© 2005 HART Communication Foundation

Copyright © 1999-2005 HART Communication Foundation. All rights reserved.

HART® is a registered trademark of the HART Communication Foundation. Any use of the word “HART” hereafter in this
document implies the registered trademark. All other trademarks used in this document are acknowledged to be trademarks of their
respective companies.

For additional information contact:

HART Communication Foundation
9390 Research Boulevard
Suite I-350
Austin, Texas 78759 USA
Tel: 512-794-0369
Fax: 512-794-3904
 HART APPLICATION GUIDE

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:
T Commissioning and installation
T Plant operations and improved quality
T 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:
T Theory of Operation3
T Benefits of HART Communications
T Getting the Most out of HART Systems
T Industry Applications

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 THEORY OF OPERATION

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

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 THEORY OF OPERATION

Communication Modes
MASTER-SLAVE HART is a master-slave communication protocol, which means that during
MODE normal operation, each slave (field device) communication is initiated by a
master communication device. Two masters can connect to each HART
loop. The primary master is generally a distributed control system (DCS),
programmable logic controller (PLC), or a personal computer (PC). The
secondary master can be a handheld terminal or another PC. Slave devices
include transmitters, actuators, 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 slave 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 slave
to stop bursting.
Use burst mode to enable more than one passive HART
device to listen to communications on the HART loop.

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 THEORY OF OPERATION

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.

20 mA

Digital
Signal

“1” “0”
“1”
“1” “0”
“1”
“0” “0”
“1”
Analog
Signal
4 mA

Time
Note: Drawing not to scale.

Figure 1: Simultaneous Analog and Digital Communication

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 THEORY OF OPERATION

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 (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.

Control System
Multiplexer or Other Host
Application

Barrier

Handheld Terminal

Field Device

Note: Instrument power is provided by an interface or external power source

that is not shown.

Figure 2: Point-to-Point Mode of Operation

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 THEORY OF OPERATION

HART Networks
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 (Figure 3). All process values are transmitted digitally. In
multidrop mode, all field device polling addresses are >0, and the current
through each device is fixed to a minimum value (typically 4 mA).
Use multidrop connection for supervisory control
installations that are widely spaced, such as
pipelines, custody transfer stations, and tank farms.

Control System or Other Host

Application

Handheld Terminal

Input/Output (I/O)
System

Field Devices

Note: Instrument power is provided by an interface or external power source that is

not shown.

Figure 3: Multidrop Mode of Operation

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 THEORY OF OPERATION

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 Common practice commands provide functions implemented by many, but

PRACTICE 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
Universal Commands Common Practice Commands Device-Specific Commands

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

Table 1: HART Commands

Note: Table 1 is a partial list of HART commands. See Appendices B, C,

and D for more detailed information.

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 THEORY OF OPERATION

HART Commands
ESTABLISHING Each HART device has a 38-bit address that consists of the manufacturer
COMMUNICATION 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
WITH A HART know the address of a field device in order to communicate successfully
DEVICE 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:
T 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.
T 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 Some HART host applications use device descriptions (DD) to obtain
DESCRIPTION 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.

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 BENEFITS OF HART COMMUNICATIONS

Benefits of HART Communications

The HART protocol is a powerful communication technology used to
exploit the full potential of digital field devices. Preserving the traditional
4–20 mA signal, the HART protocol extends system capabilities for
two-way digital communication with smart field instruments.
The HART protocol offers the best solution for smart field device
communications and has the widest base of support of any field device
protocol worldwide. More instruments are available with the HART
protocol than any other digital communications technology. Almost any
process application can be addressed by one of the products offered by
HART instrument suppliers.
Unlike other digital communication technologies, the HART protocol
provides a unique communication solution that is backward compatible
with the installed base of instrumentation in use today. This backward
compatibility ensures that investments in existing cabling and current
control strategies will remain secure well into the future.
Benefits outlined in this section include:
T Improved plant operations
T Operational flexibility
T Instrumentation investment protection
T Digital communication

© 2003 HART Communication Foundation Page 9

 BENEFITS OF HART COMMUNICATIONS

Improved Plant Operations

The HART protocol improves plant performance and increases efficiencies
in :
T Commissioning and installation
T Plant operations
T Maintenance

COST SAVINGS IN HART-based field devices can be installed and commissioned in a fraction
COMMISSIONING 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 The HART protocol supports the networking of several devices on a single
INSTALLATION twisted wire pair. This configuration can provide significant savings in
wiring, especially for applications such as tank monitoring.
Use HART multidrop mode to connect multiple instruments to
a single cable and reduce installation costs.

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.

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 BENEFITS OF HART COMMUNICATIONS

Improved Plant Operations

IMPROVED HART-communicating devices provide accurate information that helps
MEASUREMENT improve the efficiency of plant operations. During normal operation, device
operational values can be easily monitored or modified remotely. If
QUALITY 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).

Field Control Room

Device

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.
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.

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.

© 2003 HART Communication Foundation Page 11

 BENEFITS OF HART COMMUNICATIONS

Improved Plant Operations

COST SAVINGS IN The diagnostic capabilities of HART-communicating field devices can
MAINTENANCE eliminate substantial costs by reducing downtime. The HART protocol
communicates diagnostic information to the control room, which
minimizes the time required to identify the source of any problem and take
corrective action. Trips into the field or hazardous areas are eliminated or
reduced.
When a replacement device is put into service, HART communication
allows the correct operational parameters and settings to be quickly and
accurately uploaded into the device from a central database. Efficient and
rapid uploading reduces the time that the device is out of service. Some
software applications provide a historical record of configuration and
operational status for each instrument. This information can be used for
predictive, preventive, and proactive maintenance.

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 BENEFITS OF HART COMMUNICATIONS

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).

Analog
HART Interface
Digital Data
(2–3 updates
per second)

Primary Master: Power

Control System Supply
or Other Host
Application
Transmitter
Secondary Master

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 4–20 mA in hosts
with HART capability.

