HART Protocol: Digital Communication Superimposed on 4-20mA

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.

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.

HART Communication Modes

HART operates in two communication modes that are rarely fully understood even by experienced instrumentation engineers:

• Point-to-point mode: The standard installation. One device on a 4-20 mA loop, addressed at device address 0. The 4-20 mA carries the primary variable (PV); HART digital communication carries all additional variables, diagnostics, and configuration. The master polls the device for digital data while the 4-20 mA runs simultaneously. This is the mode used in 90%+ of installed HART devices.
• Multi-drop mode: Multiple HART devices on a single pair of wires, each with a unique address (1-15). In multi-drop mode, the 4-20 mA is fixed at 4 mA (the minimum) and serves no measurement function — all data is transmitted digitally. Multi-drop reduces wiring cost for non-critical monitoring points but requires a HART multiplexer or HART-capable host interface card to communicate with multiple devices. It is used for tank gauging, environmental monitoring, and other applications where many devices must be connected economically.

HART Variables: Beyond the Primary Variable

This is where HART’s value is often underutilised. Every HART device can report up to four process variables digitally: PV (Primary Variable), SV (Second Variable), TV (Third Variable), and QV (Fourth Variable). What these variables represent is device-specific:

• A Coriolis mass flow meter might report: PV = mass flow rate, SV = temperature, TV = density, QV = volume flow rate — four process variables from one instrument
• A smart pressure transmitter might report: PV = pressure, SV = sensor temperature, TV = percent of range, QV = loop current
• A valve positioner reports: PV = valve position, SV = setpoint, TV = drive signal, QV = upper/lower limit switch status

Most SCADA and DCS installations that read HART devices only capture the 4-20 mA PV. The digital variables are ignored. If your plant uses HART multiplexers or a HART-capable DCS interface (Emerson DeltaV with CHARM I/O, ABB 800xA with IDM), capturing the additional HART variables unlocks significant operational value — process insights that previously required additional instruments.

HART Device Diagnostics

Modern HART devices provide extensive diagnostic information through HART Device Status and Extended Device Status bytes, and through HART Device Description (DD) file-defined diagnostic variables. Common diagnostics include:

• Sensor degradation (sensor resistance out of range for RTDs, electrode coating for pH sensors)
• Electronics failure (amplifier fault, A/D converter error)
• Process condition alerts (plugged impulse line for differential pressure transmitters, empty pipe detection for magnetic flow meters)
• Configuration alerts (range change, damping value not optimal)

Accessing these diagnostics historically required a HART Communicator (handheld device) connected directly to the loop in the field. Modern DCS platforms with integrated asset management (Emerson AMS, Honeywell FSM, ABB IDM) can access HART diagnostics from the control room without any field visit, which dramatically reduces the cost of routine instrument health checks.

HART and Wireless: WirelessHART

WirelessHART (IEC 62591) extends the HART protocol to a wireless mesh network. WirelessHART adapters can be attached to existing wired HART devices to add wireless reporting of HART variables without replacing the device. This is particularly valuable for adding secondary HART variables to existing transmitters that are not connected to a HART multiplexer — the wireless adapter reports all HART variables to the WirelessHART gateway, which then makes them available to the DCS or asset management system.

The HART protocol data model is preserved in WirelessHART — device addresses, DD files, variable mapping, and diagnostics all work identically to wired HART. Engineers familiar with wired HART can work with WirelessHART devices without learning a fundamentally different protocol.

HART Calibration Workflow

HART simplifies the calibration workflow for smart transmitters. In traditional 4-20 mA calibration, a technician connects a calibrator to the loop, applies a known input (pressure, temperature), and trims the output mA signal using the transmitter’s local pushbuttons or with a handheld communicator. With HART, the full calibration procedure can be performed digitally:

1. Connect a HART communicator or laptop with HART modem to the loop (or use remote access via a HART multiplexer)
2. Send a HART command to put the device in calibration mode
3. Apply the reference input at the low point, trim the LRV (Lower Range Value) digitally
4. Apply the reference input at the high point, trim the URV (Upper Range Value) digitally
5. Verify the output against the reference and document the calibration record

The entire procedure can be documented automatically when using calibration management software (Beamex CMX, Fluke MET/CAL) that interfaces with HART devices. Calibration records are generated automatically, reducing documentation error and audit preparation time.