransient suppresser that filters both the power line and the telephone line isbest.

6.4.1.1 Mechanical Layout

In applications like gas pipeline monitoring the installation may need to takeinto account explosion proof or other codes. This can be easilyaccommodated as shown in Figure 7. In these situations the components notapproved for en explosion proof environment (e.g., power supply, stationcoupler, sense resistor) would be mounted in one enclosure. The HART fielddevices would be mounted in the other enclosure. The distance between thetwo enclosures (L) would be selected based on applicable codes. The onlyconnection between the two enclosures will be the single twisted pair requiredfor the HART field device power and communications.

L

Figure 7 MechanicalLayout

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