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LEARNING UNIT 2

Highway Addressable Remote Transmitter (HART)

2.1 Introduction

Industrial networks function to control physical equipment, in hostile conditions often with a high level of noise,
heat and vibrations. Their communication is characterised by transferring small data packets in a periodic or
aperiodic fashion. Industrial network applications require predictable communication (determinism) and fast
round trips between 250 microseconds and 10 milliseconds. Industrial networks must be very reliable, and failure
of the network can lead to catastrophic events in the industrial network evolved from equipment manufacturers’
proprietary communication technologies. They were later standardised into the IEC 61158, collectively known as
Fieldbus technologies. The IEC 61158 defines a fieldbus as “a digital, serial, multidrop, data bus for
communication with industrial control and instrumentation devices such as but not limited to transducers,
actuators and local controllers.” Fieldbus has 2-way communication.

Figure 2.1 compares the wiring connections between an analogue 4-20mA network and a fieldbus network. It
shows that while the analogue communication system requires that each field device is linked to the control room
using its dedicated link, the fieldbus network allows for the sharing of wiring thus cutting down the cost of
wiring. The other advantage of the fieldbus is that it offers the capability for smart instruments to transmit
their ‘process variable’ as well as additional information. In contrast, analogue communication is only capable
of carrying the process variable (PV).

Figure 2.1: Point-to-point analogue communication vs fieldbus network wiring

The communication in the analogue system is unidirectional, (i.e. either from the central controller to the field
device or the reverse). However, fieldbus allows for the bi-directional exchange of information. The fieldbus
network is thus able to facilitate a distributed control system, whereby the control functions are housed within the
field device which is not the case in the analogue communication system.

Field networks are not the only solution when plant operators want to use the advantages of smart field devices.
The HART protocol provides many possibilities even for installations that are equipped with the conventional 4
to 20 mA technique. HART devices communicate their data over the transmission lines of the 4 to 20 mA system.
This enables two-way communication and the field devices can be parameterized and started up flexibly or to read
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measured and stored data (records). All these tasks require field devices based on microprocessor technology or
intelligent devices. These devices are frequently called smart devices.

Introduced in 1989, this protocol has proven successful in many industrial applications and enables bidirectional
communication even in hazardous environments. HART allows the use of up to two masters: the engineering
console in the control room and a second device for operation on site, e.g. a PC laptop or a handheld terminal.
The most important performance features of the HART protocol include:

• proven in practice, simple design, easy to maintain and operate

• compatible with conventional analogue instrumentation
• simultaneous analogue and digital communication
• option of point-to-point or multidrop operation
• flexible data access via up to two master devices
• supports multivariable field devices
• sufficient response time of approx. 500ms
• open de-facto standard freely available to any manufacturer or user

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 backwards 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.
The HART protocol provides access to all information in multivariable devices. In addition to the analogue 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.

2.2 Operation of HART Protocol

HART is a master-slave communication protocol, which means that during normal operation, each slave (field
device) communication is initiated by a master device. Two masters can be connected to each HART loop. The
primary master is generally a distributed control system (DCS), programmable logic controller (PLC), or personal
computer (PC). The secondary master can be a handheld terminal or another PC. HART slave devices include
sensors, transmitters and various actuators. The variety ranges from two-wire and four-wire devices to intrinsically
safe versions for use in hazardous environments. Slave devices respond to commands from the primary or
secondary master.

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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 a higher rate until it instructs the slave to stop bursting. The Burst communication mode
is normally restricted to point-to-point configuration.

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) analogue signal cables to provide simultaneous analogue and digital communications as
shown in Figure 2.2.

Figure 2.2: Frequency shift keying (FSK) modulated digital communication over the analogue signal.

Because the average value of the FSK signal is always zero, the 4–20 mA analogue signal is not affected. The
digital communication signal has a response time of approximately 2–3 data updates per second without
interrupting the analogue signal. A minimum loop impedance of 230 Ω is required for communication. The digital
signal contains information from the device including device status, diagnostics, additional measured or calculated
values, etc.

The HART communication digital signal gives access to secondary variables and other data that can be used for
operations, commissioning, maintenance, and diagnostic purposes. The HART protocol utilizes the OSI reference
model. As is the case for most of the communication systems on the field level, the HART protocol implements
only layers 1, 2 and 7 of the OSI model. Layers 3 to 6 remain empty since their services are either not required or
provided by the application layer 7.

2.3 Packet Structure (HART Telegram)

The HART Packet has the structure shown in Table 2.1 below.

