What Is the HART Protocol

The Working Principle of HART Communication

The HART communication protocol uses the Bell 202 FSK (Frequency Shift Keying) standard to superimpose digital signals that are represented by two different frequencies, such as 1,200 Hz and 2,200 Hz. where the 1,200 Hz frequency represents bit 1, and the 2,200 Hz frequency corresponds to bit 0. When sinusoidal with these frequencies are placed on a DC analog signal cable, data transmission occurs. data transmission occurs. Therefore, the 4-20mA signal is not affected during the data transfer because the standard value of the frequency shift keying signal is equal to zero. The protocol supports two communication channels at the same time, e.g. a 4-20mA output analog signal and a digital signal.

The analog signal communicates the primary measurement value via the 4-20mA current loop, while additional device data is communicated via a digital signal overlaid on the analog signal. The digital signal includes device information such as device status, diagnostics, calculated values, etc. Together, the two communication channels therefore provide a very powerful and low-cost communication solution that is simple to use and configure. The protocol is often referred to as a hybrid protocol because it combines analog and digital communications.

HART technology operates on a master/slave protocol, meaning that the slave devices can only function when connected to a master device. Note that the slave device is a smart device and the master device is a computer.

What Is the HART Protocol

The HART Protocol Modes

Typically, for communication within the HART protocol, the device used in the network is a PLC or DCS (Distributed Control System), which is selected as the master, while other field devices are considered as slaves, such as sensors or actuators. However, the communication between master and slave here mainly depends on the communication mode arranged by the system. the HART protocol network communicates in two modes: master/slave mode and burst mode.

  1. Master/Slave Mode

This mode is also known as request-response mode. In this mode, once the master device sends a request, the slave device simply transmits the data. Each HART loop can accommodate two master stations. Consequently, primary master stations are typically DCS, PC (personal computer), or PLC (programmable logic controller), while secondary master stations are another PC or handheld terminal.Slave devices are actuators, controllers and transmitters that respond to commands from the master.

  1. Burst mode

Some devices that support the HART protocol only support this communication mode. This mode allows faster communication, such as updating data three to four times per second. The master device in this mode instructs the slave devices to send typical HART reply messages continuously. The master device acquires the message at a high rate of speed until it commands the slave device to stop bursting. This mode is suitable for the situation described above where one HART device needs to communicate from a HART loop.

The Difference Between HART Protocol and Mod bus

The differences between the HART protocol and Mod bus include the following.

HART Protocol Mod bus Protocol
It is a hybrid protocol It is a data communication protocol
HART is widely used in process and instrumentation systems from small automation to highly complex industrial applications. Mod bus is used to transfer signals from instrumentation and control devices to a data acquisition system or a host controller.
The protocol operates in two modes of operation such as point-to-point and multi point Mod bus is used in two transmission modes, such as ASCll mode or RTU mode.

The Advantages and Drawbacks of the HART Protocol

Advantages of the HART protocol include the following.

  • Devices enabled by the HART protocol allow the user to optimize their operational capabilities by enabling the device to use only the best data.
  • Reduces downtime due to equipment failure by recognizing potential problems before they occur.
  • It reduces equipment inventory costs and maintenance.
  • It reduces wait times for problem identification and problem resolution.
  • ItImprove safety integrity levels by using advanced diagnostics.
  • The benefits of choosing the HART protocol include: digital capability, analog capability, availability and interoperability.
  • It can be used for different devices and sensors.
  • Devices based on the HART protocol are widely accepted in the industry.
  • The protocol improves system availability, progress regularity, etc.

Drawbacks of the HART protocol include the following content elements.

  • Digital signals in HART transmission are quite slow.
  • For multipoint arrangements, analog signals are inaccessible and unavailable, and the number of devices that can split the transmission line is limited.
  • Only one process variable can be monitored at any one time.
  • Compared to other fieldbus systems such as Profibus and Foundation Fieldbus, this type of protocol is somewhat slow. As a result, this slow response time can cause some difficulties in some industrial based applications.
  • In most cases, the speed of the HART protocol is sufficient to meet the requirements of simple monitoring systems where process variables change slowly.

The Applications of HART Protocol

  • It is used worldwide to transmit and receive digital data over analog wiring between intelligent devices and control systems.
  • It is a very popular protocol used in industrial automation.
  • This protocol is mainly used to analyze smart devices.
  • It is widely used in process and instrumentation systems.
  • It is very suitable for multi variable instruments, including mass flow meters, as long as volumetric flow, mass flow, density and temperature can be transmitted to the control system over a single cable.
  • It is primarily designed for industrial process control and measurement applications.
  • It is mainly used in the process industry for communication between different devices.

There are Various types of instrumentation of BCST Group support the HART protocol, It not only retain the traditional analog signal of 4-20 mA, but also communicate with the field intelligent instrument in two-way digital communication. Which provides the best solution for on-site smart meter communication. If you are interested in our products, please log on to our web site (https://www.bcstgroup.com), and welcome to come and consult with us, we are willing to help you out.

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