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
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.