What is the HART protocol?
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By Nikhil Agnihotri
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Data communication over analog signals is a common process. In automation, the data is
communicated to and from controllers with several different process instruments. Quality
is important as the controllers and smart instruments must communicate over long Amphenol's high-current power
pin connectors provide a high-
distances. speed transmission performance
up to 10 Gb/s
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Additionally, the data communication between a device and a controller must be dictated
by a protocol (i.e. a set of digital message formats and rules). Many communication
Amphenol's pitch flex connectors
protocols are used in industrial automation, including As-i, BSAP, CIP, CAN bus, DeviceNet, are available in multiple heights
DirectNet, EtherCAT, EtherNet/IP, EGD, Powerlink, HART, Interbus, Modbus, OSGP, and are vibration-proof
Profibus, MPI, and others. Learn more
The HART (highway addressable remote transducer) protocol is one of the most popular Amphenol's FFC offer the
flexibility and performance
for use in industrial automation. Based on the legacy 4-20mA analog instrumentation associated with pitch size, length,
and thickness options
wiring, the HART protocol uses a pair of wires for data communication between industrial
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controllers and smart process instruments.
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Understanding analog transmission profile 0.60 mm stack height and
Computer systems and interfaces use digital voltage signals for data communication. The high current rating of up to 5 A
problem with voltage signals, however, is that they’re affected by wire resistance — which Learn more
makes communicating voltage signals for data communication over long distances
impractical. This is why current loops are typically used in automation.
Since the ‘80s, the 4-20 mA has been the de facto standard in automation wiring. The
value of the 4 mA represents a “live zero,” helping maintain a minimum amount of current
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for the loop-powered instruments.
These current loops use analog (continuous) signals for data communication. The digital
data is superimposed on a carrier signal (current) using a digital modulation technique. The
amplitude-shift keying (ASK), frequency-shift keying (FSK), and quadrature phase shift
keying (QPSK) are currently the most commonly used digital modulation techniques.
Understanding the HART protocol?
The highway addressable remote transducer or HART is an industrial automation protocol.
It’s a global standard for two-way analog communication between smart-process
instruments and control or monitoring host systems.
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The host system can be a laptop, a handheld device, or an industrial process-control
system (PLC/DCS). The protocol is based on the Bell 202 standard and it’s capable of
superimposing digital information on the conventional 4-20 mA analog signals using the BROWSE DESIGN GUIDES
FSK. This protocol is also maintained by the HART Communication Foundation (HCF) and
defines physical connectivity, as well as the commands used by applications.
The HART protocol communicates data at 1200 bps allowing a host to get at least two
updates from a smart-field device in one second. The protocol provides two simultaneous
channels:
1. The 4-20 mA analog signal – for communicating primary measured values EXPAND YOUR KNOWLEDGE
2. A digital signal that’s superimposed on the analog signal – for communicating additional AND STAY CONNECTED
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As the FSK signals have an average value of zero, the modulation does not interrupt the
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carrier analog signal.
The HART is a master-slave protocol that allows data communication in point-to-point or
multi-drop mode. Up to two masters (in point-to-point configuration) can share the HAVE A QUESTION?
communication bus, simultaneously. This permits handheld communicators to operate as
secondary masters while a control or monitoring system operates as the primary master.
Have a technical question about
How the HART protocol works an article or other engineering
The HART protocol makes use of the FSK. The digital data is represented by two different questions? Check out our
frequencies. The frequency of 1,200 Hz represents bit 1 and 2,200 Hz represents bit 0. engineering forums
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By superimposing two frequencies of direct-current analog signals, the HART provides
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simultaneous analog and digital communication. The 4-20 mA signal is not affected by the
shifter/translator supporting negative
digital modulation.
voltage
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For this reason, the HART is called a hybrid protocol. harvester
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Basic Electronics 06 –
Fixed Resistors:
The HART is a master-slave protocol and the entire data communication process is wirewound, foil and
semiconductor resistors
initiated and controlled by the host system. It’s the host device that requests the data from
a HART-enabled field device. This field device simply responds to the HART commands
according to the protocol.
The HART protocol operates over two wires allowing for 2-way communication between Basic Electronics 05 –
Fixed Resistors:
the host controller and the field devices. composition, film type
and cermet
There are also wirelessHART solutions available. However, the wirelessHART gateways do
not use an analog signal for data transmission. Instead, they use the digital parameters
over a wireless carrier wave. Basic Electronics 04 –
Types of resistors
The HART network
HART-enabled devices can operate in one of the two configurations:
Basic Electronics 03 –
Point-to-point: when the HART communication occurs between a HART-enabled Practical guide to
resistors
device and a host system. The primary variable (data) is communicated over an
analog signal and all additional variables are communicated using a digital signal. The
host system can include up to two masters: a primary and a secondary master.
Multi-point: when multiple HART-enabled devices are connected in parallel to a
Basic Electronics 02 –
transmission line. According to the HART v07, 64 field devices can be connected to a Common mistakes made
by electronics beginners
HART bus in a multi-drop configuration. These devices use only digital channels for
data communication while the analog signal is maintained fixed to the 4 mA. Each
device has a unique HART address and no two devices can access the transmission
line simultaneously.