Start Delimiter Command Number Data Bytes Status Checksum

Preamble Address Data

(5-20 bytes) (5 bytes) (0 to 253 bytes)

1 byte 1 byte 1 byte 0 (If Master) 1 byte

1 (If Slaves)
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Table 2.1: Structure of the HART Packet

Field Name Length (Bytes) Purpose
Synchronization and Carrier
Preamble 5-20
Detect
Start Byte 1 Specifies Master Number
Specifies slave, Specifies Master
Address 1-5 and Indicates
Burst Mode
Numerical Value for the command
Command 1 to be
executed
Number of Data
1 Indicates the size of the Data Field
Bytes
Master (0) Slave
Status Execution and Health Reply
(2)

Data 0-253 Data associated with the command

XOR of all bytes from the Start

Checksum 1 Byte to the Last byte
of Data

a) Preamble
The preamble can vary from 5 to 20 bytes and is determined by the slave’s requirements. When talking to a slave
for the first time, a master will use the longest possible preamble (e.g. 20 bytes). Once the master reads the slave’s
preamble length requirement (a stored HART parameter), it will subsequently use this new length when talking
to that slave. Different slaves can have different preamble length requirements so a master might need to maintain
a table of these values. A longer preamble means slower communication. Slave devices are now routinely designed
so that they need only a five-byte preamble, and the requirement for a variable preamble length may now be
required for use with older model transmitters.
b) Start Delimiter
This byte contains the Master number and specifies the communication packet is starting...
c) Address
Each HART device has a 38-bit address that comprises the manufacturer’s ID code, the device code, and a device-
unique identifier. This address is included in each device during manufacture and, to establish communication, it
must be known by the master-which can learn the slave address by issuing one of two commands:

• Read unique identifier (Command 0)

• Read unique identifier by tag (Command 11)
d) Command

This is a 1-byte numerical value representing which command is to be executed (e.g. Command 0 and Command
11 are used to request the device number)

e) Number of Data Bytes

Because there is no delimiter between the data and the checksum field, this field indicates the number of bytes
between this point and the checksum.
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f) Status
The status field is absent for the master and is 2 bytes for the slave. This field is used by the slave to inform the
master whether it completed the task and what its current health status is.
g) Data
Data contained in this field depends on the command to be executed.
h) Checksum
The checksum at the end of the message is used for error control and is the ’Exclusive-OR’ of all of the preceding
bytes, starting with the start delimiter. The checksum, along with the parity bit in each character, creates a message
matrix having a so-called vertical and longitudinal party. If a message is in error, a retry is usually executed.

2.3.1 Establishing Communication with HART Devices

Each HART device has a 38-bit address that consists of the manufacturer ID code, device type code and device-
unique identifier. A unique address is encoded in each device at the time of manufacture. A HART master must
know the address of a field device to communicate successfully with it. A master can learn the address of a slave
device by issuing one of two commands that cause the slave device to respond with its address: Query from
Master: 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.

Query: 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. Multi-dropping 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.

2.3.2 Summary of the 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. Host applications may implement any
of the necessary commands for a particular application.

UNIVERSAL Commands: 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
variables and units, read current output and percentage of range).

COMMON PRACTICE Commands: Common practice commands provide functions implemented by many
but not necessarily all HART communication devices (e.g., Read a selection of up to 4 variables, calibrate, Set
fixed output current etc…).

DEVICE SPECIFIC Commands: 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 (e.g., Write PID
set point, Read or write materials or construction information).

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2.3.3 Device Description

Some HART host applications use device descriptions (DD) to obtain information about the variables and
functions contained in a HART field device. The DD includes all 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 the
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 HART Communication Foundation (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 the 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.

2.4 HART Networks

HART devices can operate in one of two network configurations—point-to-point or multi-drop. In extended
systems, the number of accessible devices can be increased by using a multiplexer. In addition to that, HART
enables the networking of devices to suit special applications. Network variants include multi-drop, FSK bus and
networks for split range operation.

2.4.1 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.

The 4–20 mA analogue signal is not affected by the HART signal and can be used for control in the normal way.
Figure 2.3 below is a typical indication of point-to-point communication.

The HART master device is connected to exactly one HART field device. This connection variant requires that
the device address of the field device be always set to zero since the operating program uses this address to
establish communication.

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Figure 2.3: Point-to-point Communication

2.4.2 Multi-drop Mode

In this mode, only digital signals are used. The analogue loop current is fixed at 4 mA. In multidrop mode, it is
possible to have up to 15 instruments on one signal cable. The polling addresses of the instruments will be in the
range of 1-15. Each meter needs to have a unique address. In multi-drop mode, up to 15 field devices are connected
in parallel to a single wire pair as shown in Figure 2.4. The HART protocol was originally designed for
transmitters. The multi-drop mode was also developed for them. Figure 2.5 shows a communicator HART device
(HART hand-held device).

