HART - Communication Protocol, Summaries of Electronic Measurement and Instrumentation

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APPLICATION GUIDE

HCF LIT 34

Table of Contents

HART A PPLICATION GUIDE

Table of Contents

HART A PPLICATION G UIDE

© 1999 HART Communication Foundation Page 1

Preface

In today’s competitive environment, all companies seek to reduce operation costs, deliver products rapidly, and improve product quality. The HART® (highway addressable remote transducer) protocol directly contributes to these business goals by providing cost savings in: q Commissioning and installation q Plant operations and improved quality q Maintenance The HART Application Guide has been created by the HART Communication Foundation (HCF) to provide users of HART products with the information necessary to obtain the full benefits of HART digital instrumentation. The HART communication protocol is an open standard owned by the more than 100 member companies in the HCF. Products that use the HART protocol to provide both analog 4–20 mA and digital signals provide flexibility not available with any other communication technology. The following four sections provide you with an understanding of how the HART technology works, insight on how to apply various features of the technology, and specific examples of applications implemented by HART protocol users around the world: q Theory of Operation q Benefits of HART Communication q Getting the Most from HART Systems q Industry Applications

HART A PPLICATION G UIDE

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Theory of Operation

The following sections explain the basic principles behind the operation of HART instruments and networks: q Communication Modes q Frequency Shift Keying q HART Networks q HART Commands

THEORY OF O PERATION

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Frequency Shift Keying

The HART communication protocol is based on the Bell 202 telephone communication standard and operates using 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, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications (Figure 1). Because the average value of the FSK signal is always zero, the 4–20 mA analog signal is not affected. The digital communication signal has a response time of approximately 2–3 data updates per second without interrupting the analog signal. A minimum loop impedance of 230 Ω is required for communication.

Figure 1: Simultaneous Analog and Digital Communication

“1” “0”

“1” “1”

“1” “1”

“0”

“0”

“0”

Time

20 mA

4 mA

Digital Signal

Analog Signal

Note: Drawing not to scale.

THEORY OF O PERATION

© 1999 HART Communication Foundation Page 5

HART Multidrop 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 (Figure 2). 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.

Figure 2: Point-to-Point Mode of Operation

Control System or Other Host Application

Multiplexer

Barrier

Handheld Terminal

Field Device

Note: Instrument power is provided by an interface or external power source that is not shown.

THEORY OF O PERATION

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HART Commands

The HART command set provides uniform and consistent communication for all field devices. The command set includes three classes: universal , common practice , and device specific ( Table 1). 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.

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.

SUMMARY TABLE

Note: Table 1 is a partial list of HART commands. See Appendices B, C, and D for more detailed information.

Universal Commands Common Practice Commands Device-Specific Commands

• Read manufacturer and device type
• Read primary variable (PV) and units
• Read current output and percent of range
• Read up to four predefined dynamic variables
• Read or write eight-character tag, 16-character descriptor, date
• Read or write 32-character message
• Read device range values, units, and damping time constant
• Read or write final assembly number
• Write polling address
  • Read selection of up to four dynamic variables
  • Write damping time constant
  • Write device range values
  • Calibrate (set zero, set span)
  • Set fixed output current
  • Perform self-test
  • Perform master reset
  • Trim PV zero
  • Write PV unit
  • Trim DAC zero and gain
  • Write transfer function (square root/linear)
  • Write sensor serial number
  • Read or write dynamic variable assignments - 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 sensor calibration - PID enable - Write PID setpoint - Valve characterization - Valve setpoint - Travel limits - User units - Local display information

Table 1: HART Commands

THEORY OF OPERATION

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HART Commands

ESTABLISHING

Digital Communication

WITH A

HART DEVICE

Each HART device has a 38-bit address that consists of the manufacturer ID code, device type code, and device-unique identifier. A unique address is encoded in each device at the time of manufacture. A HART master must know the address of a field device in order to communicate successfully with it. A master can learn the address of a slave device by issuing one of two commands that cause the slave device to respond with its address: q Command 0, Read Unique Identifier —Command 0 is the preferred method for initiating communication with a slave device because it enables a master to learn the address of each slave device without user interaction. Each polling address (0–15) is probed to learn the unique address for each device. q Command 11, Read Unique Identifier by Tag - Command 11 is useful if there are more than 15 devices in the network or if the network devices were not configured with unique polling addresses. (Multidropping more than 15 devices is possible when the devices are individually powered and isolated.) Command 11 requires the user to specify the tag numbers to be polled.

DEVICE

DESCRIPTION

Some HART host applications use device descriptions (DD) to obtain information about the variables and functions contained in a HART field device. The DD 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, that combines all of the information needed by the host application into a single structured file. The DD identifies which common practice commands are supported as well as 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. DDs eliminate the need for host suppliers to develop and support custom interfaces and drivers. A DD provides a picture of all parameters and functions of a device in a standardized language. HART suppliers have the option of supplying a DD for their HART field product. If they choose to supply one, 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 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 PCs and handheld terminals. Additional information, not provided by the DD, may be required by some host applications for screen formatting and other uses.

