HART ® FIELD COMMUNICATION PROTOCOL

HART
FIELD COMMUNICATION PROTOCOL

An introduction for users and manufacturers

HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

 ®

HART
FIELD COMMUNICATION PROTOCOL

WHY COMMUNICATION ?

'4-20 mA' is a tried, tested and widely- management and utilization of 'smart'
used standard, so why do we need to instrument networks.
update it? Because only a limited
amount of information (the measured HART permits two-way
variable) is sent by a 4-20 mA signal. communications, so instrument
parameters can be interrogated and
'Smart' field devices using the HART even adjusted from anywhere on the
(Highway Addressable Remote cable.
Transducer) protocol enhance
operations because digital data is HART also has an all-digital mode
transmitted along with the 4-20 mA that allows many instruments to be
signal - without interfering with it! This connected to a single cable, cutting
has two important benefits: firstly, installation costs dramatically yet
existing cabling and current control retaining all HART advantages.
strategies remain secure; secondly,
the additional data - typically tag The HART protocol is supported by
numbers, measured variables, range major instrument suppliers and the
and span data, product information HART Communication Foundation, so
and diagnostics - can be used during interoperability is assured, along with
installation, calibration, maintenance easy integration with higher level host
and operations to cut costs systems and equipment.
substantially and improve the

Features of the HART protocol

• field proven concept that is easy to understand and use

• compatible with existing 4-20 mA systems
• simultaneous point-to-point 4-20 mA and digital communication
• alternative multi-drop mode
• measured variables, tag number, range and span settings, device
information, diagnostics and simple messages transmitted
• digital response time of 500mS; burst mode response of 300mS
• open architecture; freely available to any vendor and every user

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HART
FIELD COMMUNICATION PROTOCOL

METHOD OF OPERATION

Simultaneous analogue and digital

The HART protocol operates using
approx. +0.5 mA
the frequency shift keying (FSK)
principle, which is based on the Bell
analogue
202 [1] communication standard. The 0 signal

digital signal is made up from two

approx. -0.5 mA
frequencies - 1200 Hz and 2200 Hz,
1200 Hz 2200 Hz
representing bits 1 and 0 respectively. "1" "0"

Sine waves of these frequencies are

superimposed on the DC analog Figure 1
Because the mean harmonic signal value is zero,
signal cables to give simultaneous digital communication makes no difference to any
analogue and digital communications. existing analogue signal.

Because the average value of the

FSK signal is always zero, the 4-20
mA signal is not affected.
primary one is generally a
This produces genuine, simultaneous management system or a PC while
communication with a response time the secondary one can be a hand-
of approximately 500mS for each field held terminal or laptop computer. A
device, without interrupting any standard hand-held terminal - called
analog signal transmission that might the HART Communicator - is
be taking place. available to make field operations as
Figure 2 uniform as possible. Further
Point-to-point mode: with provision for one 4-20 mA
device and up to two masters, e.g. one
Up to two master devices may be networking options are provided by
management system and a hand-held terminal. connected to each HART loop. The gateways.

Production
management level
4-20 mA management system 4-20 mA management 4-20 mA management
system system

RB RB Controller RB
# # # # planning terminal #
# #
# # #
Gateway Bell 202-
Modem
if applicable 4-20 mA
# # #
HART
# #
barrier/
isolator DX
auxiliary power
Process management level
MUX
non- # field
multiplexer
hazardous 4-20 mA
4-20 mA 4-20 mA 4-20 mA

# # #

PD pH T #
hand-held
Field level terminal
PD pH T

Process
HART field device
a b c d

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HART
FIELD COMMUNICATION PROTOCOL

POINT-TO-POINT Production management level

n x 4 mA management system

Figure 2 shows some examples of RB

point-to-point mode. The conventional #
#
#

4-20 mA signal continues to be used #

Gateway Bell 202-
for analog transmission while Modem
measurement, adjustment and # HART
# barrier/
equipment data is transferred isolator
auxiliary power
digitally.
Process management level

The analog signal remains unaffected

and can be used for control in the
normal way. HART data gives access 4 mA 4 mA 4 mA
to maintenance, diagnostic and other # # #
operational data. pH
PD T
Field level
PD pH T

Process

HART field devices

Figure 3
With multi-drop mode, installation costs are
considerably reduced. As many as 15 field devices
can be operated from one auxiliary power supply.
Management systems and hand-held terminals can
MULTI-DROP be used.