In multi-drop operation, the devices exchange their data and measured values only via the HART protocol. The
analogue current signal serves just to energize the two-wire devices, providing a direct current of 4 mA. The host
distinguishes the field devices by their preset addresses which range from 1 to 15. Control valves cannot be used
in conjunction with multi-drop mode. The digital HART communication is too slow to pre-select set points. The
control signals for valves are therefore always transmitted as a 4 to 20 mA standardized current signal.

Figure 2.4: Multi-drop System with Primary and Secondary Masters

Figure 2.5: HART Communicator

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2.4.3 Connecting through a Multiplexer

Multiplexers can be used in applications that require interfacing with a large number of HART devices. In the
configuration shown in Figure 2.6, the multiplexer is used as the primary Input/Output (I/O) front-end for the
HART-based monitoring system in which 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. In a traditional 4-20 mA system making use of the analogue signal for measurement and control
purposes, the multiplexer can also be used to provide parallel monitoring to gain access to the digital HART
signal. Due to the cascaded multiplexer structure, the host can communicate with many (> 1000) devices, all with
the address zero.

Figure 2.6: HART Communication using a Multiplexer

The user selects a particular current loop for communication via the operating program. As long as the
communication takes place, the multiplexer connects the current loop to the host.

2.5 Transmission Time and User Data Rate

In shorter messages, the ratio between user data and control data (overhead) becomes increasingly unfavourable
so it can take up to 128 ms to transmit one user data byte. An average of 500 ms is accounted for per transaction
i.e. for both a master and a slave telegram, including additional maintenance and synchronization times. As a
result, approximately two HART transactions can be carried out per second. These values show that the HART
communication is not suitable for transmitting time-critical data. HART can be used to determine the
reference variable of a final control element in the test and start-up phases, but it is not suited to solve
control tasks.

2.6 Wiring and Installation Rules

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.

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2.7 Cable Length and Capacitance

For trouble-free transmission, the cables must have a sufficient cross-section and an appropriate 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 2.2 shows the effect 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.

Detailed information for determining the maximum cable length for any HART network configuration can be
found in the HART Physical Layer Specifications.

Table 2.2: Table 2 Cable Lengths and Cable Capacitance Length

Cable capacitance (pF/m)-Cable Length (m)
Number of 65 pf/m 95 pf/m 160 pf/m 225 pf/m
Devises
1 2769 m 2000 m 1292 m 985 m
5 2462 m 1815 m 1138 m 892 m
10 2154 m 1600 m 1015 m 769 m
15 1846 m 1415 m 892 m 708 m

2.7.1 The wiring rules can be summarized as follows:

• For short distances, simple unshielded 0.2 mm2 two-wire lines are sufficient.
• For distances of up to 1,500 m, individually twisted 0.2 mm² wire pairs with a common shield over the
cable should be used.
• For distances of up to 3,000 m, individually twisted 0.5 mm2 two-wire lines shielded in pairs are required.

Most of the wiring in the field meets these requirements and can therefore be used for digital communication.

2.7.2 Plug connectors

An essential benefit is that HART integrates the existing wires and no additional wiring or changes to wiring need
to be made. The HART specification does not prescribe the use of a specific type of plug connector. Since the
polarity does not influence the frequency evaluation, HART signals are usually connected via simple clamp
terminals.

2.8 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.

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2.8.1 Intrinsic Safety Devices

HART-communicating devices work well in applications that require IS 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 analogue instrumentation, energy to the field can be limited with or without a ground connection by
installing one of the following IS devices:

• A Shunt-diode (Zener) barrier that uses a high-quality safety ground connection to bypass excess energy
(Figure 2.7).
• Isolators (Galvanic Isolators), which do not require a ground connection, that repeat the analogue
measurement signal across an isolated interface in the safe-side load circuit (Figure 2.8)

Both Zener barriers and Galvanic 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. Designing an IS direct-
current loop simply requires ensuring that a field device has sufficient voltage to operate, taking into account
Zener barrier resistance, the load resistor, and any cable resistance.

Figure 2.7: Zener Barrier to provide a bypass for Excess Energy

Figure 2.8: IS Using Galvanic Isolators

2.8.1.1 When designing an IS loop using shunt-diode barriers, two additional requirements must be
considered:

• 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.
• 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
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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 1

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. While it is difficult to
meet the two requirements noted above for a network using shunt diode barriers, it can be done. The 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 due to cable resistance. System
operation at 22 mA requires reducing the 20 mA voltage by 0.7 V (340 Ω × 2 mA).

2.8.1.2 Designing an IS system using Isolators

The implementation of HART loops in an IS system with isolators requires more planning. An isolator is designed
to recreate the 4–20 mA signal from the field device in the safe-side load circuit. Most of the older isolator designs
will 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).

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.

2.8.1.3 Multi-drop IS Networks

HART multi-drop networks are particularly suitable for intrinsically safe installations. With a multi-drop
configuration, fewer barriers or isolators are required. Each field device consumes only 4 mA (for a total of 16
mA in a four-device loop). Plain Zener barriers can therefore 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.

