HIGHWAY ADDRESSABLE REMOTE TRANSDUCER (HART)

HART BASIC TRAINING FROM HART INSTITUTE

HART Communication is the global standard for smart process instrumentation and is the world’s most used digital communications protocol. HART technology is installed in more than 15 million process devices worldwide and is included in 70 percent of all smart process measurement and control instruments installed each year. Over the past few years, industry engineers have begun to re-examine this powerful technology—and have realized that the intelligent capabilities of the HART-smart field devices already installed in industrial plants can provide the valuable information promised by newer technologies---and are an untapped resource that can lower operating costs, reduce downtime and maximize assets. HART Basics is the first of three mini-courses designed to provided the basic knowledge required to better understand and benefit from HART technology. In this course, you will learn the basic concepts of HART including the technology, basic wiring, Device Descriptions, HART commands, diagnostic and alert information and other fundamental concepts.

Introduction Many plants and mills around the world have been using HART instrumentation for years. These important assets for controlling and monitoring the process have typically been providing only a fraction of the value of which they are capable. Users are looking for ways to lower their operating cost, reduce downtime and maximize their existing or new assets and HART technology can help you meet these goals by using the full capabilities of HART devices and system interfaces that you may have already in place.

HART stands for Highway Addressable Remote Transducer. It is an open protocol developed in the late 1980's to facilitate communication with Smart field devices. Over the years, it has become the global standard for industrial field communications between intelligent devices and control, safety and asset management systems. With more than 150 companies supporting more than 800 different registered HART devices, there are HART-enabled devices for practically every application. What is new about HART? HART delivers value 24/7 or full-time. This is a change from the traditional thinking of HART being used as just a configuration tool or just as a troubleshooting tool. Move from part-time, limited communication applications to full-time communication with data integrated into control and asset management applications and you will be maximizing your assets and providing valuable information to plant operations and will provide significant benefits.

An information gap exists between the plant floor and the control system. The information contains valuable data as to the health and status of your field devices. By using the power or intelligence in HART-smart devices & systems, the information gap can be closed by providing the device diagnostic information users are looking for to lower maintenance cost, improve process availability and increase operating profits.

As you complete these courses you will learn about the following:

1) The fundamental concepts that embody the HART communication protocol 2) How to apply HART instruments and integrate them with your system and asset management applications 3) How to use your knowledge of HART technology to reduce the time and cost for installing and commissioning loops, improve your operations and reduce maintenance costs, improve your ability to manage these assets, and improve the safety of your plant.

We believe that you will find HART technology is SIMPLE and easily understood, is EASY to install and live with, is INEXPENSIVE to use and is also FEATURE RICH providing you with capabilities that you thought wouldn't be possible with existing technology.

Benefits HART Instruments

Digital Instruments

Accuracy and stability Reliability Multivariable Computations Diagnostics Multiple sensor inputs Ease of commissioning Tag ID Remote configuration Loop checks Adjustable operational parameters

There are many new and exciting ways to unleash or put the power of HART to work. With more than 15 million HART capable devices installed throughout the world, it's important to understand how one goes about using this power - maximizing the benefits and value of HART.

Theory of Operation

HART communication occurs between two HART-enabled devices, typically a field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices. HART provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA current loop - the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal. The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc. Together, the two communication channels provide a complete field communication solution that is easy to use and configure, is low-cost and is very robust. Since HART devices support the standard 4-20mA mode, they can be used with any system that can work with 4-20mA

devices. This method has been field proven and used by plants worldwide. However, this method does not take advantage of all the benefits of digital communication that HART offers.

Communication Modes

There are two communication modes available with HART technology: Request-Response Mode and Burst Mode.

Request - Response Mode The digital communication part of the communication is a request-response communication protocol, which means that during normal operation, each device communication is initiated by a request from a host device - sometimes known as a master. Two masters can connect to each HART loop. The Primary Master is generally a distributed control system (DCS), programmable logic controller (PLC), asset management system or a personal computer (PC) running an application. The Secondary Master is generally a handheld terminal or another PC with an appropriate HART-enabled application. HART-enabled devices include but are not limited to transmitters, actuators, flowmeters, valve positioners, analyzers and controllers that respond to commands from the primary or secondary master.

Burst Mode Some HART devices support the optional burst communication mode. Burst mode enables faster communication (3-4 data updates per second). In burst mode, the Master instructs the device to continuously broadcast a standard HART reply message (e.g., the value of the process variable). The Master receives the message at the higher rate until it instructs the device to stop bursting. This mode allows multiple HART-enabled devices to be connected in a multidrop-wiring scheme.

Frequency Shift Keying

The HART Communication Protocol is based on the frequency shift keying (FSK) principle. The digital signal is made up of two frequencies - 1,200 Hz and 2,200 Hz representing bits 1 and 0. Sine waves of these two frequencies are superimposed

on the direct current (dc) analog signal wire to provide simultaneous analog and digital communications. Because the average value of the FSK signal is always zero, the 4-20mA analog signal is not affected. A minimum loop impedance of 230 ohms is required for communication.

HART Networks

HART devices can operate in one of two network configurations-point-to-point or multidrop.

Point-To-Point In point-to-point mode, the traditional 4-20 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART Protocol. The 4-20 mA analog signal is not affected by the HART signal and can be used for control in the normal way. The HART Communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance, and diagnostic purposes. The instrument power is provided by an I/O interface or external power source.

Typical Point-To-Point Wiring

Multidrop The multidrop mode of operation requires only a single pair of wires and, if applicable, safety barriers and an auxiliary power supply for up to 15 field devices. All process values are transmitted digitally. In multidrop mode, all field devices are assigned polling addresses and the current through each device is fixed to a minimum value (typically 4 mA).

Use miltidrop connection for supervisory control and installations that are widely spaced, such as pipelines, custody transfer stations, and tank farms.

The Handheld Terminal in the graphic below can be connected anywhere on the segment; at the I/O terminals, at any of the devices or anywhere on the wiring segment.

