HART (Highway Addressable Remote Transducer) is the most widely used digital protocol in process instrumentation. Over 80 million HART-enabled devices are installed worldwide. Despite being introduced in 1986, HART remains the dominant protocol for smart field instrument communication in process plants.

HART PROTOCOL AT A GLANCE

Dual-Signal Architecture

4-20
mA Analog Signal

1200
Hz Mark Frequency

2200
Hz Space Frequency

80M+
Installed Devices

Variable Abbreviation Carried Via
Primary Variable PV 4-20mA + Digital
Second Variable SV Digital only
Third Variable TV Digital only
Fourth Variable QV Digital only

How HART Works

HART uses Frequency Shift Keying (FSK) to superimpose a digital signal on the conventional 4-20mA analog signal. The digital signal uses two frequencies: 1200 Hz for a binary 1 (mark) and 2200 Hz for a binary 0 (space). Because the FSK signal is zero-mean, it does not affect the DC 4-20mA process variable reading.

This clever design means: the 4-20mA analog signal continues to carry the primary process variable to the DCS analog input card, while simultaneously, a handheld communicator or asset management system communicates digital information with the device over the same two wires.

HART Command Types

  • Universal Commands (0-30): Supported by all HART devices. Read device identity, primary variable, PV range, engineering units.
  • Common Practice Commands (32-127): Common functions not implemented by all devices. Output current trim, PV loop test, self-test.
  • Device-Specific Commands (128-255): Manufacturer-specific functions. Access proprietary features unique to that device.

HART Variables

Every HART device can transmit up to four variables:

  • PV (Primary Variable): The main measurement. Always transmitted on the 4-20mA loop.
  • SV (Second Variable): A secondary measurement. Temperature in a DP flow transmitter, for example.
  • TV (Third Variable) and QV (Fourth Variable): Additional device parameters accessible only digitally.

HART Multidrop Mode

Up to 15 HART devices can share a single pair of wires in multidrop mode. In multidrop, all devices set their output current to 4mA fixed, and all communication is digital. This eliminates the analog measurement and is used in some remote monitoring applications where wiring is expensive.

HART in DCS and Asset Management

DCS analog input cards with HART capability can extract the digital HART variables while receiving the 4-20mA analog signal. Asset management systems (Emerson AMS, Honeywell FSC) use HART to monitor device health, calibration status, and diagnostics – predictive maintenance data without sending a technician to the field.