© 2003 HART Communication Foundation Page 13

 BENEFITS OF HART COMMUNICATIONS

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

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 As HART field devices are upgraded, new functions may be added. A basic
HART REVISIONS 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.

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 BENEFITS OF HART COMMUNICATIONS

Digital Communication
A digital instrument that uses a microprocessor provides many benefits.
These benefits are found in all smart devices regardless of the type of
communication used. A digital device provides advantages such as
improved accuracy and stability. The HART protocol enhances the
capabilities of digital instruments by providing communication access and
networking (Table 2).

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 ✓
Access to historical data ✓
Multidrop networking ✓
Access by multiple host devices ✓
Extended communication distances ✓
Field-based control ✓
Interoperability ✓
Table 2: Digital Instruments Versus HART Instruments

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 GETTING THE MOST OUT OF HART SYSTEMS

Getting the Most out of HART Systems

To take full advantage of the benefits offered by the HART communication
protocol, it is important that you install and implement the system correctly.
The following sections contain information that can help you to get the
most from your HART system:
T Wiring and Installation
T Intrinsic safety
T HART multidrop networks
T Control system interfaces
T Multiplexers
T Reading HART data into nonHART systems
T Universal handheld communicator
T PC configuration software
T Commissioning HART networks
T Device status and diagnostics
T Connecting a PC to a HART device or network
T PC application development tools
T Control in field devices

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 GETTING THE MOST OUT OF HART SYSTEMS

Wiring and Installation

In general, the installation practice for HART communicating devices is the
same as 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 affect communication.
The minimum conductor size is 0.51 mm diameter (#24 AWG) for cable
runs less than 1,524 m (5,000 ft) and 0.81 mm diameter (#20 AWG) for
longer distances.

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. Table
3 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.

Cable Capacitance – pf/ft (pf/m)

Cable Length – feet (meters)

No. Network 20 pf/ft 30 pf/ft 50 pf/ft 70 pf/ft

Devices (65 pf/m) (95 pf/m) (160 pf/m) (225 pf/m)

9,000 ft 6,500 ft 4,200 ft 3,200 ft

1
(2,769 m) (2,000 m) (1,292 m) (985 m)

8,000 ft 5,900 ft 3,700 ft 2,900 ft

5
(2,462 m) (1,815 m) (1,138 m) (892 m)

7,000 ft 5,200 ft 3,300 ft 2,500 ft

10
(2,154 m) (1,600 m) (1,015 m) (769 m)

6,000 ft 4,600 ft 2,900 ft 2,300 ft

15
(1,846 m) (1,415 m) (892 m) (708 m)

Table 3: Allowable cable lengths for 1.02 mm (#18 AWG)

shield twisted pair

© 2003 HART Communication Foundation Page 17

 GETTING THE MOST OUT OF HART SYSTEMS

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 HART-communicating devices work well in applications that require IS

DEVICES 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:
T Shunt-diode (zener) barriers that use a high-quality safety ground
connection to bypass excess energy (Figure 6)
T 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 7 on page 19)
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.

HAZARDOUS SIDE SAFE SIDE

Zener Barrier

Power Supply

1–5 V Output
Transmitter Signal

250 Ω Load Resistor

Figure 6: 4–20 mA Loop with a Zener Barrier

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 GETTING THE MOST OUT OF HART SYSTEMS

Intrinsic Safety
HAZARDOUS SIDE SAFE SIDE

Power
Isolator Supply

1–5 V Output
4–20 mA Signal
Transmitter

250 Ω Load Resistor

Figure 7: 4–20 mA Loop with Isolator

DESIGNING AN IS Designing an IS direct-current loop simply requires ensuring that a field

SYSTEM USING device has sufficient voltage to operate, taking into account zener barrier
resistance, the load resistor, and any cable resistance.
SHUNT-DIODE
When designing an IS loop using shunt-diode barriers, two additional
BARRIERS
requirements must be considered:
T The power supply must be reduced by an additional 0.7 V to allow
headroom for the HART communication signal and yet not approach
the zener barrier conduction voltage.
T The load resistor must be at least 230 Ω (typically 250 Ω).

Depending on the lift-off voltage of the transmitter (typically 10–12 V),

these two requirements can be difficult to achieve. The loop must be
designed to work up to 22 mA (not just 20 mA) to communicate with a
field device that is reporting failure by an upscale, over-range current. The
series resistance for the same zener barrier may be as high as 340 Ω. To
calculate the available voltage needed to power a transmitter, use the
following equation:
Power Supply Voltage – (Zener Barrier Resistance + Sense Resistance) ×
Operating Current (mA) = Available Voltage
Example: 26.0 V – (340 Ω + 250 Ω) × 22 mA = 13.0 V
Any cable resistance can be added as a series resistance and will reduce the
voltage even further. In addition, the power supply to the zener barrier must
also be set lower than the zener barrier conduction voltage. For example, a
28 V, 300 Ω zener barrier would typically be used with a 26 V power
supply.

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 GETTING THE MOST OUT OF HART SYSTEMS

Intrinsic Safety
While it is difficult to meet the two requirements noted above for a network
using shunt-diode barriers, it can be done. Following are two possible
solutions to the problem:
1. Shunt the load resistor with a large inductor so that the load resistor
impedance is still high (and mainly resistive) at HART signal
frequencies, but much lower at direct current. This solution, while it
does work, is physically somewhat inconvenient.
2. Use an IS isolator rather than a shunt-diode barrier. The output voltage
on the hazardous side is usually specified as greater than X Vdc at
20 mA (typically 14–17 V). This value already includes the voltage
drop due to the internal safety resistor, so the only extra voltage drop is
that due to cable resistance. System operation at 22 mA requires
reducing the 20 mA voltage by 0.7 V (340 Ω × 2 mA).