HART Commands The HART Command Set provides uniform and consistent communication for all HART-enabled devices. The command set includes three classes: universal, common practice, and device specific (Table 1). There are 35-40 data items that are standard in every HART registered device. Host applications may implement any of the necessary commands for a particular application.

Universal All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).

Common Practice

Common practice commands provide functions implemented by many, but not necessarily all, HART communication devices. The HART specifications recommend devices to support these commands when applicable.

Device Specific Device specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on device-specific commands is available from device manufacturers or in the Field Device Specification document.

SUMMARY TABLE

Universal Commands

Common Practice

Commands

Device-Specific

Commands

Identify device Read manufacturer

ID code, device type, device ID, extended device status, configuration change counter

Read primary variable (PV) and units

Read loop current and percent of range

Read dynamic variables and loop current

Read dynamic variables classification

Read of write up to 32- character tag, 16-character descriptor, date

Read or write 32-character message

Read PV transducer information

Read or write final assembly number

Read or write polling address and loop current modes

Read device variables

Write PV damping value

Write PV range values

Set PV upper range value, lower range value

Enter / exit fixed current mode

Perform self-test Perform device reset Read additional

device status Set PV zero Write PV units Trim loop current

zero and gain Write PV transfer

function Write PV transducer

serial number Read or write

dynamic variables assignments

Burst mode control, write burst mode command number

Lock device, read lock state

Squawk, find device Transfer service,

transfer control Catch device

variable

Read or write low-flow cut-off

Start, stop, or clear totalizer

Read or write density calibration factor

Choose PV ( mass, flow, or density )

Read or write materials or construction information

Trim transducer calibration

PID enable Write PID setpoint Valve

characterization Valve setpoint Travel limits User units Local display

information

Field Device Specification The Field Device Specification is written as a reference document and includes the listing of the device specific commands for each device and is written by the device developer. The Field Device Specification specifies the capabilities, features, and operation of the Field Device as viewed from a HART perspective.

The Field Device Specification is an important reference document for Host Application designers, System Integrators, and Users. This document can be used as a reference guide to identify all commands available in a specific device including the device specific commands and functions that are unique to that device. These commands access setup and calibration information, as well as information about the construction of the device.

The Field Device Specifications are available for download for some products in the Product Catalog section of the HART Communication Foundation website.

Device Descriptions

Device Description (DD) files are the HART standard for interoperability of HART devices with host systems. The HART device suppliers provide a DD file for each HART device and the DD file combines all of the information needed by the host application into a single structured file. HART host applications can use device descriptions (DD) to obtain information about the variables and functions contained in a HART field device. The DD file includes all of the information needed by a host application to fully communicate with the field device.

HART Device Description Language (DDL) is used to write the DD. The DD identifies which commands (universal, common practice and device specific) are supported by a device as well as contains the format and structure of all device-specific commands. A DD for a HART field device is roughly equivalent to a printer driver for a computer. DD files eliminate the need for host suppliers to develop and support custom interfaces and drivers for supporting each device. A DD provides a picture of all parameters and functions of a device in a standardized language. The DD will provide information for a DD-enabled host application to read and write data according to each device's procedures. DD source files for HART devices resemble files written in the C programming language. DD files are submitted to the HART Communication Foundation (HCF) for registration in the HCF DD Library. Quality checks are performed on each DD submitted to ensure specification compliance, to verify that there are no conflicts with DDs already registered, and to verify operation with standard HART hosts. The HCF DD Library is the central location for management and distribution of all HART DDs to facilitate use in host applications such as Distributed Control Systems (DCS), safety systems, asset management systems, PCs and handheld terminals. There is a DD revision associated with each device revision to support its full capabilities. If the current DD revision is not available, the functionality of the previous level will be available. The Table below indicates that a device of a certain revision working with a DD of a previous revision will only provide the device functionality of the previous revision.

Device Revision DD Revision Device Functionality

5 5 5 5 4 4

Enhanced Device Descriptions Working in cooperation with Fieldbus Foundation, PROFIBUS and the OPC Foundation, the HART Communication Foundation is currently developing the enhanced Device Description Language (eDDL). The new HART eDDL extends the capabilities of DDL to provide an industry-standard solution for advanced visualization of intelligent device information to maintain the proven integrity of existing DD technology across all three communications technologies.

HART eDDL enhancements include an improved user interface with support for menus, windows, tabs and groups and added graphic support for graphs, trends, charts and dial indicators. The eDDL further standardizes the user interface for managing intelligent devices and eliminates the need for Windows resource files, supplemental files, and other DD modifications previously required by some applications.

The HART eDDL Specification has been successfully validated and is near completion.

The graphic above shows actual examples of what can be displayed using Enhanced Device Descriptions.

Multi-variable Device

Most HART devices provide multiple process variables, which can be used for monitoring or control to gain a better insight into the process. For example, a pressure transmitter provides pressure as its primary value (which is usually the 4-20 mA signal) but also could provide temperature as a HART digital Secondary Value. These variables can be used for control or monitoring applications. Using this additional HART data eliminates the need for additional instrumentation that may have been required to achieve a similar result.

Examples of multi-variable information by product type:

Multivariable Type

Functions

DP Transmitter Pressure, Differential Pressure, Body Temperature, Environmental Temperature, Square Root, Mass Flow

Magnetic Flowmeter

Flow Rate, Body Temperature, Integrated Flow, Totalization

pH Analyzer pH Value, Liquid Temperature, Cell Constants

Valve Positioner Valve Position, Actual Position, Actuator Temperature, Supply Pressure, Output Pressure

Level Level, Interface level, Temperature, Delta level

Coriolis Mass Flow Mass flow, Volumetric flow, Temperature, Density, Totalizer

Device Status and Diagnostics

One of the many benefits of HART technology is the ability to diagnose potential device problems either at the device, at the control system I/O, within the control system or anywhere in between. Each HART device has a 38-bit address that consists of the manufacture ID code, device type code and a device unique identifier. Two bytes of status, also called response code, are included in every message from a HART device. These two bytes convey three very useful types of information:

Communication errors Command response problems Field device status

Field Device Status HART devices provide valuable status information, which not only indicates when the device is malfunctioning but also provides a description of the failure (like a temperature transmitter indicating "sensor break" as the reason for the malfunction). Device diagnostics and eight status alerts are automatically returned with every communication message. These status alerts may provide early warning of a potential device problem, can automatically detect range or configuration changes and validates the PV with the loop current thereby increasing the integrity of the control system information. This feature saves a lot of effort involved in debugging of process and instrumentation issues allowing troubleshooting and repair to be done quickly.