This mode requires only a single pair

of wires and, if applicable, safety power supply is required by the field
barriers and an auxiliary power supply device and the master can be many
for up to 15 field devices (see Figure kilometres away. However, most
3). European countries do not permit Bell
202 signals to be used with national
Multi-drop connection is particularly carrier equipment so HART products
useful for supervising installations should not be used in this way.
that are widely spaced, such as
pipelines, feeding stations and tank Any number of field devices can be
farms. operated on leased lines, as long as
they are individually supplied with
HART instruments can be used in auxiliary power, independently of the
either mode. In point-to-point communication. If only one power
operations, the field device has supply is used for all the field devices,
address 0, setting the current output the number is limited to 15.
to 4-20 mA. In multi-drop mode, all
device addresses are greater than
zero and each device sets it’s output
current to 4 mA. For this mode of
operation, controllers and indicators
must be equipped with a HART
modem.

HART devices can communicate

using company-leased telephone
lines [2]. In this situation only a local

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HART
FIELD COMMUNICATION PROTOCOL

HART Protocol Structure Layer 1, the Physical layer, operates

on the FSK principle, based on the
Bell 202 communication standard:

HART follows the basic Open Data transfer rate: 1200 bit/s
Systems Interconnection (OSI) Logic '0' frequency: 2200 Hz
reference model, developed by the Logic '1' frequency: 1200 Hz
International Organization for
Standardization (ISO) [3]. The OSI The vast majority of existing wiring is
model provides the structure and used for this type of digital
elements of a communication system. communication. For short distances,
The HART protocol uses a reduced unshielded, 0.2 mm2 two-wire lines
OSI model, implementing only layers are suitable. For longer distances (up
1, 2 and 7 (see Figure 4). to 1500m), single, shielded bundles of
0.2 mm2 twisted pairs can be used.
Beyond this, distances up to 3000m
can be covered using single,
shielded, twisted 0.5 mm2 pairs.

A total resistance of between 230

OSI reference model ohms and 1100 ohms must be
Open Systems Interconnection available in the communication
circuit, as indicated in Figures 2 and 3
Layer Function HART
by RB.
7 Application provides formatted data HART instructions

6 Presentation converts data

Layer 2, the Link layer, establishes
the format for a HART message.
5 Session handles the dialogue
HART is a master/slave protocol. All
4 Transport
secures the
transport connection
the communication activities originate
establishes network from a master, e.g. a display terminal.
3 Network connections
This addresses a field device (slave),
establishes the data
2 Link link connection HART protocol regulations which interprets the command
1 Physical connects the equipment Bell 202
message and sends a response.

The structure of these messages can

Figure 4 be seen in Figure 5. In multi-drop
The HART protocol implements layers 1, 2 and 7
from the OSI model. mode this can accommodate the
addresses for several field devices
and terminals.

A specific size of operand is required

Structure of a HART message to enable the field device to carry out
the HART instruction. The byte count
Preamble SD AD CD BC Status Data Parity
indicates the number of subsequent
status and data bytes.
Field device and communication
status (ONLY from field device
to master) Layer 2 improves transmission
Byte count reliability by adding the parity
HART instruction character derived from all the
Display terminal and field
device addresses preceeding characters; each
Start character character also receives a bit for odd
parity.
Figure 5
The HART message structure offers a high degree
of data integrity.

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HART
FIELD COMMUNICATION PROTOCOL

The individual characters are:

Classes of instruction and classes of conformity
1 start bit
8 data bits Classes of instruction Classes of conformity for display terminals
for field devices
1 bit for odd parity Data flow Read measurement
1 stop bit variable 1 single measurement variable

identifier effective range

Read universal description effective limits
information 1A messages attenuation
Universal date serial number
Layer 7, the Application layer, brings commands start and end of scale
Write standard attenuation
the HART instruction set into play. parameters 2 phys. devices
Common practice circuit test
The master sends messages with commands
Read
requests for specified values, actual device-specific 3
information
Device-specific
values and any other data or commands
Write selected
parameters available from the device. parameters 4

The field device interprets these

Read and write
instructions as defined in the HART entire database 5
protocol. The response message
provides the master with status
information and data from the slave.

