What is HART and HART Multiplexer?


What is HART?

HART stands for highway addressable remote transducer. HART is a communication protocol for the process instrumentation. It is the global communication standard in process instrumentation. A transducer is a device that is capable to convert the physical variable into a similar electrical signal. HART communication protocol is the most used digital communication protocol in the world. HART is utilized with millions of field devices for effective communication. Most industrial plants around the world are utilizing HART instrumentation for several years. The major purpose of utilizing the HART technology in an industrial plant is to increase productivity. Maintenance costs can be reduced and also downtime by utilizing the HART communication. The highly addressable remote transducer or HART is an open protocol that can easily communicate with the smart field devices.


HART is capable to transmit and receive digital signals through Analog wires. Thus it would facilitate the communication between the field devices and the control system such as DCS, or PLC. HART is capable to do hybrid communication, so we can monitor or control the device remotely. Due to the digital networking feature of the HART, we can access the field device or the controller from anywhere in that network. It utilizes the conventional 4-20mA network so that it can be easily used with the existing system as well as in the current system.

What is a HART device?

A HART device can be described as a field device or smart device. A HART device is a device that communicates by utilizing the HART communication protocol. HART devices are better than the Fieldbus devices. Almost all the smart instruments utilize HART communication.

HART devices are known as smart devices, here the smart refers to signal modulation and rearranging transducer. Smart devices are intelligent transducers that are composed with integrated electronics. This smart device would also have a transduction element and it is also capable to do signal conditioning.

Major functions of the Smart device:

•          It is capable to perform the logic functions

•          It can do two-way communication

•          It would be able to take decisions

How does HART Communicates or how HART works?

HART communication would take place between two HART-enabled devices. As we discussed earlier this communication would take place between a field device and the control system. Communication would take place through the instrumentation grade wire. The HART utilizes the Bell 202 frequency shift keying to overlaying the digital signal on the conventional 4-20mA signal. The digital information would be encoded on top of the 4-20mA Analog signal. There would be two simultaneous channels for the communication the 4-20mA Analog signal and the digital signal.

The 4-20mA Analog signal current loop would be utilized to do the primary value communication. Digital signals would be utilized to do any kind of additional data communication. The digital signal would contain several information such as device status, calculated value diagnostics, etc.


HART is a master-slave communication protocol the communication of the slave device would be initiated by the master device. The slave will be a field device in the process industry like a transmitter, actuator, etc, and the master would be a control system. The control system could be a DCS or PLC. In this, two masters can be connected to each HART loop.

Burst mode:

There is another mode of communication which is the burst mode in this the slave would continuously send messages to the master till the master transmits a signal to stop the bursting. This is the fastest way of HART communication, in this, the master would command the slave to transmit continuously till the master instructs it to stop.

What are the different types of HART network protocols?

Point to point:

In this type, the 4-20mA signal is utilized to do the communication of one process variable. The additional process variable and other data would be transmitted digitally. This type of communication would be carried out between two devices.


In this type, a single pair of wires would be utilized to do the operation and communication takes place digitally. 

What is HART Multiplexer?

A HART multiplexer can be described as a communication path. It would allow or route the transmission between the field device and the control system. It would store all the information regarding the HART devices in the network. The HART multiplexer would provide online access from a control system. The control system would be able to configure, calibrate, maintain, and diagnose the field device. Multiplexer supports both point-to-point and multi-drop communication.


What are the applications of HART Multiplexer?

•          Automatic addressing

•          It would provide an alarm if there is any fault

•          Autobaud rate detection

•          Field device can be checked with the help of a multiplexer

•          HART communications can be easily carried out through the Ethernet

•          Field instruments can be easily inspected from anywhere in the plant or it can also be done remotely with the help of the internet

What are the factors that must be considered while selecting a HART multiplexer?

•          It should be selected based on the number of HART channels that it supports

•          Should have a burst mode

•          Operating temperature

•          Should support Multidrop

•          Type of communication with the control system

•          Should consider the number of the HART channel that shares a HART modem

What are the different types of HART Communicators available in the market?

•          Emerson AMX Trex

•          Druck DPI 620

•          Yokogawa YHC5150X

•          E+H Field expert SMT70

•          ProComSol PC HART

What are the advantages of using the HART device?

By utilizing a HART we can determine the basic HART variables such as percent travel, set point, temperature, and supply pressure. These data can be utilized for feedback within the system.

•          Accuracy and stability

•          Reliability

•          Multivariable

•          Commissioning can be easily done

•          It is supported by almost all the industrial devices and suppliers

•          Remote configuration

•          Operational parameters can be easily adjustable

•          Digital communication

•          Operational flexibility

•          Less maintenance

•          Downtime can be reduced

•          Measurement quality can be improved

•          It can be used in a harsh and noisy environment

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