Most HART field instruments provide both status information and diagnostic information. The HART protocol defines basic status information as information that is included with every message from a field device. Basic status information enables the host application to immediately identify warning or error conditions detected by the field device. Status messages also enable the user to differentiate between measurements that are outside sensor or range limits and actual hardware malfunctions. Examples of status messages are:

Field device malfunction Configuration changed Cold start More status available Analog output current fixed Analog output saturated Non-primary variable out of limits Primary variable out of limits

HART instruments can implement extensive, device-specific diagnostics. The amount and type of diagnostic information is determined by the manufacturer and varies with product and application. Diagnostic information can be accessed using the HART communication protocol. Host applications using DD files can interpret and display diagnostic information. Applications not using DD technology may require product-specific software modules to interpret diagnostic information. Many manufacturers offer special software applications for their own products. Some modules allow you to customize for specific products. Manufacturers of valve actuators have made extensive use of this capability to provide preventative and predictive diagnostic information that greatly enhances the value of their products as compared to conventional actuators. Several software applications are available that provide continuous communication with field devices using a HART-compatible multiplexer and HART I/O. These applications provide real-time monitoring of status and diagnostic information.

Field Device Status Codes

Bit No. Definition Bit 7 Device Malfunction — The device detected a serious error

or failure that compromises device operation. Bit 6 Configuration Changed — An operation was performed

that changed the device’s configuration. Bit 5 Cold Start — A power failure or Device Reset has occurred. Bit 4 More Status Available — More status information is

available via Command 48, Read Additional Status Information.

Bit 3 Loop Current Fixed — The Loop Current is being held at a fixed value and is not responding to process variations.

Bit 2 Loop Current Saturated — The Loop Current has reached its upper (or lower) endpoint limit and cannot increase (or decrease) any further.

Bit 1 Non-Primary Variable Out of Limits — A Device Variable not mapped to the PV is beyond its operating limits.

Bit 0 Primary Variable Out of Limits — The PV is beyond its operating limit.

Note: Voltage Mode Field Devices must use "Loop Current Fixed" and "Loop Current Saturated" to indicate the signaling voltage is fixed or saturated.

HART 6

Since 1989, all HART-enabled devices were designed to the HART 5 specification. To better support the continuously growing functionality offered by HART devices as they become more intelligent, the HART protocol has evolved to HART 6.

The HART 6 protocol is 100% backward compatible - so current users of HART instruments and systems will have the assurance that those devices will be fully compatible with HART 6 products, protecting their investments. The ongoing support of HART technology and its enhancements by major global suppliers and Users paves the way for HART devices to continue to remain a significant contributor to automation solutions by providing additional device information to higher level systems. This information can then be used for asset management, device diagnostics, remote communication of parameter settings, multi-variable information and general device health intending to lower operating cost, improve efficiencies and enhance asset management.

A HART 6 device includes HART 5 as a subset, so the migration from HART 5 to HART 6 is painless for the User:

When connected to a HART 5 Master, a HART 6 device behaves like a HART 5 device and the HART 5 functions are fully available.

When connected to a HART 6 Master, the extra functionality provided by HART 6 becomes available.

Major enhancements in HART 6 include:

Improved host integration by providing device family definition

Expanded data and device status information improving host ability to act on diagnostic information

32 character tags for more flexibility and consistency with plant standards

A updated Block Data Transfer to support the transfer of large blocks of data between masters and field devices

New common practice commands to support commissioning and troubleshooting of HART devices in multi-drop and multi-pair cable installations

Wiring and Installation

In general, the installation practice for HART communicating devices is the same as that for conventional 4-20mA instrumentation. Individually shielded twisted pair cable, either in single pair or multi-pair varieties, is the recommended wiring practice. Unshielded cables may be used for short distances, if ambient noise and cross-talk will not effect communication. The minimum conductor size is 0.51mm diameter (#24 AWG) for cable runs less than 1,524 meters (5,000 ft.) and 0.81mm diameter (#20 AWG) for longer distances.

Typical Wiring Connection using a HART Field Device, a PC with a PC-based Application and a Handheld Terminal. Also shown is a required HART Interface or Modem - either RS232 or USB version.

Grounding

To prevent interference by external signals, it is important to ground the system properly. In particular, the signal loop should be grounded, if at all, at one point only. The cable screening must be connected to ground, at one point only, and must not be connected to instrument or junction box cases unless these are isolated from ground. The single ground point will usually be at or near the host (for example, the control system).

Power supply

Power for a two-wire instrument loop is typically 24V d.c. As always, the voltage must be sufficient to provide the necessary lift-off voltage for the field device, taking into account voltage drops in the cable and load resistor, and a passive IS barrier if one is present. Smart devices may take up to 22 mA to indicate an alarm condition; use this value to calculate the worst loop voltage drop.

There are additional communication-related specifications for the power supply for a HART loop; these are shown in the table below. The ripple and noise specifications are designed to prevent direct interference with the HART signals. The impedance limit ensures that HART signals see the power supply as a low impedance path, and prevents inadvertent coupling and crosstalk between multiple HART loops powered from a common supply. (The resistance of output fuses, if any, must be included, when measuring this value.)