DESIGNING AN IS The implementation of HART loops in an IS system with isolators requires

SYSTEM USING more planning. An isolator is designed to recreate the 4–20 mA signal from
the field device in the safe-side load circuit. Most older isolator designs will
ISOLATORS not carry the high frequencies of HART current signals across to the safe
side, nor will they convey HART voltage signals from the safe side to the
field. For this reason, HART communication through the isolator is not
possible with these older designs. (It is still possible to work with a
handheld communicator or PC with an IS modem on the hazardous side of
the isolator.) When retrofitting HART instruments into an existing
installation, inspect the system for isolators that may have to be replaced
(any isolators that will not support HART signals).
Major suppliers of IS isolators have introduced
designs that are fully HART compatible. Modern IS
isolators provide trouble-free design and operation
and transparent communication in both directions.

IS device suppliers can assist with certification and performance

specifications for their HART-compatible products. Field device
manufacturers will also supply certification details for their specific
products.

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 GETTING THE MOST OUT OF HART SYSTEMS

Intrinsic Safety
MULTIDROP IS HART multidrop networks are particularly suitable for intrinsically safe
NETWORKS installations. With a multidrop configuration, fewer barriers or isolators are
required. In addition, because each field device takes only 4 mA (for a total
of 16 mA in a four-device loop), plain zener barriers can be used. With a
250 Ω load, 25 V – (340 + 250 Ω) × 16 mA = 15.5 V, which is well above
the transmitter lift-off voltage and leaves a margin for cable resistance.

IS OUTPUT LOOPS For output devices such as valve positioners, direct-current voltage
considerations will vary depending on the drive requirements of the device.
Zener barriers may be possible. If not, modern HART-compatible output
isolators are appropriate.

IS CERTIFICATION If the HART loop contains an IS-approved handheld communicator or

CONSIDERATIONS modem, slight changes may be needed to meet IS installation certification
rules. Handheld communicators and modems add the HART signal voltage
to the voltage level coming from the zener barrier or isolator. For example,
a handheld communicator typically adds a maximum of 2 V to the loop.
Therefore, when used with a 28 V zener barrier, a total of 30 V may
theoretically be present in the loop. The allowable capacitance must be
reduced by about 15% to account for this increase in voltage.

IS NETWORK The cable length calculation must include the resistance of both the zener
CABLE LENGTH barrier and the load resistor.
CALCULATIONS

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 GETTING THE MOST OUT OF HART SYSTEMS

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.

Master Device

Modem

Auxiliary Power
Supply

Transmitters

Figure 8: Multidrop Configuration

Standard HART commands are used to communicate with field instruments
to determine process variables or device parameter information (see HART
Commands on page 7). 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 (see Water Treatment Facility
Upgrade on page 45).

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 GETTING THE MOST OUT OF HART SYSTEMS

HART Multidrop Networks

MULTIDROP WITH HART field controllers can also be wired in a multidrop network
HART FIELD (Figure 9). Each analog output signal from the transmitter/controllers is
isolated from every other output signal, which provides a cost-effective
CONTROLLERS HART network configuration. In this case, the analog signals are not fixed
and are used for the output signal to the controlled device.

Handheld
Terminal Computer or
DCS
Power HART Interface
Supply Power Supply
Impedance

4–20 mA

+ – + – + – + – + – + –

HART
Transmitter

Control Valve

Figure 9: HART Controllers with Multidrop

APPLICATION Connecting HART field devices in a multidrop network can provide

CONSIDERATIONS significant installation savings. The total cable length in a multidrop
network is typically less than the maximum cable length in point-to-point
connections because the capacitance of the additional devices reduces the
distance that the HART signal can be carried (see Wiring and
Installation on page 17).
To save on installation costs, use HART multidrop
networks for remote monitoring stations, tank farms,
pipeline distribution systems, and other monitoring
applications in which fast update rates are not required.

CONFIGURING Using the polling address structure of the HART protocol, up to 15 devices
DEVICES FOR can be connected in a multidrop network. The analog current of a HART
device can be fixed by setting its polling address to a number other than
MULTIDROP zero. With the HART protocol, each field instrument should be configured
OPERATION with different polling addresses or tag numbers before being connected to a
multidrop network—otherwise, the master will not be able to establish
communication with the slave devices.

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 GETTING THE MOST OUT OF HART SYSTEMS

Control System Interfaces

When you change your existing 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 functionality.
Control systems such as a DCS, PLC, or SCADA/RTU (remote terminal
unit) implement only the functionality required for a given application. For
example, a flow-control system may only read the primary variable of a
device and provide no additional support for viewing or changing
configuration information. Other control-system interfaces provide
comprehensive HART support, maintaining complete configuration records
for all connected devices.
Contact your system supplier for specific details on their HART
interface(s). Use the form in Appendix A to obtain information from
control-system suppliers to identify specific characteristics of their
products.

HART I/O Many HART-compatible I/O subsystems have multiple analog channels on
SUBSYSTEMS each I/O card. Suppliers choose whether to provide one HART interface per
channel or to share one HART interface among several channels. The
number of shared channels per HART interface impacts the frequency of
data updates from a HART field device and the HART functionality that is
supported.

HART I/O FOR For the best performance and flexibility, one HART interface should be
MULTIDROP dedicated to each I/O channel. Systems that share only one HART interface
among several I/O channels may not support multidrop networks. The
SUPPORT effective update rate of a multiplexed interface is slow enough that the
performance of multiplexed multidrop networks would not be practical.
Some suppliers enable multidrop support by fixing the HART interface to
one specific I/O channel. However, the other channels on that card may
then not be available for HART communications.

HART I/O FOR Burst mode is an optional implementation in a field device. Receiving burst
BURST MODE mode messages is optional in a host as well. To take full advantage of burst
mode, the I/O system should have one HART interface for each channel. If
SUPPORT the HART interface is shared by more than one channel, 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 terminal or other configuration tool is required.

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 GETTING THE MOST OUT OF HART SYSTEMS

Control System Interfaces

DATA HANDLING All HART-compatible control systems can read the digital primary variable
from a slave device. However, some system architectures may not be able
to accommodate textual data (e.g., tag and descriptor fields). In these cases,
the controller is able to read the process variable, but may not have direct
access to all other data in the HART device.

PASSTHROUGH Some control systems are integrated with a configuration or instrument-

FEATURE 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 passthrough feature of the control 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.