To make interaction between HART

compatible devices as efficient as Figure 6
possible, classes of conformity have Classes of instruction and classes of conformity
been established for masters, and
classes of commands for slaves.
There are six classes of conformity
for a master as seen in Figure 6. For
slave devices, logical, uniform
communication is provided by the
following command sets:

Universal commands
understood by all field devices. Examples of all three command
sets can usually be found in a field
Common practice commands device, including all universal
provide functions which can be commands, some common-
carried out by many, though not practice commands and any
all, field devices. Together, these necessary device-specific
commands comprise a library of commands.
the most common field device
functions.

Device-specific commands
provide functions which are
restricted to an individual device,
permitting special features to be
incorporated that are accessible
by all users.

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HART
FIELD COMMUNICATION PROTOCOL

OPERATING CONDITIONS
The HART standard [4] requires level frequencies in both directions (see
3 resistance to interference in the Figure 2). As can be seen in Figure 3,
lines in accordance with in multi-drop mode it is also possible
IEC 801-3 and -4. This satisfies the to interconnect field devices in
general requirement for noise accordance with DIN VDE 0165.
resistance.

Connecting or disconnecting a user,

or even a breakdown of
communication does not interfere with
transmission between the other units.

Intrinsically safe applications deserve

special mention. Barriers or isolators
must be able to transmit the Bell 202

HART AT A GLANCE

● Simultaneous analog and digital ● Multi-drop connection is permitted. four of which are transmitted on
communication. The analog signal Several 'smart' devices can be demand. Thus, devices with several
contains the process information; the connected to a single twisted pair of measurement functions (e.g. Coriolis
digital is used for two-way wires, reducing wiring costs. mass flow meters) can transmit
communication of both process and several variables in each message.
device information. ● Can be used with leased telephone
lines, so multi-drop connections can
● Accepts other analog units. With cover great distances using
simultaneous analog and digital inexpensive interface technology.
communication, you can also use
analog indicators, recorders and ● Provides an open message
controllers while communicating with structure. This enables new HART
'smart' field devices. devices with new features to be
added, thereby retaining compatibility
● A process management system and with existing units.
a hand-held terminal can both
communicate simultaneously, as ● The protocol allows up to 256
shown in Figure 2. variables in each field device, any

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HART
FIELD COMMUNICATION PROTOCOL

TECHNICAL DATA
DATA TRANSMISSION HARDWARE
RECOMMENDATIONS:
Type of data transmission:
Frequency shift keying (FSK) in Type of connection and length For an in-depth examination of
accordance with Bell 202, relating to limitations: whether a particular hook-up will
the transfer rate and the frequency for work, refer to the specification for the
bit information '0' and '1'. Distance Line type min. conduct.
(m) area Physical layer in the HART document
AWG/(mm2) [4].
Transfer rate: ≤ 1.500 multiple 2-wire, 24/0.2
1200 bit/s twisted, common
shielding
'0' bit information frequency:
2200 Hz > 1.500 single 2-wire, 20/0.5
≤ 3.000 twisted, shielded

'1' bit information frequency:

1200 Hz
The following rule of thumb for
Signal structure: determining the max. line length for a
1 start bit, 8 data bits, 1 bit for odd particular application can be taken
parity, 1 stop bit from the restrictions governing the
signal:
Transfer rate for simple variables:
approx. 2/s (poll/response) l = 65 . 106 - (Cf + 10.000)
approx. 3/2 (burst mode, optional) (R . C) C
where
Maximum number of units in bus
mode: l length in meters,
with a central power supply: 15 R resistance in ohms,
load plus internal resistance
Multiple variable specification: from the barrier/isolator,
max. number of variables per field C line capacity in pF/m,
unit: 256 Cf maximum internal capacitance for
max. number of variables per the Smart field units in pF.
message: 4
Consider the example of a pressure
Maximum number of master transducer, a control system and a
systems: simple shielded pair with
two R = 250 ohms,
C = 150 [pF/m],
Data integrity: Cf = 5.000 [pF]

Physical layer: l = 65 . 106 - (5.000 + 10.000)

error rate destination circuit: 1/105 bit (250 . 150) 150

Link layer:
l = 1.633 [m]
recognizes: all groups of up to three Then as is:
corrupt bits and practically all longer In intrinsically safe applications, there
and multiple groups. may be further restrictions.

Application layer:
Communication status transmitted in
a response message.

