The HART Protocol
- A Solution Enabling Technology -
ĒšIntroduction
ĒšThe HART Protocol - An Overview
ĒšThe Benefits of Fieldbus Technologies Today!
ĒšHART Technology Support
By Ron Helson, Director of the HART Communication
Foundation
 The HART® Field Communications Protocol is widely
accepted in the industry as the standard for digitally
enhanced 4-20mA communication with smart field
instruments. A wide range of products from an
increasing number of suppliers are available today,
and many more are in development. The enhanced
two-way communication capability of instruments
using the HART protocol can significantly improve plant
information management, provide solutions to today's business
challenges, and yield substantial cost savings. Initial
installation/commissioning savings of $400 to $500 per
instrument and annual maintenance/operations savings of $100
to $200 per instrument are commonly reported.
For many years, the field communication standard for process
automation equipment has been a milliamp (mA) analog current
signal. The milliamp current signal varies within a range of
4-20mA in proportion to the process variable being
represented. In typical applications a signal of 4mA will
correspond to the lower limit (0%) of the calibrated range and
20mA will correspond to the upper limit (100%) of the
calibrated range. If the system is calibrated for 0-100 PSI, then
an analog current signal of 12mA (50% of range) will
correspond to a pressure of 50PSI. Virtually all installed
systems use this international standard for communicating
process variable information between process automation
equipment.
HART Field Communications Protocol extends this 4-20mA
standard to enhance communication with smart field
instruments. The HART protocol was designed specifically for
use with intelligent measurement and control instruments which
traditionally communicate using 4-20mA analog signals. HART
preserves the 4-20mA signal and enables two-way digital
communications to occur without disturbing the integrity of the
4-20mA signal. Unlike other digital communication
technologies, the HART protocol maintains compatibility with
existing 4-20mA systems, and in doing so, provides users with
a uniquely backward compatible solution. HART Communication
Protocol is well-established as the "de facto" industry standard
for digitally enhanced 4-20mA field communication.
HART Field Communications Protocol is an extremely
important technology for both end users and suppliers. HART
technology is a major step in the evolution of process control,
and as an enabling technology is bringing significant innovation
in the capabilities of field instrument systems. The enhanced
communications capability of intelligent field instruments
employing the HART protocol, offers significantly greater
functionality and improved performance over traditional
4-20mA analog devices. The HART protocol permits the
process variable to continue to be transmitted by the 4-20mA
analog signal and additional information pertaining to other
variable, parameters, device configuration, calibration, and
device diagnostics to be transmitted digitally at the same time.
Thus, a wealth of additional information related to plant
operation is available to central control or monitoring systems
through HART communications.
A wide breadth of products supporting the HART protocol are
available from major instrumentation suppliers, and the number
of products and suppliers incorporating the technology continues
to grow. The HART protocol provides many benefits today that
are promised by fieldbus technologies in the future, and yet
retains the compatibility and familiarity of existing 4-20mA
systems. The HART protocol is field proven, simple to
implement, use and maintain. HART technology is being used in
a wide variety of applications worldwide to gain significant
improvements in plant performance, provide solutions to
regulatory compliance issues (ISO 9000, OSHA, EPA, DOT,
etc.) and realise substantial cost savings in initial
installation/commissioning and ongoing maintenance/operations.
Current estimates exceed 4 million installations worldwide.
 HART is an acronym for
"Highway Addressable Remote
Transducer". The HART
protocol makes use of the Bell
202 Frequency Shift Keying
(FSK) standard to superimpose
digital communication signals at
a low level on top of the
4-20mA as show in Figures 1
and 2. This enables two-way
field communication to take
place and makes it possible for
additional information beyond just the normal process variable
to be communicated to/from a smart field instrument. The
HART protocol communicates at 1200 bps without interrupting
the 4-20mA signal and allows a host application (master) to
get two or more digital updates per second from a field device.
As the digital FSK signal is phase continuous, there is no
interference with the 4-20mA signal.
HART is a master/slave protocol which means that a field
(slave) device only speaks when spoken to by a master. The
HART protocol can be used in various modes for
communicating information
to/from smart field instruments
and central control or
monitoring systems. HART
provides for up to two masters
(primary and secondary) as
show in Figure 3. This allows
secondary masters such as
handheld communicators to be
used without interfering with
communications to/from the
primary master, i.e. control/monitoring system. The most
commonly employed HART communication mode is
master/slave communication of digital information simultaneous
with transmission of the 4-20mA signal as shown in Figure 4.
The HART protocol permits all digital communication with field
devices in either point-to-point or multidrop network
configuration. Figure 5 highlights the optional "burst"
communication mode where a single slave device can
continuously broadcast a standard HART reply message. Higher
update rates are possible with this optional digital
communication mode and use is normally restricted to
point-to-point topologies.
Considerable installation savings are possible with the multidrop
networking capability of HART which allows multiple field
devices to be connected to the same pair of wires. In multidrop
applications, communication with field devices is restricted to
digital only as the loop current is fixed at a minimum value and
loses any meaning relative to the process. From an installation
perspective, HART signals are carried over the same wiring as
is typically used for conventional 4-20mA analog instruments
today. As one might expect, cable run lengths can be longer for
individually shielded twisted pairs, than for multiple twisted
pairs with an overall shield as reflected in Figure 6. For short
distance, unshield cables may be used. HART compatible
intrinsic safety barriers and isolators are also available which
pass the HART signals for use in hazardous areas.