Power supply specifications

Maximum ripple (47 to 125 Hz) 0.2 V p-p Maximum noise (500 Hz to 10 kHz) 1.2 mV rms

Maximum series impedance (500 Hz to 10 kHz) 10 Ω

Cable Length Most installations are well within the 3,000-meter (10,000 ft) theoretical limit for HART Communication. However, the electrical characteristics of the cable (mostly capacitance) and the combination of connected devices can affect the maximum allowable cable length of a HART network. The table below shows the affect of cable capacitance and the number of network devices on cable length. The table is based on typical installations of HART devices in non-IS environments, i.e. no miscellaneous series impedance. Detailed information for determining the maximum cable length for any HART network configuration can be found in the HART Physical Layer Specifications.

The allowable cable length depends upon the cable capacitance and the number of network devices.

Cable Length - feet (meters)

No. Network Devices

Cable Capacitance – pf/ft (pf/m)

20 pf/ft (65 pf/m)

30 pf/ft (95 pf/m)

50 pf/ft (160 pf/m)

70 pf/ft (225 pf/m)

1 9,000 ft (2,769 m)

6,500 ft (2,000 m)

4,200 ft (1,292 m)

3,200 ft (985 m)

5 8,000 ft (2,462 m)

5,900 ft (1,815 m)

3,700 ft (1,138 m)

2,900 ft (892 m)

10 7,000 ft (2,154 m)

5,200 ft (1,600 m)

3,300 ft (1,015 m)

2,500 ft (769 m)

15 6,000 ft (1,846 m)

4,600 ft (1,415 m)

2,900 ft (892 m)

2,300 ft (708 m)

Allowable cable lengths for 1.02 mm (#18 AWG) shield twisted pair

Intrinsic Safety Intrinsic Safety (IS) is a method of providing safe operation of electronic process-control instrumentation in hazardous areas. IS systems keep the available electrical energy in the system low enough that ignition of the hazardous atmosphere cannot occur. No single field device or wiring is intrinsically safe by itself (except for battery-operated, self-contained devices), but is intrinsically safe only when employed in a properly designed IS system.

Intrinsic Safety Devices HART communicating devices work well in applications that require IS operation. IS devices (e.g., barriers) are often used

with traditional two-wire 4-20 mA instruments to ensure an IS system in hazardous areas. With traditional analog instrumentation, energy to the field can be limited with or without a ground connection by installing one of the following IS devices:

Shunt-diode (zener) barriers that use a high-quality safety ground connection to bypass excess energy (Figure 1). Isolators (which do not require a ground connection) that repeat the analog measurement signal across an isolated

interface in the safe-side load circuit (Figure 2).

Both zener barriers and isolators can be used to ensure an IS system with HART-communicating devices, but some additional issues must be considered when engineering the HART loop.

Figure 1.

Figure 2.

HART Multidrop Networks The HART communication protocol enables several instruments to be connected on the same pair of wires in a multidrop network configuration (Figure 8). The current through each field device is fixed at a minimum value (typically 4 mA) sufficient for device operation. The analog loop current does not change in relation to the process and thus does not reflect the primary variable. Communications in multidrop mode are entirely digital.

Multidrop Configuration

Standard HART commands are used to communicate with field instruments to determine process variables or device parameter information. The typical cycle time needed to read information on a single variable from a HART device is approximately 500 milliseconds (ms). For a network of 15 devices, a total of approximately 7.5 seconds is needed to scan and read the primary variables from all devices. Reading information from multivariable instruments may take longer, as the data field will typically contain values for four variables rather than just one.

The typical multidrop network enables two-wire measurement devices to be connected in parallel. Two-wire loop-powered and four-wire active-source devices can be connected in the same network. If both two-and four-wire devices are used in the same network, three wires must be used to properly connect the devices.

Summary

In Course 1 - HART Basics, we learned:

HART technology enables two-way digital communication to occur between intelligent devices and connected host or control systems.

That HART is an enabling technology meaning that it can be applied in many different application including control, monitoring, safety, asset management, etc.

HART provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The digital signal contains information from the device including device status, diagnostics, additional measured or

calculated values, etc. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA

current loop. The digital communication part of the communication is a request-response communication protocol, which means

that during normal operation, each device communication is initiated by a request from a host device. The digital signal is made up of two frequencies - 1,200 Hz and 2,200 Hz representing bits 1 and 0. Sine waves of

these two frequencies are superimposed on the direct current (dc) analog signal wire to provide simultaneous analog and digital communications.

A minimum loop impedance of 230 ohms is required for communication. HART devices can operate in one of two network configurations-point-to-point or multidrop. The HART Command Set includes three classes: universal, common practice, and device specific. There are 35-40 data items that are standard in every HART registered device. Host applications may implement

any of the necessary commands for a particular application. The HART device suppliers provide a DD file for each HART device and the DD file combines all of the

information needed by the host application into a single structured file. The DD file includes all of the information needed by a host application to fully communicate with the field device

and access all of the information available from the device. The new HART enhanced Device Description Language extends the capabilities of DDL to provide for advanced

visualization of intelligent device information. HART Protocol is backward compatible with the installed base of instrumentation and control systems in use today.

A new HART-enabled device can replace an existing 4-20mA analog-only device of similar measurement capability without change to the host system or wire.

Most HART devices provide multiple process variables, which can be used for monitoring or control to gain a better insight into the process.

Device diagnostics and status information are returned with every communication message. The installation practice for a HART communicating device is similar for a conventional 4-20mA instrument. HART communicating devices can be used in applications that require IS operation.

Future lessons will include: Control System Interfaces, Data Handling, Gateways, SCADA/RTU Systems, Multiplexers, Reading HART Data into Non-HART Systems, PC-based Configuration Software and how HART delivers benefits in all phases of the Plant Life Cycle.

HART – GETTING CONNECTED TRAINING COURSE II INTRODUCTION

This course is the second in the three part series and follows HART Basics, which should be taken first. Course I covered basic HART technology and presented and identified wiring and installation concepts. Course II identifies the many possible ways to Get Connected to the HART data and information sitting in a HART-enabled device.

