# HART (Highway Addressable Remote Transducer)

**URL:** <https://forumautomation.com/t/hart-highway-addressable-remote-transducer/204>\
**Category:** Industrial Automation\
**Tags:** instrumentation, interviewquestions, hart\
**Created:** [December 27, 2014, 2:11am UTC](https://forumautomation.com/t/hart-highway-addressable-remote-transducer/204 "2014-12-27T02:11:06Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![denizen](https://forumautomation.com/letter_avatar_proxy/v4/letter/d/848f3c/32.png) [@denizen](https://forumautomation.com/u/denizen)\
**Post date:** [December 27, 2014, 2:11am UTC](https://forumautomation.com/t/hart-highway-addressable-remote-transducer/204/1 "2014-12-27T02:11:06Z")

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**HART is a communication protocol used for industrial process measurement and control applications.** HART is a hybrid protocol. Analog + Digital communication. **HART Analog Communications** -use industry standard 4-20 mA signal only –only one parameter (Process Variable or Controller Output) –communicated to the host system. **HART Digital Communications** –Added information (Device Status and Diagnostic Alerts;Process Variables and Units;Loop Current and % Range, Basic Configuration Parameters, Manufacturer and Device Tag etc.,) – 35- 40 information items. HART Digital Communications – Low level modulation superimposed on the standard 4-20 mA current loop –BELL 202 MODEM- Frequency Shift Keying (FSK) – Logic 1 – 1200 Hz and Logic 0 –2200 Hz (with an amplitude of –0.5mA to +0.5mA) – Device configuration, non-real-time diagnostics, and status monitoring. Two-way communication Master/Slave Protocol (Message Rate –2) HART burst mode communications (Message Rate- 3) Multivariable Instruments. HART Multidrop Mode. Interoperability . **Benefits of HART Communication** Improved Plant operation Commissioning and Installation Plant operation and improved quality Maintenance Operational Flexibility Instrument Investment Protection Digital Communication Control in Field Devices Hand-Held Communicator

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [September 6, 2024, 5:09pm UTC](https://forumautomation.com/t/hart-highway-addressable-remote-transducer/204/2 "2024-09-06T17:09:57Z")

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Highway Addressable Remote Transducer Communicator is a handheld device used to configure, calibrate, and diagnose HART-enabled field instruments, such as transmitters, valves, and sensors. The HART protocol allows for both analog (4-20 mA) and digital communication over the same wiring, enabling operators to gather device status, perform diagnostics, and configure parameters without needing to interrupt the process.

Working Principle:

The HART protocol uses Frequency Shift Keying (FSK) as its working principle for digital communication. FSK allows digital signals to be superimposed on the standard 4-20 mA current loop, enabling simultaneous analog and digital communication. HART uses Frequency Shift Keying (FSK) modulation, where a 1200 Hz signal represents a logical “1” and a 2200 Hz signal represents a logical “0”. This FSK signal is superimposed on the 4-20 mA current loop without affecting the analog signal.

Why we set mostly HART Range from 4-20 mA and not in volts?

The 4-20 mA current loop is an industry standard because it is less susceptible to electrical noise over long distances, and current signals are independent of wire resistance. The range starts at 4 mA (not 0 mA) to provide a “live zero” signal, allowing the system to detect a zero-current (0 mA) condition as a potential fault.

Calibration in mA vs Volts:

Calibration in milliamps (mA) rather than volts ensures that the measurement is independent voltage drops across long wires, connections, or varying load resistances. Current remains constant in a series loop, making it a more reliable and accurate way to measure and calibrate instruments.

Live Zero Concept:

The “live zero” concept refers to the practice of setting the zero point of an instrument at 4 mA rather than 0 mA. This allows for the detection of faults, such as a broken wire or disconnected device, as these would result in a 0 mA signal. The range of 4-20 mA provides a clear indication of both normal operation (4-20 mA) and fault conditions (0 mA).

How we know Hart properly Connects to Measuring Device:

Check for Communication: The HART communicator should display the device information, such as the tag number, device type, and manufacturer details, once connected.

- Verify Response: Navigate the communicator’s menu and attempt to read or configure a parameter.

Check Status Indicators: Many HART communicators have status indicators or error messages that can alert you if the connection is not established correctly.