The HART Command Set is organised into three groups and
provides read/write access to the wealth of additional
information available in smart field instruments employing this
technology. Universal Commands must be implemented by all
HART devices and provide interoperability across the large and
growing base of products from different suppliers supporting the
HART technology. Universal Commands provide access to
information that is useful in normal plant operation such as the
instrument manufacturer, model, tag, serial number, descriptor,
range limits, and process variables. Common Practice
Commands provide access to functions which can be carried out
by many devices though not all, and Device Specific Commands
provide access to functions which may be
unique to a particular device. Figure 7 highlights the type of
information that can be obtained from these devices. The
integrity of HART communication is very secure as status
information is included with every reply message and extensive
error checking occurs with each transaction. Up to four process
variables can be communicated in one HART message and each
device may have up to 256 variables.
 Device Description
Language (DDL), a recent
enhancement to the HART
technology, extends
interoperability to a higher
level than provided through
the Universal and common
Practice Commands. As
reflected in Figure 8, DDL
provides a field device
(slave) product developer
with the means to create a complete description of their
instrument and all relevant characteristics, such that it can talk
to any host device using the language. This is analogous to a
printer driver in the personal computer world which enables an
application to talk with a printer such that what gets printed on
the page is what was expected by the application. Universal
hand-held communicators capable of configuring any
HART-based instrument through DDL are available today.
Broader application in other types of host systems is expected.
The HART Communication Foundation manages the centralised
library of all registered Device Descriptions and DDL is being
supported by all members of the Foundation.
The relative simplicity of the HART protocol makes it easy for
both end users and suppliers to gain experience and benefit from
the enhanced two-way communication capability of smart field
instruments using this technology. Powerful multiparameter
instruments, efficiency with remote communication, field device
diagnostics, cost effective control in field devices, installation
savings with multidrop networking, and flexible/accurate digital
data transmission are all achievable today with instruments that
use HART Field Communications Protocol. Users can achieve
many of the benefits promised by fieldbus technologies in the
future with the HART protocol today. And, the compatibility
with 4-20mA makes it easy for users to incrementally add
HART speaking instruments incrementally protecting their
investment in existing systems and gaining the benefits of
enhanced field communication.
The advantages of enhanced field communication enabled by
HART technology have been quantified in terms of maintenance,
installation, and commissioning cost savings in a wide variety of
applications in industries. Initial installation commissioning
savings of $400-$500 per instrument and annual
maintenance/operations savings of $100-200 per instrument
are commonly reported. Tremendous operational benefits are
also being achieved. And today, the large and growing base from
major instrumentation suppliers supporting the HART protocol,
provides the freedom to choose the right product for the job in
integrating with existing plant systems.
Instrumentation products equipped with the HART protocol are
being used in a wide range of applications and industries to
provide cost saving benefits and improved plant performance
worldwide. From chemical/refining operations, to gas/liquid
distribution systems, and remote/off-shore monitoring stations
current installations are addressing virtual all aspects of
control, data acquisition, and maintenance. Installation
estimates already exceed 1,400,000 and independent
projections forecast explosive growth over the next decade.
Some of the many reasons cited for the tremendous growth and
acceptance of HART technology include:
- The wide variety and increasing number of products available
today from a growing list of major instrumentation suppliers
around the world, HART is the only "open" communications
protocol of its type and in "defactor" industry standard. Users
have the freedom to choose the right product for their
application and interoperability is assured by the common
command and data structure.
- Relatively easy to understand and use, the HART protocol
provides access to the wealth of additional information
(variables, diagnostics, calibration, etc.) available in smart field
devices employing this technology. HART enables field
instrument suppliers to incorporate powerful features into their
products such as PID control algorithms, diagnostics, and
additional process measurements. User access to these
features is provided through the enhanced communication
capability of HART.
- HART is a no risk solution for enhanced field communication.
For maintenance and operations people, HART is a relatively
easy transition especially in point to point applications. Any
fears about being able to keep the plant running are small as the
4-20mA signal can still carry the process variable (as with
traditional instruments), and the enhanced two-way field
communication capability of HART can provide real benefit for
improved plant performance.
- And, in applications where appropriate, the multidrop
capability of HART provides the opportunity to connect several
field instruments on the same pair of wires, substantially
reducing installation costs.
As owner of the technology, the independent HART
Communication Foundation maintains the standard and ensures
ongoing support for the HART protocol. The Foundation is a
not-for-profit corporation, specifically organised to coordinate
and support the application of HART technology worldwide.
Educating the industry on the capabilities and value of this
important technology is a key role. Operating costs are offset by
membership and training support service fees. Membership is
open to all suppliers, end-users, and others interested in use of
the HART protocol.
The HART Communication Foundation supports the industry,
and its members, as the official source of information regarding
HART technology worldwide. Additional functions include:
providing training and support for application of the technology,
directing quality assurance programs to ensure the
interoperability of HART devices, managing the centralised
library and controls distribution of all registered Device
Descriptions, and coordinating trade show exhibits and
promotional activities for educating industry on the technology.
The Foundation maintains a forum section on CompuServe
devoted to the HART protocol (GO HARTCF) and a World Wide
Web site located at http://www.hartcomm.org
This article was written and supplied by kind permission by Ron
Helson, director of the HCF.
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