2.8.1.4 IS Output Loops

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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.

2.8.1.5 IS Certification Considerations

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

2.9 Wireless HART

The latest release of HART, release 7, contains an implementation of both the network and transport layer. This
has allowed for the implementation of Wireless HART aimed at connecting remote points in a mesh topology as
shown in Figure 2.9. It provides a secure, reliable and robust method of connecting devices wirelessly for
industrial applications.

Wireless HART is a self-organizing wireless mesh network technology based on the IEEE 802.15.4 standard. It
operates within the 2.4 GHz ISM band and has a data rate of 250kbps. This is much faster than the 1.2kbps in the
traditional FSK-based HART. Wireless HART provides 3 main advantages over its wired counterpart:

• Lower cost: No cabling costs are involved since it is wireless.

• Easy installation: It is possible to mount instruments in environmentally difficult positions.
• Flexible: The nodes can be moved around without the need to reconfigure the network.

Figure 2.9: Wireless HART mesh network

A basic wireless HART network consists of Wireless HART field devices, at least one gateway, a 5-network
manager and a security manager. These components are connected into a wireless mesh network supporting
bidirectional communication.

The roles and descriptions of the Wireless HART network are:

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A Gateway device connects the wireless network to the plant host application situated in the wired plant’s
automation network. The gateway connects the wireless devices via an in-build access point. Moreover, the
gateway also tends to incorporate the network and security manager functions.

The Network manager is a software application that manages the mesh network and devices. It connects new
devices, forms redundant routing paths of the mesh network, sets the communications schedule for the devices
and monitors the network for any changes.

The Security manager is also a software application that works in close association with the network manager.
It allows only authorized devices to join the network and ensures that data is securely transmitted within the
network by using encryption and authorization keys. There can be only one security manager per network and one
security manager can serve multiple networks.

A Wireless HART field device is a battery-powered low-power field device with built-in capability of receiving,
transmitting and relaying data messages in radio frequency.

Wireless HART adapter allows existing traditional HART devices to be integrated into the Wireless HART
network. It provides an alternative communication path for the HART device’s existing 4-20mA current loop.

A WHART router is used to extend the WHART’s network coverage. It is thus used for message forwarding.
Occasionally the WHART can have a mobile terminal device used for commissioning, monitoring and
maintenance of the devices.

Wireless HART’s physical layer is based on the IEEE 802.15.4 operating in the 2.4 GHz ISM band, which
includes 15 of 16 possible Radio frequency channels. To avoid signal jamming WHART uses FHSS (Frequency
Hopping Spread Spectrum) technique.

The Wireless HART data link layer (DLL) is also based on the IEEE 802.15.4-2006 Media Access Control
protocol. WHART uses both the CSMA/CA and TDMA for arbitration. TDMA (Time Division Multiple Access)
synchronizes the network participants using 10ms timeframes/time slots as indicated in Figure 2.10 The two
communicating nodes can each transmit and the other acknowledges receipt of message in one time slot. This
enables a very reliable (collision-free) network and reduces the lead and lag times during which a station must be
active.

Figure 2.10: Wireless HART mesh network.

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Ref # Vaal University of Technology/Electrical Engineering
 PROCESS CONTROL & INSTRUMENTATION EIDCS2A – DIGITAL CONTROL SYSTEMS 2
HIGHWAY ACCESSIBLE REMOTE TRANSMITTER (HART) LEARNING UNIT 2

Forming a Wireless HART network – The network manager is initially pre-configured with the network ID and
password. On the factory floor, the field devices are configured with the network ID and password for them to be
part of the network. Following this, the network manager sends an ‘advertisement’ to the newly configured device
and the device will respond with a ‘join request’.

In the third step, the network manager authorizes the network device, gives it a unique key from the security
manager, schedules its data transmitting in the TDMA frame and updates its routing paths. After this, the field
device can then begin to send/publish data using the network.

2.10 Benefits of HART communication

I. Improved plant operations:

• If uploaded to a software application, digital data can be used to automate record-keeping for regulatory
compliance (e.g., environmental, validation, ISO9000, and safety standards).
• The HART protocol provides access to all information in multivariable devices. In addition to the
analogue 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.
II. Increase operational flexibility:
• The HART protocol ensures interoperability 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.
III. Lowering maintenance costs
• 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.
IV. Lowering the cost of installation and commissioning
• A HART network can be installed and commissioned in a fraction of the time required for a traditional
analogue-only system. Operators who use HART digital communications can easily identify a field
device by its tag and verify that operational parameters are correct.
• 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.

Page 14 of 14
Digital Control Systems 2
Ref # Vaal University of Technology/Electrical Engineering