SCADA/RTU RTUs used in SCADA systems use a special telemetry to communicate

SYSTEMS with the control system. RTUs have the same considerations regarding
multidrop 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 multidrop, burst mode, and multivariable instruments),
HART data are only available to the central host system if the telemetry
protocol supports the transfer of HART commands or specific HART data
(see Multidrop for Tank Farm Monitoring on page 40).

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 GETTING THE MOST OUT OF HART SYSTEMS

Multiplexers
HART-compatible multiplexers are ideal for users who want to interface
with a large number of HART devices. Multiplexers can be modular and are
capable of supporting both point-to-point and all-digital (multidrop) HART
communication modes. Communication between a multiplexer and a host
application depends on the multiplexer capabilities (e.g., RS232C, RS485,
Modbus, and TCP/IP Ethernet).
When installing HART multiplexer systems, the following capabilities
should be considered:
T Number of HART channels supported
T Number of HART channels that share a HART modem
T Burst mode support
T Multidrop support
T Method of communication with the host computer or control system

MULTIPLEXER AS HART multiplexers can be used as the primary I/O front end for a
THE PRIMARY I/O HART-based control or monitoring system (Figure 10). Typically, a PC acts
as the host, providing the human-machine interface and performing other
SYSTEM 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.
Multiplexer

Field
Devices

SCADA

Field Device

Figure 10: HART Multiplexer as the Primary I/O System

PARALLEL When a traditional 4–20 mA control system is using the analog signals for
MONITORING WITH measurement and control outputs, a HART multiplexer can be added to the
network to gain access to the digital HART signal. Using a multiplexer
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 4–20 mA signal.

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 GETTING THE MOST OUT OF HART SYSTEMS

Multiplexers
Use a HART multiplexer to gain access to the digital
HART 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 (Figure 11).

Automation and
Display System
Supervisory
Controllers Computer

I/O

Transmitter Multiplexer

Control Valves

Figure 11: HART Multiplexer with Existing I/O

A multiplexer can also be an integral part of the control system as a
third-party I/O (Figure 12). As an I/O system, the multiplexer can include
IS barriers and other filtering capabilities and provide services to the field
device, such as galvanic isolation or power. For this type of installation, no
additional terminations or space are required. The multiplexer can also act
as a gateway to convert the HART messages to another protocol such as
Modbus, PROFIBUS, or Ethernet.
.

Automation and Supervisory

Display System Computer

Controller

I/O

Transmitter Control Valve

Figure 12: HART Multiplexer Integrated with I/O

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 GETTING THE MOST OUT OF HART SYSTEMS

Reading HART Data into NonHART Systems

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 digitally. However, existing
controllers or 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 analog or contact information, which enables any
traditional analog/digital I/O to take full advantage of the benefits of
HART-communicating devices. The Rosemount Inc. Tri-Loop module and
the Moore Industries Site Programmable Alarm (SPA) are two such
products.

HART The Tri-Loop module monitors a HART loop for a bursting message and
DATA-CONVERSION converts three of the four possible variables in HART command number
three to analog outputs (Figure 13). The conversion enables the field device
PRODUCTS to provide a total of four analog signals over a single pair of wires run from
the field.
.

Channel 1
4–20 mA
Channel 2 Signals for
Secondary
Channel 3 Variables

Field Terminals

Rail-Mounted
Tri-Loop Module Control System

Figure 13: Tri-Loop Module

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 GETTING THE MOST OUT OF HART SYSTEMS

Reading HART Data into NonHART Systems

The SPA module continuously communicates with any HART-capable
device and provides contact closure outputs (alarm trips) based on the
information received (Figure 14). For example, the SPA can be configured
to monitor the device-status information inherent in the HART
communication protocol and trigger events such as local on/off applications
or alarms. The SPA can also initiate emergency shutdown action if
problems are detected with a field device in critical loop applications.

HART 4–20 mA and HART

Transmitter HART Digital Communicator
Signals

Control
System

Process
and
Diagnostic
Data

Annunciator

HART Master Shutdown

Controls

Event
Recorder

Figure 14: SPA Module

Both HART Tri-Loop and SPA provide multivariable product support on a
loop-by-loop basis.

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 GETTING THE MOST OUT OF HART SYSTEMS

Universal Handheld Communicator

The 275 Universal HART Communicator is available from major
instrumentation suppliers around the globe and is supported by all member
companies in the HCF. Using HART DDL, the communicator can fully
communicate with and configure any HART device for which it has a DD
installed. If the communicator does not have the DD for a particular
network device installed, it can still communicate with that device using the
universal and common practice commands (see HART Commands on
page 7). The HCF provides centralized control and registration for all DDs
that can be loaded into the communicator. An index of registered DDs can
be found on the world wide web at <http://www.hartcomm.org>.
Use the 275 Universal HART Communicator to
communicate with and configure any
HART-communicating device.

Figure 15: 275 Universal Handheld Communicator

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PC Configuration Software
Many instrument manufacturers, as well as some independent software
developers, offer HART communication software for PCs with capabilities
similar to and beyond those offered by a HART handheld communicator.
Use special software applications to continuously
monitor the status of connected field devices and log
status changes as they occur, which may help reduce
the costs of regulatory compliance.

The software packages listed in Table 4 are used for configuration

management, parameter tuning, and data acquisition with a HART device.
The list is not comprehensive, and all software applications are not
functionally equivalent. A number of product-specific software
applications are also available for diagnostics. An RS232 HART interface
or other interface device connects the PC running the HART application
software to the field devices.
SUMMARY TABLE
OF HART Software Application Manufacturer

SOFTWARE Asset Management Configuration and calibration

Fisher-Rosemount
Solutions (AMS) management
CONF301 HART
Configuration management Smar International
Configurator
CONFIG Configuration management Krohne
Cornerstone Base Configuration and calibration Applied System
Station management Technologies
Cornerstone Applied System
Instrument configuration
Configurator Technologies
Configuration management
H-View Arcom Control Systems
and data acquisition
IBIS Configuration management EB Hartmann & Braun
IBIS Configuration management Samson
K-S Series Configuration management ABB
Mobrey H-View Configuration management KDG Mobrey
UTSI International
Pacemaker Configuration management
Corporation
SIMATIC PDM Configuration management Siemens
EB Hartmann & Braun/
Smart Vision Configuration management
Bailey Fischer & Porter
XTC Configuration
Configuration management Moore Products Co.
Software

Table 4: HART Software

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 GETTING THE MOST OUT OF HART SYSTEMS

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 Before installation, manufacturers usually enter device tags and other
VERIFICATION 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 terminal 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 can be important 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 (see Multiplexers on page 26). 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 Once a field instrument has been identified and its configuration data
CHECK 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.