Basic Electronics 01 –
Beginners guide to
setting up an electronics
The HART protocol lab
The data in a HART protocol is transmitted in packets. Each message packet contains the
following data fields:
Preamble – includes 5 to 20 bytes with the hex value, “FF,” which is transmitted from the
host system to the field device(s). Most of the host systems only transmit five preamble RECENT ARTICLES
bytes. These bytes are transmitted to help the field device synchronize with the data
stream. How to design a LoRa-based asset-
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Start character – after the preamble bytes, a start byte is transmitted from the master management
(host) to the field device. This byte can indicate one of several values and, often, contains Power Integrations’ plug-and-play
the master number. The start character is the indicator of the beginning of the HART gate drivers simplify design for wind,
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Aside from the master number, it may contain indicators for the:
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Message format – such as the master-to-slave, slave-to-master, or the burst
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message from a slave
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Address format – a short or long frame
Number of bytes in the expansion field
Address – may have 1 to 5 bytes. It contains both a master and a slave address. The
indication for the master address is a single bit, where 1 indicates a primary master and 0
indicates a secondary master. The indication of a slave address is a 4-bit polling address in EE ENGINEERING
a short frame and a 38-bit unique identifier in the long frame. One bit is used to indicate TRAINING DAYS
the burst mode from the slave.
Expansion – may contain 0 to 3 bytes. The number of bytes in an expansion field includes
bits 5 and 6 of the start character.
Command – has a single byte that contains a HART command.
The universal HART commands range from 0 to 30
The practice commands range from 32 to 126
The device-specific commands range from 128 to 253
From the HART v06 onwards, the 16bit extended commands for the device families are also
supported. These commands begin with the 0x1F as a command byte, followed by a 2-byte
command. SUBMIT A GUEST POST
Byte count – has a single byte indicating the number of bytes in the status and data fields.
It let the receiver field device know when the message packet ends.
Status – is only present in the messages transmitted from a slave to the master. It
contains two bytes indicating communication errors, the status of a received command, or
the status of the receiver field device.
Data – may contain 0 to 255 bytes. The presence of the data field depends on a particular
command. All universal and common practice commands have 33-byte data fields.
Checksum – a HART message will end with a checksum byte. It’s the XOR of all the
previous bytes. To detect a communication error, a parity bit is attached to each byte. The
checksum byte helps in maintaining the overall integrity of the HART message.
The HART modes
The HART protocol can operate in two modes:
Master-slave mode: where the host system initiates and controls the entire data
communication. There can be two masters, including the primary and secondary ones
in the HART loop.The primary master can be a distributed control system (DCS), a
programmable logic controller (PLC), or a workstation. The secondary master can
include handheld communicators or a laptop/PC. The slave devices can be HART-
enabled sensors, actuators, controllers, or transmitters.
Burst mode: only available in a point-to-point configuration. In this mode, the slave
device continuously broadcasts the standard HART reply messages to the
master(s).Once a master instructs the slave device to broadcast the messages, the
slave keeps sending responses without the need for polling. It stops only when it
receives further instructions from the host system. This is why this mode allows for
faster data communication with up to 3 or 4 updates per second.
The HART applications
When using the HART protocol, four or more variables can be transmitted in a single
message. Multi-variable instruments can take the most advantage of this protocol. In fact,
there are now millions of HART-enabled devices that are used in process automation and
other industrial applications.
There are also more than 250 member companies of the HART Communication Foundation
that have their HART-enabled products in the market. Several of these products are used
in DCS and PLC applications.
As the HART uses the older 4-20 mA standard, it has become the de facto standard for
data communication for smart-field devices and grid instrumentation. Smart-field devices
and sensors use this protocol for communicating device configuration, diagnostic data,
device status, device troubleshooting, and measurement values.
The wirelessHART is also widely used in instrumentation and automation. It allows for the
communication of sensor data without complex multiplexing and cable installations.
Device descriptions
Generally, host systems use high-level language applications for data communication
when using the HART protocol. The protocol is implemented by software on the host
device. Since the host system might need to communicate with several different HART
devices, the software must “know” the HART commands supported by a particular device.
Similar to how computer peripherals have device drivers written for them to communicate
with different computer systems, HART devices have device descriptions (DD).
The device descriptions are written in the HART device description language (DDL) and
their source files are submitted by the device manufacturer to HCF for registration in the
HCF DD library. After verification, these source files are available for host applications.
The host applications use these DD files to identify common practice and device-specific
commands supported by a particular device. By using DDs, the host suppliers need not
build custom interfaces and drivers for every different HART device.
The pros and cons
The HART protocol offers many advantages in automation and instrumentation segments.
Perhaps the greatest one is that it’s based on the conventional 4-20 mA standard. This
reuse of the 4-20 mA wiring not only means the protocol is cost-effective, but it also
allows it to have wide acceptance among industrial manufacturers and vendors.
Today, there’s a broad selection of HART products available with compatible host
applications and devices. The protocol is highly accurate, reliable and it’s platform-
independent — allowing for full-interoperability in a multi-vendor environment. As a hybrid
protocol, it’s also the preferred choice for multi-variable instruments. Thanks to its ability
to self-diagnosis with quick device configuration, the HART protocol is now commonly
used with smart-field devices.
There are some limitations, however. For example, the digital signal in the HART
transmission is relatively slow. Also, for a multi-drop configuration, the analog signal is not
available and the number of devices that can share the transmission line is limited. These
limitations often mean integration with other protocols and standards is necessary.
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