There are many different control and data acquisition systems installed throughout the world. Suppliers of HART-enabled devices have created many cost-effective solutions that allow HART data to be used by virtually all of these systems. HART technology’s simultaneous use of two communication channels (analog and digital) allows Users to tap into valuable information from HART-enabled intelligent field devices regardless of the age or capability of the system.

In addition, Distributed Control System (DCS) suppliers have developed upgrades or enhancements that provide alternative ways of retrieving, using and benefiting from the vast amount of information in HART-enabled field devices.

Getting Connected

Today, most process automation system suppliers offer a variety of HART interface solutions to support integration with their control systems. Many have intelligent I/O subsystems and system interfaces for direct connection to HART devices. Most allow the HART data to be used in real-time for operator display, alarm and control functions.

In addition, third-party I/O systems and interface products are available to support integration with legacy control systems that might not be easily upgraded for continuous HART communication. Gateway interface solutions are also available for linking HART devices to systems based on network protocols such as Ethernet, Modbus, and PROFIBUS.

Real-time HART integration with plant control, safety and asset management systems unlocks the value of connected devices and extends the capability of systems to detect problems with the device, its connection to the process or faults distorting the 4-20mA current loop.

The justification for integration is two-fold; (1) cost reduction as a result of improved operations and increased efficiencies, and (2) cost avoidance because early warning to impending problems drastically reduces process disruptions and unplanned shutdowns.

The Argument

In order to meet corporate objectives to lower operating costs and increase plant availability, it is imperative that you maximize your assets and utilize every bit of data you have. HART technology makes the access of such information easy and efficient, but you must integrate the intelligence in your devices into your current system architecture. Start with a single loop or a single device or start with a select group of devices, the option is yours.

Why integrate HART data from intelligent field devices?

Every HART-enabled device is very intelligent and contains information that helps lower operating costs, improve product quality and extend process uptime.

The information within every HART-enabled device is available 24 hours a day, 7 days a week. All you have to do is start using it!

The information includes device specific status, process information, multi-variable measurements, alerts, diagnostics, and more.

Real-time diagnostic alerts provide early warning of device or process problems and reduces time from problem identification to problem resolution.

Early warning of pending problems enables proactive action to minimize negative impact. Remote diagnostics reduce unnecessary trips into the field. Advanced diagnostics increases safety integrity level (SIL). Intelligent multivariable devices minimize the need for additional sensor points. Provides continuous validation of control information and loop integrity. Facilitates automated record keeping and documentation for compliance requirements. Facilitates automated safety shutdown testing (i.e. partial stroke testing). Enables detection of previously undetected problems. Increases asset productivity and system availability. Possibly avoid the high cost of unscheduled shutdowns or process disruptions. Leverage intelligent device capabilities for operational improvements including Lean Manufacturing and Six Sigma.

Multiple Use Cases Most large process plants are controlled and monitored by Distributed Control Systems (DCSs). These DCSs are typically connected to a large number of HART-enabled instruments and control valves via 4-20mA signals. Every HART-enabled device has a simultaneously transmitted digital signal encoded on the same wire as the 4-20mA signal. The digital information in each device is available for you to use and contains a wealth of useful process and device diagnostic information.

Most of the major DCS and control system vendors have HART Communication capability in their newer control systems, and many have upgrade paths to enable HART Communication for their older systems.

HART Communication can be utilized in:

Asset Management applications and systems Legacy control systems Monitoring and SCADA applications New construction projects Plant expansions Plant upgrades Regulatory Compliance Safety Systems and Safety-related applications

Accessing Your HART Data

You can access HART data in two ways – Point-To-Point and Multiple Point – on a temporary or a permanent basis. Point-To-Point Access of HART Data

Point-To-Point - Temporary Connections

This method involves connecting one of several types of communicators directly to a HART-enabled device. There are many reason why you would want to communicate with a specific device. They include:

Device Configuration Device Diagnostics Device Troubleshooting Loop Integrity Check Process Troubleshooting

Two standard methods of connecting to a HART-enabled device on a temporary basis are:

Universal Handheld Communicators PC Based Device Management Tools

Universal Handheld Communicators

HART Handheld Communicators are available from major instrumentation suppliers around the globe and are supported by HCF member companies. Using Device Description (DD) files (see Course I), the communicator can fully configure any HART device for which it has a DD installed. If the communicator does not have the DD for a specific device, it will still communicate and configure the device using the HART Universal and Common Practice commands. (See HART Commands in Course I.)

There are 35-40 standard data items in every registered HART device. The data can be accessed by any approved communicator / configuration tool. These items do not require the use of a DD and typically include the basic functionality for all devices. These are the Universal and Common Practice commands required of every registered HART device (see HART Commands in Course I). To access the device specific data, a current DD is required and provides the communicator with the information it needs to fully access all the device specific capabilities.

The HCF provides centralized control and registration for all DDs that can be loaded into the communicator. Because field devices and their DDs are updated often, it is important to be sure that the handheld communicator contains the latest DD for devices that are being used in your plant.

A HART handheld communicator, if equipped, can also facilitate record keeping of device configurations. As-installed device configuration data can be stored in memory or on a disk for later archiving or printing. There are many types of handheld communicators available today. Be sure you review their features and ability to meet your specific requirements.

When connecting to a device, be sure to follow the wiring instructions outlined in Course I and observe all plant area safety requirements. Remember, the connections to a device can be made anywhere on the control loop. The connections do not have to be physically located at the device.

Figure1. HART Handheld

PC Based Device Management Tools - Temporary

You can configure a HART-enabled device with a desktop or laptop PC by using a PC-based software application and a HART interface modem. The advantages of using a PC include an improved screen display and support for more DDs and device configurations due to additional PC-based storage capacity. Due to the critical nature of device configurations in the plant environment, PCs can also be used as backup storage for data from handheld communicators.