AS-INSTALLED A HART configurator also facilitates record keeping. As-installed device

RECORD KEEPING configuration data can be stored in memory or on a disk for later archiving
or printing.

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APPENDICESAPPENDICESAPPENDICES
GETTING THE MOST OUT OF HART SYSTEMS

Device Status and Diagnostics

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:
T Field device malfunction
T Configuration changed
T Cold start
T More status available
T Analog output current fixed
T Analog output saturated
T Nonprimary variable out of limits
T 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 (see Multiplexers on page 26). These applications provide
real-time monitoring of status and diagnostic information.

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 GETTING THE MOST OUT OF HART SYSTEMS

Connecting a PC to a HART Device or Network

PCs are commonly used for HART host applications for configuration and
data acquisition. A specially designed device (Table 5) allows the HART
network to be connected to the RS232C serial port or PCMCIA slot of a PC
(Figure 16).
Product Name Manufacturer

Commubox Endress + Hauser

FSK-Modem EB Hartmann & Braun

HT311 RS232 Interface Smar International

VIATOR PCMCIA HART

MACTek
Interface

VIATOR RS232 HART Interface MACTek

Table 5: HART Interfaces

PC/Host
Application
RS232 HART
Interface Handheld Terminal

Field
Device
Power
Supply

Figure 16: RS232 HART Interface

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 GETTING THE MOST OUT OF HART SYSTEMS

PC Application Development Tools

Software drivers are available to assist in the development and integration
of PC applications with HART networks. Table 6 shows a partial list of
products available.
Product Name Description Manufacturer

Hview Provides DDE server Arcom Control Systems

HRT VBX 16-bit Visual Basic driver Borst Automation

HRT OCX 32-bit ActiveX Control Borst Automation

HART OPC HCF (via member

OPC Server
Server companies)

HL-LinkPro HART driver for LabVIEW Cardiac Systems Solutions

Table 6: PC Development Tools

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 GETTING THE MOST OUT OF HART SYSTEMS

Control in Field Devices

Microprocessor-based smart instrumentation enables control algorithms to
be calculated in the field devices, close to the process (Figure 17). Some
HART transmitters and actuators support control functionality in the
device, which eliminates the need for a separate controller and reduces
hardware, installation, and start-up costs. Accurate, closed-loop control
becomes possible in areas where it was not economically feasible before.
While the control algorithm uses the analog signal, HART communication
provides the means to monitor the loop and change control setpoint and
parameters.

PC-Based Operator
Interface

Modbus Link
(RS232) Muiltiplexer (HART Master)

4–20 mA to
Position Valve

HART Transmitter Control Valve

with PID Slave

Figure 17: Transmitter with PID (HART Slave)

Placing control in the field enhances control functionality. Measurement
accuracy is maintained because there is no need to transmit data to a
separate controller. Control processing takes place at the high update rate of
the sensor and provides enhanced dynamic performance.

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 GETTING THE MOST OUT OF HART SYSTEMS

Control in Field Devices

HART FIELD A HART field controller takes advantage of the HART protocol’s
CONTROLLER simultaneous analog and digital signaling by converting the transmitter’s
traditional analog measurement output into a control output. The analog
IMPLEMENTATION signal from the smart transmitter (controller) is used to manipulate the field
device (Figure 18). The analog output signal also carries the HART digital
signal, which is used for monitoring the process measurement, making
setpoint changes, and tuning the controller.

Bypass
Capacitor

+ Smart
Power Transmitter
Supply

Control
Resistor Valve

Figure 18: Smart Transmitter with PID

The communication rate of the HART protocol (2–3 updates per second) is
generally perceived as too slow to support closed-loop control in the central
host. With control in the field, the control function no longer depends on
the HART protocol’s communication rate. Instead, the control signal is an
analog output that is updated at a rate that is much faster than can typically
be processed in a conventional control system. Processing rates vary from
2–20 updates per second, depending on the product. The HART digital
communication rate remains sufficient for monitoring the control variable
and changing setpoint values.

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 INDUSTRY APPLICATIONS

Industry Applications
Many companies in a wide variety of industries have already realized the
advantages of using the HART communication protocol. This section
describes some applications in detail and outlines the tangible benefits that
result. The applications have been grouped into the following sections:
T Inventory-management applications
T Cost-saving applications
T Remote-operation applications
T Open-architecture applications

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 INDUSTRY APPLICATIONS

Inventory-Management Applications
Accurate measurements for inventory management are essential in all
industries. The HART communication protocol enables companies to make
sure inventory management is as efficient, accurate, and low cost as
possible.

HART MULTIDROP Tank level and inventory management is an ideal application for a HART
NETWORK FOR multidrop network (Figure 19). The HART network digital update rate of
two PVs per second is sufficient for many tank-level applications. A
TANK LEVEL AND multidrop network provides significant installation savings by reducing the
INVENTORY amount of wiring from the field to the control room as well as the number
MANAGEMENT of I/O channels required. In addition, many inexpensive
process-monitoring applications are commercially available to further cut
costs.

Transmitters

Storage
Tanks

HART Field
Multiplexer

Figure 19: Inventory Management with Multidrop

One company uses a HART multiplexer to digitally scan field devices for
level-measurement and status information. The information is forwarded to
the host application using the Modbus communication standard.
Multivariable instruments further reduce costs by providing multiple
process measurements, such as level and temperature, which reduces the
wiring and number of process penetrations required.