THEORY OF O PERATION

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Improved Plant Operations

The HART protocol improves plant performance and provide savings in: q Commissioning and installation q Plant operations and improved quality q Maintenance

COST SAVINGS IN

COMMISSIONING

HART-based field devices can be installed and commissioned in a fraction of the time required for a traditional analog-only system. Operators who use HART digital communications can easily identify a field device by its tag and verify that operational parameters are correct. Configurations of similar devices can be copied to streamline the commissioning process. A loop integrity check is readily accomplished by commanding the field transmitter to set the analog output to a preset value.

COST SAVINGS IN

INSTALLATION

The HART protocol supports the networking of several devices on a single twisted wire pair. This configuration can provide significant savings in wiring, especially for applications such as tank monitoring.

Multivariable devices reduce the number of instruments, wiring, spare parts, and terminations required. Some HART field instruments embed PID control, which eliminates the need for a separate controller, and results in significant wiring and equipment cost savings.

Use HART multidrop mode to connect multiple instruments to a single cable and reduce installation costs.

Benefits of HART Communication

Improved Plant Operations...........................................................................................................................

© 1999 HART Communication Foundation Page 11

IMPROVED

MEASUREMENT

QUALITY

HART-communicating devices provide accurate information that helps improve the efficiency of plant operations. During normal operation, device operational values can be easily monitored or modified remotely. If uploaded to a software application, these data can be used to automate record keeping for regulatory compliance (e.g., environmental, validation, ISO9000, and safety standards). Numerous device parameters are available from HART-compatible instruments that can be communicated to the control room and used for control, maintenance, and record keeping (Figure 4).

Figure 4: Examples of Device Parameters Sent to Control Room Some HART devices perform complex calculations, such as PID control algorithms or compensated flow rate. Multivariable HART-capable instruments take measurements and perform calculations at the source, which eliminates time bias and results in more accurate calculations than are possible when performed in a centralized host.

Some HART field devices store historical information in the form of trend logs and summary data. These logs and statistical calculations (e.g., high and low values and averages) can be uploaded into a software application for further processing or record keeping.

The HART protocol provides access to all information in multivariable devices. In addition to the analog output (primary variable), the HART protocol provides access to all measurement data that can be used for verification or calculation of plant mass and energy balances.

Field Control Room Device

BENEFITS OF HART COMMUNICATION

Tag Message Descriptor Range Values Diagnostic Information Four Process Variables Construction Materials 128 Device Variables Serial Numbers Damping Date

© 1999 HART Communication Foundation Page 13

Operational Flexibility..................................................................................................................................

The HART protocol allows two masters (primary and secondary) to communicate with slave devices and provide additional operational flexibility. A permanently connected host system can be used simultaneously, while a handheld terminal or PC controller is communicating with a field device (Figure 5).

Figure 5: Multimaster System The HART protocol ensures interoperablility among devices through universal commands that enable hosts to easily access and communicate the most common parameters used in field devices. The HART DDL extends interoperability to include information that may be specific to a particular device. DDL enables a single handheld configurator or PC host application to configure and maintain HART-communicating devices from any manufacturer. The use of common tools for products of different vendors minimizes the amount of equipment and training needed to maintain a plant.

HART extends the capability of field devices beyond the single-variable limitations of 4- 2 0mA in hosts with HART capability.

Transmitter Secondary Master

Primary Master: Control System or Other Host Application

Analog Digital Data (2–3 updates per second)

HART Interface

Power Supply

BENEFITS OF HART C OMMUNICATION

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Instrumentation Investment Protection

Existing plants and processes have considerable investments in wiring, analog controllers, junction boxes, barriers, marshalling panels, and analog or smart instrumentation. The people, procedures, and equipment already exist for the support and maintenance of the installed equipment. HART field instruments protect this investment by providing compatible products with enhanced digital capabilities. These enhanced capabilities can be used incrementally.

At the basic level, HART devices communicate with a handheld terminal for setup and maintenance. As needs grow, more sophisticated, on-line, PC-based systems can provide continuous monitoring of device status and configuration parameters. Advanced installations can also use control systems with HART I/O capability. The status information can be used directly by control schemes to trigger remedial actions and allow on-line reranging based on operating conditions and direct reading of multivariable instrument data.

COMPATIBILITY OF

HART REVISIONS

As HART field devices are upgraded, new functions may be added. A basic premise of the HART Protocol is that new HART instruments must behave in precisely the same manner as older versions when interfaced with an earlier revision host system.

The HART communication protocol enables you to retain your previous investments in existing hardware and personnel.

BENEFITS OF HART COMMUNICATION