Figure 2. Connecting a PC to a HART device

Today, handheld PCs, PDAs and other portable computers are expanding the available options for accessing HART data. PC software applications can perform all of the functions of a handheld communicator and more. However, caution should be taken when using a PC in hazardous areas of the plant.

Software applications are available from many suppliers. It is important to review their features to determine ease of use, ability to add or download DDs and general functionality.

Figure 3. Sample configuration application

Permanent Digital Connections

Continuously monitoring both of the two HART communication channels improves system performance and reduces unwanted downtime.

Reliable and accurate communication is Critical to Good Control Smart Instrument Applications using only the 4-20mA signal reduces system performance

– Communication limited to One Data Item – PV – Device Diagnostics, Secondary Process Variables, Status Information… Lost

Integrity of the Control Loop is NOT Assured – Faults distorting the 4-20mA current loop may not be detected – Problems if 4-20mA set-up in device and control system not aligned

Point-to-Point - Permanent Connections

Many HART products are able to perform more than one measurement or output function (e.g., make multiple process measurements, calculate process information, and provide positioner feedback information). All of this information can be easily accessed through the digital channel. However, existing controllers and interface equipment may not have the ability to read digital HART data. Products are available that can read HART digital signals and convert them to additional analog outputs or contact closure outputs, which enables any traditional analog/digital I/O to take full advantage of the benefits of HART-enabled devices.

For example, HART loop monitors connect transparently onto a HART loop and read the simultaneous HART digital message encoded on the analog signal wires. The loop monitor converts three of the four digital variables to three separate analog outputs (see Figure 4). The conversion allows the user to extract additional variables within a device for use with traditional analog I/O. HART monitors are typically located in the control room or marshalling cabinet to take advantage of the simultaneously transmitted HART digital signal.

Figure 4

The HART monitor continuously communicates with any HART-enabled device and provides contact closure outputs (alarm trips) based on the information received (see Figure 4). For example, the device can be configured to monitor the device-status information inherent in the HART-enabled device and to trigger events such as device malfunction or in-range failure. The monitor can also initiate emergency shutdown action when problems are detected on a critical loop.

USE CASE: PARTIAL STROKE TESTING On-line testing of Emergency Shutdown (ESD) valves can be accomplished using a HART loop monitor to verify that the valve is operating within specification without the process disruption of completely closing the valve (which is the traditional way to verify ESD valve operation). (See Figure 5.)

Figure 5

Partial Stroke testing can be performed by applying a control signal from a PLC or a DCS. Have the control system output a signal equivalent to 90% (18.4mA). When the valve reaches the 90% set point, the contact closure in the HART monitor will trip verifying that the valve has reached 90%. The control signal should then be returned to 100% value by the Control System, and the valve reopened. A second contact closure is tripped at 100% (full open) travel to ensure that the valve did reopen completely after the test.

This procedure verifies that the valve reached 90%, proving that the valve is not stuck. Because the valve was immediately reopened, the test has not substantially impeded the process flow reducing any process disruption. Additional analog outputs from the HART monitor can be programmed to provide status information for other important valve parameters such as valve travel and valve output pressure. PC Based Devices Management Tool - Permanent Connections PC based device management tools are sometimes used to permanently monitor specific devices. This is done using the same equipment and techniques mentioned above. Long-term monitoring of a critical control loop or a device suspected of being a potential problem may provide the information necessary to correct the problem.

Multiple Point Access To HART Data

Applications that require multi-point access to HART devices and their data require permanent or continuous monitoring of all devices. The full-time connection to HART devices by control, asset management and safety systems enables the full-time monitoring of potential device or process problems.

Regardless of the age or vintage of your control system, there are cost-effective solutions available to access HART data either on a full-time or part-time basis. These HART-enabled solutions include:

HART-enabled I/O Gateways Multiplexers Remote I/O

Many process automation control systems and instrumentation suppliers are currently investing heavily in HART technology. Most suppliers now offer control system interfaces, I/O systems and PC-based software applications that leverage the intelligence in HART-enabled field devices to deliver continuous, real-time device diagnostics, multi-variable process information and much more.

Integrating this information into DCS architectures enables the user to get the full benefit from intelligent devices, making HART Communication an important part of plant applications for control, safety, asset productivity and more. Continuous communication, between the field device and control system, enables problems with the device, its connection to the process, or inaccuracies in the 4-20mA control signal to be detected automatically so that corrective action can be taken to avoid process disruptions.

When you consider changing or upgrading your control system by adding a HART interface, it is important to understand the complete functionality offered by the HART interface. While several control system suppliers offer HART interfaces, not all interfaces provide the same level of integration or functionality.

There are several techniques used by control interfaces to access the HART data. In addition to controlling the 4-20mA control loop, HART modems are added to the interface to access the digital HART data. The way the modem is implemented has a direct relationship to the speed of access to all connected inputs. A common method of access to HART data uses a multiplexer design, which assigns one HART modem to a specific number of inputs. HART-Enabled I/O Many HART-enabled I/O subsystems have multiple analog channels on each I/O card. Suppliers choose whether to provide one HART interface (a modem) per channel or to share one HART interface among several channels. The number of shared channels per interface impacts the frequency of data updates from each HART field device and the HART functionality supported.

For example, some designs assign one HART modem to 2, 4, 8, or more analog inputs significantly increasing the time it takes for all HART-enabled devices connected to the interface to be monitored. It is important to understand the functionality or throughput of the HART I/O depending on each application.

For the best performance and flexibility, one HART interface should be dedicated to each I/O channel. Systems that share only one HART interface among several I/O channels may not support multi-drop networks. The effective update rate of a

multiplexed interface is slow enough that the performance of a multiplexed, multi-drop network might not be practical. Some suppliers enable multi-drop support by fixing the HART interface to one specific I/O channel. However, the other channels on that I/O card may not be available to support HART Communication.

HART I/O For Burst Mode Applications Burst mode is an optional implementation in a HART-enabled field device. Receiving burst mode messages is optional in a host system. To take full advantage of burst mode, the I/O system should have one HART interface for each channel. If the HART interface is shared by more than one channel, the messages sent by the field device may not be detected by the control system. If the system does not have the ability to configure burst mode in the field device, a handheld communicator or other configuration tool can be used.