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 INDUSTRY APPLICATIONS

Inventory-Management Applications
MULTIDROP FOR In one tank farm application, 84 settlement tanks and filter beds on a very
TANK FARM large site (over 300,000 m2) are monitored using HART multidrop
networks and HART RTUs (see SCADA/RTU Systems on page 25). The
MONITORING HART architecture required just eight cable runs for 84 tanks, with 10–11
devices per run (Figure 20). Over 70 individual runs of over 500 m each
were eliminated. Cable savings were estimated at over $40,000 when
compared to a conventional installation. RTU I/O was also reduced, which
resulted in additional hardware and installation savings. The total installed
cost was approximately 50% of a traditional 4–20 mA installation.

HART Multiplexer Control Room

Storage
Storage Tanks
Tanks

Figure 20: Tank Farm Monitoring with Multidrop

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 INDUSTRY APPLICATIONS

Inventory-Management Applications
UNDERGROUND Underground salt caverns are frequently used for crude oil storage. One
PETROLEUM customer pumps oil from barges into the storage caverns. An ultrasonic
flowmeter records the total flow. To get the oil out of the caverns, a brine
STORAGE WITH solution is pumped into the cavern through a magnetic flowmeter. Brine
HART and crude oil flowing in both directions are measured and reported to the
COMMUNICATION DCS using the HART communication protocol for accuracy. The DCS
tracks flow rate and total quantity to maintain a certain pressure inside the
FOR ACCURACY
caverns (Figure 21).

HART Transmitter
Interface

HART Transmitter
Interface

Oil Caverns Field Instruments

Note: Digital accuracy for flow accuracy and flow totals

Figure 21: Underground Petroleum Storage

© 2003 HART Communication Foundation Page 41

 INDUSTRY APPLICATIONS

Cost-Saving Applications
Use HART multidrop networking to reduce
installation and maintenance costs.

WASTEWATER A Texas wastewater treatment plant replaced stand-alone flowmeters and

TREATMENT PLANT chart recorder outstations that required daily visits for totalization with a
HART system. HART-based magnetic flowmeters were multidropped into
UPGRADE HART RTUs to create a cost-effective SCADA network. The use of HART
technology reduced system and cable costs, enhanced measurement
accuracy, and eliminated time-consuming analog calibration procedures.
A system of 11 HART multidrop networks was used to connect 45
magnetic flowmeters from different plant areas. Each flowmeter
communicated flow rate and a totalized value over the HART network.
Multidrop networks eliminated the need for additional hardware and PLC
programming while providing a more accurate totalized value. Complex
and costly system integration issues were also avoided—for example, there
was no need for synchronization of totals between the host and field PLCs.
Multidrop networking further reduced the installation cost by reducing the
required number of input cards from the traditional 45 (for point-to-point
installations) to 11. Maintenance was simplified because of access to
instrument diagnostic and status data.

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 INDUSTRY APPLICATIONS

Cost-Saving Applications
APPLIANCE A consumer appliance manufacturer used the networking capability of the
MANUFACTURING HART protocol to measure level, flow, and pressure. HART multidrop
provided substantial wiring and installation savings as well as digital
WITH MULTIDROP accuracy with the elimination of the analog to digital (A/D) and digital to
analog (D/A) conversions of the instrument and PLC I/O. Figure 22 shows
pressure transmitters connected to a PLC via smart transmitter interface
multiplexers.

Storage
Tanks
Highway

PLC

Communication
Module

Smart Transmitter Terminal

Interface Block Module

Figure 22: Multidrop Network Example

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 INDUSTRY APPLICATIONS

Cost-Saving Applications
REMOTE The benefits of remote monitoring and rezeroing of smart transmitters
REZEROING IN A using the HART protocol are dramatically illustrated in this example of two
smart transmitters that control the fluid level in lauter tubs in a brewhouse
BREWERY application. Similar benefits would be realized in any application involving
a closed vessel.
Two smart transmitters are installed on each lauter tub—one on the bottom
of the tank and the other about nine inches from the bottom. The bottom
transmitter is ranged ±40 inH2O; the upper transmitter is ranged
0–30 inH2O. As the lauter tub is filled, the bottom transmitter senses level
based on pressure. When the level reaches the upper transmitter, that point
is marked as the new zero-level point, and the upper transmitter becomes
the primary sensing instrument for the lauter-tub level. The nine-inch
zero-level offset from the bottom of the tank is necessary to accommodate
loose grain that settles in the bottom of the tank.
Transmitters that are coordinated and working together control fluid level
in each lauter tub to within a few barrels. However, the upper transmitter
requires periodic maintenance or replacement and rezeroing. An undetected
false upper-transmitter level reading can cause a tank level error of up to
40 gallons.
The usual procedure for transmitter rezeroing takes about 95 minutes and
has been required as frequently as twice a day. Rezeroing a transmitter
using configuration software and PLC interface modules eliminates the
need to locate and identify the problem at the site as well as the need for
verification by control-room personnel and greatly reduces the chance for
inadvertent errors. Estimated total time to rezero each transmitter is
reduced to 15 minutes.
Through the configuration software’s instrument-status and diagnostic
capabilities, a false level indication can be automatically detected while a
lauter tub fill is in progress. The affected transmitter can then be
automatically rezeroed by programming logic in the programmable
controller to issue the appropriate command to the instrument.

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 INDUSTRY APPLICATIONS

Cost-Saving Applications
WATER HART transmitters and a control system with HART capability were
TREATMENT chosen to upgrade a water treatment facility. The completed installation
reduced capital, engineering, and installation costs. The process dynamics
FACILITY of the water treatment facility allowed the HART instruments to be used in
UPGRADE all-digital mode without compromising plant performance.
The water treatment plant is divided into two areas, each with 14 filters.
Each area is controlled by a separate control system for complete
autonomy. A HART network monitors each filter for filter level, filter bed
differential, and filter outlet flow. The multidrop installation used a
three-wire system in order to accommodate both the two-wire and the
four-wire devices (magnetic flowmeters) in use (Figure 23)
(see Multidrop on page 6).