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HART
FIELD
FIELD COMMUNICATION
COMMUNICATION PROTOCOL
PROTOCOL

HART COMMUNICATION FOUNDATION

The HART Communication The Foundation provides a

Foundation is an independent, non- cooperative forum for ensuring the
profit corporation, organized to serve interoperability of field and control
growing industry interest in the HART devices using the HART protocol.
Protocol and the needs of HART Technical committees guide protocol
users. enhancements and maintain its open
structural standards.
The Foundation’s mission is to
coordinate, promote, and support the Foundation operation is guided by a
application of HART technology member elected Board of Directors,
worldwide. Educating the industry on an Executive Committee, and a full
this important technology is a key time Administrative Director. Key
role. issues are decided by members
through the General Assembly.
Foundation operating costs are offset
by membership and training/support For information on the HART Protocol
service fees. Membership is open to specifications, development tools,
all suppliers, end users, and others training courses, or Foundation
interested in promoting use of the membership please contact the
HART Protocol. address on the next page.

Bibliography [1] Bell System Technical Reference: PUB 41212,

"Data Sets 202S and 202T Interface Specification",
[3] DIN ISO 7498: Informationsverarbeitung,
Kommunikation Offener Systeme, Basis-
July 1976. Referenzmodell, Beuth Verlag, Berlin.

[2] Appendix to Bell System Technical Reference [4] HART Smart Communications Protocol
PUB 41004, "Data Communications Using Specification, Rev. 5.1.4, January 1991.
Voiceband Private Line Channels", October 1973.

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HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

HART
®

FIELD COMMUNICATIONS PROTOCOL

This introduction reflects the current state

of the commitment and lays no claim to completeness.

HART® Field Communications Protocol

Editor and contact addresses:

HART Communication Foundation

9390 Research Blvd, Suite II-250
Austin, Texas 78759 USA
Tel : +1 512 794 0369
Fax : +1 512 794 8893
or any member company

revision date: 10/1995

© HART Communication Foundation. All Rights Reserved
 HART COMMUNICATION FOUNDATION - HCF
MEMBERSHIP July 1996

ABB Masoneilan
Adaptive Instruments Corporation Measurement Technology Ltd
Allen-Bradley Meridian Instruments B.V.
Analog Devices MESCO Engineering GmbH
Anderson Instrument Co., Inc. Micro Motion, Inc.
Apparatebau Hundsbach GmbH Milltronics Ltd.
Applied System Technologies, Inc. Moore Industries-International Inc.
Arcom Control Systems Moore Products Co.
Bailey - Fischer & Porter MTS Systems Corporation
Beamex Oy, Ab Neles-Jamesbury
BESTA AG Peek Measurement Ltd.
Brooks Instrument Pepperl + Fuchs
BTG Källe Inventing AB Pondus Instruments AB
Delta Controls Limited Princo Instruments, Inc.
Drägerwerk AG Raytek, Inc.
Drexelbrook Engineering Company Rittmeyer Ltd. Measuring Control
Elcon Instruments sr. Robertshaw Tennessee
EMCO Flowmeters Rosemount Analytical Inc.
Endress + Hauser GmbH Rosemount Inc.
Endress + Hauser Ltd Rössel Messtechnik GmbH & Co.
Fieldbus International AS Saab Tank Control
Fisher Controls Samson AG
Fisher-Rosemount Systems Inc. Schlumberger Industries
Flowdata, Inc. Siemens AG
Foxboro-Eckardt Smar International Corporation
Fuji Electric Co., Ltd. SMC Corporation
Harold Beck & Sons, Inc. Softing GmbH
Hartmann & Braun AG Solartron Transducers
Helios AG SOR, Inc.
Hersey Measurement Company Southwest Research Institute
Honeywell, Inc. Sparling Instruments Co., Inc.
Honeywell Loveland Controls Company The Foxboro Company
Instrumenfirman INOR AB Toshiba Corporation - Principle Office
Johnson Yokogawa Corp Two Technologies, Inc.
K-TEK Corporation Valmet Automation Inc.
Kamstrup A/S Valtek International
Kay-Ray/Sensall, Inc. VEGA-Grieshaber KG
KDG Mobrey Ltd. Viatran Corporation
Knick Elektronische Meßgeräte GmbH & Co. W. C. Groenuijzen
Krohne Mebtechnik GmbH & Co. Westlock Controls Corporation
Leeds & Northrup Whessoe Varec, Inc.
MACTek Corporation Wireless Scientific, Inc.
Magnetrol International Yamatake-Honeywell Co., Ltd.

HART® is a registered trademark of

The HART Communication Foundation