Pass-through Feature Some control systems are integrated with a configuration or instrument-management application. In these systems, the control system passes a HART command, issued by the management application, to the field device via its I/O interface. When the control system receives the reply from the field device, it sends the reply to the management application. This

function is referred to as a pass-through feature of the control system. The main advantage of a pass-through feature is less hardware and wiring as such IO systems eliminate the need for an external Smart Multiplexer system.

Gateways Gateways can be used to bring HART digital data into control systems that do not support HART-capable I/O. Some systems support HART gateways with communication protocols such as Modbus, PROFIBUS DP, or TCP/IP Ethernet. The typical HART gateway supports all universal commands and a subset of the common practice commands. Support varies depending on the gateway supplier. Some gateways support access to device-specific information.

The following is an example of a HART-to-Modbus gateway that uses a multi-drop HART segment as input and Modbus RTU Data Link as the output to a legacy control system.

Figure 1

SCADA / RTU Systems RTUs used in SCADA systems use a special telemetry to communicate with the control system. RTUs have the same considerations regarding multi-drop and burst mode support as other systems. However, implementation is made more complex because RTUs often communicate to an upper-level host using a communication protocol other than HART (e.g., Modbus). While there are many benefits to implementing HART in an RTU (support of multi-drop, burst mode, and multivariable instruments), HART data is available to the central host system only if the telemetry protocol supports the transfer of HART commands or specific HART data.

Multiplexers

HART-compatible multiplexers are ideal for users who want to interface with a large number of HART devices connected to a legacy analog system or where the I/O is not capable of direct digital communications with the HART device. Multiplexers can be modular and are capable of supporting both point-to-point and all-digital (multi-drop) HART Communication modes. Communication between a multiplexer and a host application depends on the multiplexer capabilities but often includes RS232C, RS485, Modbus, TCP/IP Ethernet or others.

When installing HART multiplexer systems, the following capabilities should be considered:

Number of HART channels supported Number of HART channels that share a HART modem Burst mode support Multi-drop support Method of communication with the host computer or control system

Multiplexers As The Primary I/O System

HART multiplexers can be used as the primary I/O front end for a HART-based control or monitoring system (see figure). Typically, a PC acts as the host, providing the human-machine interface and performing other high-level functions. The multiplexer continuously monitors the field devices, reports the current readings and instrument status to the host, and passes HART commands from the host computer to the field devices.

Figure 1. HART Multiplexer as the Primary I/O System

Parallel Monitoring With A Multiplexer

When a traditional 4–20mA control system is using the analog signal for measurement and control, a HART multiplexer can be added to the network to gain access to the digital HART signal. Using a multiplexer enables a supervisory computer to monitor diagnostics and device status, access configuration information, and read any additional process inputs or calculations not provided by the 4–20mA signal. Two types of multiplexers are used in conjunction with a control system. A multiplexer wired in parallel with the field wiring is commonly used when the control system wiring is already in place (see Figure 2).

Figure 2. HART Multiplexer with existing I/O

In this case, the 4-20mA control and monitoring signals are allowed to pass-through the multiplexer unchanged and are passed directly into the traditional control system. The HART signal (red dash lines in Figure 2) is simultaneously translated into the required protocol and passed to an asset management or other type of monitoring system. This allows the legacy system to remain unchanged but still allows access to the HART information sitting in the intelligent field devices.

Remote I/Os

Remote I/O solutions behave like I/O cards that are located or placed in the field and are connected via a communication link or fieldbus to the Control System. They provide signal conversion (A/D or D/A conversion) for the analog signal, HART Protocol conversion and may include a power supply for the loop, IS barriers, galvanic isolation and filtering capabilities.

The Remote I/O also acts as a gateway by embedding the HART information into another industrial protocol such as PROFIBUS, INTERBUS, ControlNET or Ethernet / PROFINET. Because the Remote I/O is located in the field – near or at a large concentration of inputs - it provides significant cabling cost reductions (see Figure 3).

Figure 3. HART Remore I/O

Commissioning HART Networks

HART-based instruments have several features that significantly reduce the time required to fully commission a HART network (loop). When less time is required for commissioning, substantial cost savings are achieved.

Device Verification

Before installation, manufacturers usually enter device tags and other identification and configuration data into each field instrument. After installation, the instrument identification (tag and descriptor) can be verified in the control room using a configurator (handheld communicator or PC). Some field devices provide information on their physical configuration (e.g., wetted materials)—these and other configuration data can also be verified in the control room. The verification process is important for safety and in conforming to governmental regulations and ISO quality requirements.

The commissioning process can be further streamlined by connecting a PC configurator to each HART loop online, either by integration with the control system or by using one of the many available HART multiplexing I/O systems. With this centralized approach, there is no need to move the configuration device from one termination point to the next while commissioning all devices on the network.

Loop Integrity Check

Once a field instrument has been identified and its configuration data confirmed the analog loop integrity can be checked using the loop test feature, which is supported by many HART devices. The loop test feature enables the analog signal from a HART transmitter to be fixed at a specific value to verify loop integrity and ensure proper connection to support devices

such as indicators, recorders, and DCS displays. Use the HART Protocol loop test feature to check analog loop integrity and ensure a proper physical connection among all network devices.

Additional integrity can be achieved if the analog value is compared to the digital value being reported in a device. For example, someone might have provided an offset to the 4-20mA analog value that has not been accounted for in the control system. By comparing the digital value of the Primary Variable to the analog value, the loop integrity can be verified.

As-Installed Record Keeping

Many HART configurators also facilitate record keeping. As-installed device configuration data can be stored in memory or on a disk for later archiving or printing.

Getting It Right While on a superficial level it might seem like every vendor offers a similar level of support for HART Communication in their instruments and systems, in fact capabilities vary widely. Consequently, it is recommended that you question vendors carefully about their products’ “HARTability.”