4 mA Pressure
Transmitters

12 mA Main
Power

Magnetic
Flowmeter
4 mA

Figure 23: Multidrop Networks with 2-Wire and 4-Wire Devices

Because the water treatment facility had a modular design, the use of
HART instruments allowed the configuration from the one filter network to
be copied to the others, which reduced the implementation time.
Engineering, system configuration, drafting, commissioning, maintenance,
and documentation were simplified. A reduced I/O card count also saved
money.

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 INDUSTRY APPLICATIONS

Cost-Saving Applications
IMPROVED A cleaning materials supplier required periodic checkup of the instrument
DIAGNOSTICS condition and configuration information as compared to the initial
installation. The field transmitters provided a historical record of status
changes along with current configuration information. Periodic download
of this information was made possible using PLC ladder logic developed
for HART instruments.

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 INDUSTRY APPLICATIONS

Remote-Operation Applications
UNMANNED Choosing the HART communication protocol for all-digital communication
OFFSHORE GAS in a wide-area network enabled one company to have real-time monitoring
and control, access to diagnostics, and maintenance capabilities—all from a
PRODUCTION WITH remote location.
HART NETWORKS
Over half of the 500 transmitters on 15 platforms could be multidropped
with update rates of three seconds (six devices), which resulted in
substantial savings in wiring, I/O, and installation. The remaining devices
(flowmeters) required a faster response and were wired point to point using
digital HART communications to transmit the process data. The flowmeters
used the optional burst mode, which provided an update rate of 3.7 times
per second. All-digital communications provided maximum accuracy and
eliminated potential errors from input scaling, conversion, and drift (see
Multidrop on page 6).
Radio Antennae

Primary Standby RTU

RTU

Modbus Link

HART
Multiplexers

Transmitters

Transmitters

Figure 24: RTU Application

Each platform’s RTU provided a link to approximately 50 temperature,
pressure, and flow transmitters (Figure 24). The RTU used the multimaster
capability of the HART protocol to enable the second RTU to act as a hot
standby, which monitored activity and was able to take over if a failure
occurred. The RTUs provided links with the emergency and safety systems
and a local interface for maintenance personnel. The Modbus protocol was
used for communication to the central SCADA system.

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 INDUSTRY APPLICATIONS

Remote-Operation Applications
VENEZUELA In a Venezuela gas-lift project, HART multidrop technology was used for
GAS-LIFT PROJECT remote operation of offshore gas-lift production wells at considerable
savings (Figure 25):
T 30% decrease in installation costs
T 16:1 reduction of input modules
T Reduced cost of I/O cards in the RTU
T Remote reranging
T Remote access to the transmitter status for improved process uptime

Radio Antennae Microwave Towers

Configuration
and
Maintenance
Tools
Control
Room
Electric Valve HART Transmitters

Figure 25: Offshore Gas-Lift Project

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 INDUSTRY APPLICATIONS

Open-Architecture Applications
OIL REFINERY The best way to judge the openness of a communication protocol is by the
EXPANSION number of products supported. By this standard, the HART protocol is
perhaps the most open of any field-communication protocol available
today.
In a major refinery expansion, an oil company weighed the advantages of
using either a proprietary system or a HART-based system. The results
indicated that the company could use HART digital instruments in 92% of
their applications, compared to only 33% with the proprietary system.
Choosing HART products resulted in an incremental $23,000 in savings
due to commissioning efficiencies and ongoing maintenance and diagnostic
capabilities.
The oil company used a traditional control system with analog I/O and
supplemented the control capability with an online maintenance and
monitoring system. All of the HART field devices were monitored from a
central location (Figure 26).

Ethernet Link Maintenance

Control Station
Display
System

Controller

HART I/O I/O

Multiplexer

HART
Transmitter Control Valve
Fisher
Fisher

Figure 26: Online Implementation

© 2003 HART Communication Foundation Page 49

 INDUSTRY APPLICATIONS

Open-Architecture Applications
HART WITHIN A HART field devices can be seamlessly integrated with PROFIBUS DP
PROFIBUS networks using the HART/DP Link, which enables the connection of four
HART devices and facilitates the passthrough of HART commands to host
NETWORK applications on the DP network (Figure 27). The HART/DP Link supports
IS installations.

PCs with HART Applications

PLC

PROFIBUS
DP

Remote I/O DP/PA

Link
DP/ASI Link
HART/DP
Link

Profibus
PA
HART
Instruments

Figure 27: HART Within a PROFIBUS Network

Page 50 © 2003 HART Communication Foundation

 INDUSTRY APPLICATIONS

Open-Architecture Applications
HART/DDE Cost-effective level- and temperature-monitoring systems can be designed
SERVER using HART multidrop networks and commercially available HART/DDE
interface software. HART/DDE interface software allows any compliant
application (e.g., spreadsheet) to directly read the process data and status
information available in HART field devices. A HART interface module
connected to the PC’s serial port is needed for this HART monitoring
application (Figure 28).

Spreadsheet
Data Logging

RS232 HART
Interface

Power Supply

Transmitter

Figure 28: Multidrop Network

© 2003 HART Communication Foundation Page 51

 WHERE TO GET MORE INFORMATION

Where To Get More Information

WHAT To serve the growing interest in HART-related products, the HCF publishes
INFORMATION IS a library of additional documents, articles, and overviews. The following
information is currently available:
AVAILABLE? T HART specifications
T Technical overview
T Application notes
T Technical assistance
T Training classes

WHERE TO FIND By Mail

INFORMATION HART Communication Foundation
9390 Research Blvd, Suite I-350
Austin, TX 78759 USA

By Phone
Call 512-794-0369.

By Fax
Send correspondence to 512-794-3904.

By E-mail
Send correspondence to <hcfadmin@hartcomm.org>.

Online
Visit the HCF website at <http://www.hartcomm.org>.

