HART Technology in Industrial Automation

Introduction

HART (Highway Addressable Remote Transducer) is one of the most influential communication technologies in process automation.

Its core innovation is combining:

• Traditional 4–20 mA analog transmission
• Digital communication
• Device diagnostics
• Remote configuration

on the same pair of wires.

The key value of HART technology is not simply communication itself, but how it extends the functionality of industrial instruments without replacing existing analog infrastructure.

Through HART technology, instruments gained the ability to provide:

• Secondary process variables
• Device health monitoring
• Calibration information
• Diagnostic data
• Remote parameter configuration

while still maintaining compatibility with traditional analog control systems.

History and Development of HART

HART protocol was born from the economic need to combine digital communication with existing:

4–20 mA

current loop systems.

The protocol was originally developed by:

Emerson Electric Co.

during the 1980s.

At that time, industrial plants already had massive installed bases of analog instrumentation. Replacing all wiring and control systems with fully digital networks would have been extremely expensive.

HART solved this problem by allowing digital communication to coexist with analog signaling on the same cable.

Later, the protocol became an open standard, and ownership was transferred to:

FieldComm Group

in 2015.

FieldComm Group was formed through the merger of:

• HART Communication Foundation
• Fieldbus Foundation

Its mission is to promote standardized digital integration for industrial field devices and process automation systems.

Today, HART remains one of the most widely deployed communication protocols in industries such as:

• Oil & gas
• Chemical processing
• Water treatment
• Power generation
• Food & beverage
• Pharmaceutical manufacturing

Core Technical Principle of HART

The fundamental innovation of HART is:

Simultaneous analog and digital communication over the same wire.

Traditional analog loops only transmit one process variable.

For example:

• Pressure
• Flow
• Temperature
• Level

However, HART overlays digital communication on top of the analog current signal.

As described by Ortmans (1996):

“The HART protocol has the possibility to communicate analog and digital.”

The protocol includes specifications for:

• Physical transmission
• Message structure
• Data formatting
• Transaction procedures
• Communication commands

This hybrid communication architecture became the foundation of modern smart instrumentation.

HART FSK Communication Technology

HART uses:

FSK

(Frequency-Shift Keying) communication technology.

The digital signal is superimposed onto the existing:

4–20 mA

analog current loop.

Two frequencies are used to represent binary values:

1200 Hz=1

and

2200 Hz=0

The communication speed is:

1200 bps

The slave device can transmit information to the master, and the master can also send commands back to the device.

This transmission method is based on the Bell 202 telephone communication standard.

One of the most important engineering characteristics is:

• The average value of the FSK signal is zero

Therefore, the digital communication does not interfere with the analog current signal.

This allows:

• Analog process control
• Digital diagnostics

to operate simultaneously.

How HART Enhances Instrument Functions

HART transformed traditional field instruments into smart devices.

Instead of transmitting only a single process variable, instruments could now provide:

• Secondary variables
• Device diagnostics
• Sensor status
• Calibration data
• Maintenance information

For example:

A pressure transmitter could transmit:

• Primary pressure value
• Sensor temperature
• Electronics status
• Device alarms
• Calibration condition

through the same communication loop.

HART communication is widely used to access:

• Secondary variables
• Diagnostics
• Device status

In some industrial systems:

The primary value is read using both analog and digital methods to cross-check analog signal health and verify digital-to-analog conversion performance.

This greatly improves:

• Reliability
• Maintenance efficiency
• Process safety
• Predictive maintenance capability

Multidrop Communication

HART supports:

• Point-to-point mode
• Multidrop mode

In multidrop mode:

• Multiple HART devices share one communication line
• Each device has a unique address
• Communication becomes primarily digital

This reduces:

• Wiring complexity
• Installation cost
• Cable infrastructure

especially in large industrial installations.

HART Command System

HART communication is built around a command-based architecture.

The HART command byte contains an integer range from:

0∼253

Commands above 255 use:

• Extended command structures

using command:

31 (0x1F)

to indicate extended command operation.

HART commands are divided into five categories:

• Universal commands
• Common-practice commands
• Non-public commands
• Device-specific commands
• WirelessHART commands

Universal Commands

Universal commands are supported by all HART-compliant devices.

These commands provide standard device functions such as:

• Reading primary variables
• Reading device information
• Accessing diagnostics
• Device identification

A commonly used command is:

#0

which is often used at the beginning of communication to determine required preamble bytes.

Another important command is:

#48

which reads additional device status information.

The command architecture helped standardize communication between:

• Controllers
• Asset management systems
• Field devices

from different manufacturers.

Alternative HART Physical Layer Technologies

Although FSK is the most common HART implementation, alternative physical layers also exist.

RS-485 HART

HART can operate using:

RS-485

instead of analog current loops.

This abandons analog transmission but enables faster communication speeds:

19.2∼38.4 Kbps

making it comparable to fully digital protocols such as:

• Foundation Fieldbus
• PROFIBUS PA

C8PSK HART

Another alternative is:

$C8PSK$

(Coherent 8-Way Phase Shift Keying)

which enables speeds up to:

9.6 Kbps

supporting approximately:

10 transactions/second

If communication quality degrades, devices can automatically revert to standard FSK communication.

Industrial Problems Solved by HART

HART solved several major industrial automation problems.

Preserving Existing Infrastructure

Industrial plants could upgrade to digital instrumentation without replacing:

• Existing cables
• PLC analog cards
• Analog control systems
• Installed instruments

This dramatically reduced modernization costs.

Adding Diagnostics to Analog Systems

Traditional 4–20 mA systems provided almost no diagnostics.

HART enabled:

• Device health monitoring
• Remote diagnostics
• Calibration management
• Predictive maintenance

without changing analog process control architectures.

Improved Asset Management

HART enabled centralized monitoring of:

• Instrument condition
• Sensor drift
• Calibration status
• Maintenance history

This improved operational reliability and reduced downtime.

HART in Industry 4.0

Modern HART technology continues evolving alongside Industry 4.0 systems.

Today, HART-enabled instruments can integrate with:

• Ethernet gateways
• Asset management systems
• WirelessHART networks
• Cloud monitoring platforms
• IIoT architectures
• Predictive maintenance systems

Modern HART devices now support:

• Remote diagnostics
• Real-time monitoring
• Digital twins
• Predictive analytics

while still maintaining compatibility with traditional:

4–20 mA

infrastructure.

This backward compatibility remains one of HART’s strongest industrial advantages.

Future Development Trends of HART

Future HART development will likely focus on:

• Faster digital communication
• Wireless connectivity
• Improved cybersecurity
• Cloud integration
• Advanced diagnostics
• Integration with Ethernet-APL and OPC UA

Although Ethernet-based industrial communication is rapidly expanding, HART will likely remain important because industrial facilities continue operating enormous installed bases of analog instrumentation.

Rather than disappearing, HART increasingly serves as a bridge between:

• Traditional instrumentation
• Modern digital industrial architectures

Conclusion

HART fundamentally changed industrial instrumentation by enabling:

• Analog and digital communication on the same wire
• Smart diagnostics over analog loops
• Remote configuration and calibration
• Improved asset management
• Cost-effective digital modernization

Its hybrid communication architecture allowed industries to modernize existing analog systems without abandoning installed infrastructure.

Even in the Industry 4.0 era, HART continues to play a critical role in connecting traditional field instrumentation with modern digital automation systems.