Evaluating Smart I/O

Possible questions for your I/O supplier include:

How much HART capability is built into the I/O and how smart is it? Can the I/O validate and secure the 4-20mA signal? Is there one HART modem per channel, or is the I/O multiplexed? How fast can it update the HART digital values? In what ways does the system support access to multivariable HART data from multivariable devices? Can you merely “push a button" on the I/O to calibrate the loop current and/or check the range? Does the I/O support multi-drop? Does the I/O automatically scan and monitor the HART-enabled field devices or is the scanning only possible using

"pass through"?

Evaluating HART-enabled Control System

Possible questions for your control system suppliers include:

Does the system make it easy to use all HART capabilities? How much training is required to learn how to get and use HART data? Review the configuration of a HART device using the control system. Can the system use secondary digital process variables? Does it understand the HART device status change? Can the system detect configuration changes? Does the system do notification by exception? How does the system detect changes in configuration and status?

How is the HART device status communicated to the operators? How do you perform tests when there is an error in the device? How open is the system to third party software?

Train Your Staff in HART Capabilities

Experienced instrument technicians and process engineers will have little trouble familiarizing themselves with HART Protocol capabilities or putting them to work. It is unlikely they will require much – if any – additional training. However, if you want an in-depth look at HART technology and the way it is used, you can obtain training from many of the vendors who sell HART-enabled devices and systems. In addition, the HART Communication Foundation offers numerous HART technology workshops throughout the world, educational webcasts and a CD-based library of information.

Tapping into HART

The potential benefits of HART Communication go way beyond taking readings with handheld communicators. You may be thinking about making some careful steps toward tapping into your HART-enabled field instruments for their added value. But like most journeys, the first steps are the most intimidating. Where to begin? What to expect?

Based on the information presented in this course, you can implement HART technology a little at a time. So, look for projects with the most potential value--start small, expand gradually, and be willing to take a few prudent risks. Here are some tips to consider.

Become the HART Champion

Users involved in all aspects of plant operations have a role to play in fully implementing and attaining the benefits of HART Communication. In most companies, no single group has total ownership of this powerful technology. Consequently, to make things happen you need to be the HART champion in your organization. If there are areas in which you think HART technology can be advantageous to your business, why not step up and drive the effort?

Select a Proving Ground

The fact that even the smallest HART implementation can yield huge benefits with minimal investment can relieve some of the risk and anxiety that naturally accompany upgrade projects. Consider starting small and carefully evaluating areas ripe for HART-driven improvements.

Evaluate Your Expenditures

As mentioned earlier, nearly every plant has at least some of the components it needs to begin realizing HART-enabled benefits. Before investing in additional hardware and software, evaluate what you already have and determine how much more you actually need to invest.

Use the Benefits

Be sure to evaluate how you want to use the additional HART data. Using the data may involve a change in the way people look at maintenance since HART data can significantly influence decisions, scheduling of outages and start-ups. Investing in HART technology on at least a small scale can produce big benefits. Users have indicated that getting the project approved is easier when you can show the ability to implement HART Communication without having to shut down processes, the ease of migration to HART smart devices, and the low risk and high benefit HART can provide.

HART Applications

Now that you are connected full-time to the HART data in your HART-enabled field devices, you need to consider different applications. Additional application information is discussed in detail in Course III.

Review

In Course II we learned:

1. Regardless of the age or capability of your control, safety or asset management system, HART technology can provide additional information to help extend time between shutdowns, lower maintenance cost and improve process performance.

2. The two most common tools used to temporarily connect to a HART-enabled device are: a handheld communicator or a PC running a configuration software application and connected via an external modem.

3. A handheld communicator or a properly equipped PC can be connected at any point on the loop to access HART Communication data – eliminating the need to be located at or connected to the terminals on the specific device.

4. In order to access all of the intelligent information in a specific device, the handheld communicator or the PC-based software application need to contain the current device description (DD) that matches the model and revision level of your device.

5. There are 35-40 standard data items in every registered HART device that can be accessed without using the DD. 6. Once a device has been configured, the configuration can be saved to facilitate device replacement or maintenance /

regulatory record keeping. 7. HART technology can be used on a Point-to-Point basis – monitoring the information from a single device – or on a

Multipoint basis – simultaneously monitoring the information from many devices. 8. The information from intelligent HART-enabled devices can be monitored for device status alerts on a temporary

basis (daily scan of all devices on the network) or can be monitored full-time and the information integrated into the control or monitoring system.

9. The translation of the digital HART Protocol signal can be done by an external (or stand alone) device or can be done using a HART-enabled I/O system that allows the termination of the 4-20mA signal to an I/O card. (The Primary Variable is converted to a digital signal and the HART data is translated into digital information, which can be used by the system.)

10. The actual conversion or translation of HART Protocol information can be done in the field – close to the HART-enabled device – or in the control or relay room.

11. Many products and tools are available to retrofit existing systems to access HART data. With these tools, you can easily replace wire termination boards (in some cases pin-for-pin replacements), which allows the 4-20mA signal to be passed into the control system – unchanged. The HART data is passed to the HART-enabled asset management system.

12. Since HART data is read using a modem, it is important to know the ratio of modems to input signals in the HART-enabled I/O. This item is one of the main factors used to determine the scan rate of each signal connected to the I/O card.

13. Gateways are used to convert HART Protocol information to other standardized protocols such as HART to Modbus, PROFIBUS, Ethernet, TCP/IP, etc.

14. HART multiplexers can be used as the primary I/O front end for a HART-based control or monitoring system 15. Not all suppliers implement HART Protocol the same way. It is important to understand how your suppliers

implement this enabling technology so that you will be better able to predict performance and results. 16. Gaining access and using HART data can be done one loop at a time. Migrating to larger applications will provide

full-time monitoring of more devices, therefore increasing the benefit. This scalability offers the User the ability to determine the size and scope of the use of valuable HART Protocol information.