Page 52 © 2003 HART Communication Foundation

 GLOSSARY

Glossary
275 HART A handheld master device that uses the HART communication protocol and
Communicator DDL to configure or communicate with any HART smart device

Bell 202 A U.S. telephone standard that uses 1,200 Hz and 2,200 Hz as 1 and 0,
respectively, at 1,200 baud; a full duplex communication standard using a
different pair of frequencies for its reverse channel; HART uses Bell 202
signals but is a half-duplex system, so the reverse channel frequencies are
not used

Burst (Broadcast) Mode A HART communication mode in which a master device instructs a slave
device to continuously broadcast a standard HART reply message
(e.g., value of a process variable) until the master instructs it to stop
bursting

Cable Capacitance Per The capacitance from one conductor to all other conductors (including the
Unit of Length shield if present) in the network; measured in feet or meters

Cable Resistance Per The resistance for a single wire; meausred in feet or meters
Unit of Length

Closed-Loop Control A system in which no operator intervention is necessary for process control

Communication Rate The rate at which data are sent from a slave device to a master device;
usually expressed in data updates per second

DCS See Distributed Control System.

DD See Device Description.

DDL See Device Description Language.

Device Description A program file written in the HART Device Description Language (DDL)
that contains an electronic description of all of a device’s parameters and
functions needed by a host application to communicate with the device

Device Description A standardized programming language used to write DDs for

Language HART-compatible field devices

Distributed Control Instrumentation (input/output devices, control devices, and operator

System interface devices) that permits transmission of control, measurement, and
operating information to and from user-specified locations, connected by a
communication link

Page 70 © 2003 HART Communication Foundation

 GLOSSARY

Glossary
Field The area of a process plant outside the control room where measurements
are made, and to and from which communication is provided; a part of a
message devoted to a particular function (e.g., the address field or the
command field)

Field Device A device generally not found in the control room; field devices may
generate or receive an analog signal in addition to the HART digital
communication signal

Frequency Shift Keying Method of modulating digital information for transmission over paths with
poor propagation characteristics; can be transmitted successfully over
telephone systems

FSK See Frequency Shift Keying.

Gateway A network device that enables other devices on the network to

communicate with a second network using a different protocol

HART Command Set A series of commands that provide uniform and consistent communication
for all master and slave devices; includes universal, common practice, and
device-specific commands

HART Communication Highway Addressable Remote Transducer communication protocol; the

Protocol industry standard protocol for digitally enhanced 4–20 mA communication
with smart field devices

HART Loop A communication network in which the master and slave devices are
HART smart or HART compatible

Host Application A software program used by the control center to translate information
received from field devices into a format that can be used by the operator

Interoperability The ability to operate multiple devices, independent of manufacturer in the

same system, without loss of functionality

Intrinsic Safety A certification method for use of electrical equipment in hazardous

(e.g., flammable) environments; a type of protection in which a portion of
an electrical system contains only intrinsically safe equipment that is
incapable of causing ignition in the surrounding environment

Intrinsic Safety Barrier A network or device designed to limit the amount of energy available to the
protected circuit in a hazardous location

IS See Intrinsic Safety.

© 2003HART Communication Foundation Page 71

 GLOSSARY

Glossary
Master Device A device in a master-slave system that initiates all transactions and
commands (e.g., central controller)

Master-Slave Protocol Communication system in which all transactions are initiated by a master
device and are received and responded to by a slave device

Miscellaneous Series The summation of the maximum impedance (500 Hz–10 kHz) of all
Impedance devices connected in series between two communicating devices; a typical
nonintrinsically safe loop will have no miscellaneous series impedance

Modem Modulator/demodulator used to convert HART signals to RS232 signals

Multidrop Network HART communication system that allows more than two devices to be
connected together on a single cable; usually refers to a network with more
than one slave device

Multimaster Multimaster refers to a communication system that has more than one
master device. The HART protocol is a simple multimaster system
allowing two masters; after receiving a message from a slave device, the
master waits for a short time before beginning another transmission, which
gives the second master time to initiate a message

Multiplexer A device that connects to several HART loops and allows communication
to and from a host application

Multivariable Instrument A field device that can measure or calculate more than one process
parameter (e.g., flow and temperature)

Network A series of field and control devices connected together through a

communication medium

Parallel Device The summation of the capacitance values of all connected devices in a
Capacitance network

Parallel Device The parallel combination of the resistance values of all connected devices
Resistance in the network; typically, there is only one low-impedance device in the
network, which dominates the parallel device-resistance value

Passthrough A feature of some systems that allows HART protocol send-and-receive

messages to be communicated through the system interface

PID Proportional-integral-derivative

PID Control Proportional-plus-integral-plus-derivative control; used in processes where

the controlled variable is affected by long lag times

Page 72 © 2003 HART Communication Foundation

 GLOSSARY

Glossary
Point to Point A HART protocol communication mode that uses the conventional
4–20 mA signal for analog transmission, while measurement, adjustment,
and equipment data are transferred digitally; only two communicating
devices are connected together

Polling A method of sequentially observing each field device on a network to

determine if the device is ready to send data

Polling Address Every HART device has a polling address; address 0 is used for
point-to-point networks; addresses 1–15 are used in multidrop networks

Process Variable A process parameter that is being measured or controlled (e.g., level, flow,
temperature, mass, density, etc.)

Protocol A set of rules to be used in generating or receiving a message

PV See Process Variable.

Remote Terminal Unit A self-contained control unit that is part of a SCADA system

RTU See Remote Terminal Unit.

SCADA See Supervisory Control and Data Acquisition.

Slave Device A device (e.g., transmitter or valve) in a master-slave system that receives
commands from a master device; a slave device cannot initiate a transaction

Smart Instrumentation Microprocessor-based instrumentation that can be programmed, has

memory, is capable of performing calculations and self-diagnostics and
reporting faults, and can be communicated with from a remote location

Supervisory Control and A control system using communications such as phone lines, microwaves,
Data Acquisition radios, or satellites to link RTUs with a central control system

Zener Type of shunt-diode barrier that uses a high-quality safety ground

connection to bypass excess energy

© 2003 HART Communication Foundation Page 73