Instrumentation · Communication Protocols · HART
HART Protocol: How It Works and How to Use a HART Communicator
A complete beginner-to-intermediate guide to HART: what it is, how FSK works, point-to-point vs multidrop, what the four process variables are, and a step-by-step guide to using a HART field communicator in the plant.
Walk into any modern process plant and you will find HART instruments on almost every control loop. Pressure transmitters, temperature transmitters, level instruments, flow meters, control valve positioners. The vast majority of smart field devices installed in oil and gas, chemical, water and power plants communicate using the HART protocol.
Yet many instrumentation technicians and engineers use HART every day without fully understanding how it actually works. They know how to connect a HART communicator, but they do not always understand why it needs a 250 ohm resistor, what FSK means, or what data is actually flowing through that 4-20 mA loop alongside the process variable.
This guide explains HART from the beginning: what it is, the clever physics behind how it works, what information it carries, and exactly how to use a HART communicator in the field to read diagnostics, configure transmitters and perform calibration. By the end you will have a complete working understanding of HART that goes well beyond just knowing how to press buttons on a communicator.
What Is HART Protocol?
HART stands for Highway Addressable Remote Transducer. It is a digital communication protocol that allows two-way communication between smart field instruments and host systems (DCS, PLC, handheld communicator) over the same two-wire cable that carries the standard 4-20 mA analogue signal.
In simple terms, HART allows a smart transmitter to do two things at the same time over the same pair of wires:
- Send the primary process measurement as a standard 4-20 mA analogue signal (which the DCS reads in the normal way)
- Simultaneously communicate digital information: secondary process variables, device status, diagnostics, configuration settings and more, to a HART communicator or HART-capable host system
This is the core genius of HART. It was designed to give you the benefits of digital communication without requiring you to change your existing 4-20 mA wiring or your existing DCS input cards. The plant's control system continues to read the 4-20 mA signal exactly as before. The HART digital signal rides on top of it invisibly.
A brief history
HART was originally developed by Rosemount Inc. in the late 1980s, based on the Bell 202 telephone modem standard, the same technology that brought caller ID to landline telephones. The protocol was made open and vendor-independent in 1993 when the HART Communication Foundation was established. Today it is governed by the FieldComm Group and is installed in more than 40 million devices worldwide, making it the most widely deployed digital field communication protocol in the process industries.
Watch: HART Protocol Explained (Video)
Before diving deeper into the technical detail, this short video from RealPars gives an excellent visual explanation of how HART works alongside the 4-20 mA signal:
How HART Works: Frequency Shift Keying (FSK) Explained
The key to understanding HART is understanding how a digital signal can sit on top of an analogue current signal without corrupting it. The answer is a technique called Frequency Shift Keying (FSK).
Here is the physics in plain language:
- The 4-20 mA analogue signal is a DC (direct current) signal. Its value at any moment represents the process measurement.
- The HART digital signal is an AC (alternating current) sine wave signal superimposed on top of the DC loop current.
- The digital signal uses two frequencies: 1,200 Hz represents a binary 1 and 2,200 Hz represents a binary 0.
- Critically, the HART sine wave is symmetrical: the positive half of each cycle exactly cancels the negative half. The net effect on the DC current level is zero. The average of the AC signal adds nothing to the DC value.
- This means the 4-20 mA analogue reading is completely unaffected by the HART digital signal riding on it.
Figure 1: HART uses Frequency Shift Keying (FSK) to superimpose a digital signal on the 4-20 mA loop. Binary 1 = 1,200 Hz and Binary 0 = 2,200 Hz. Because the sine wave averages to zero, the DC analogue reading is completely unaffected.
The HART digital signal communicates at 1,200 baud (bits per second). This allows two or more digital updates per second from a smart field device. It is slow by modern standards, but it is fast enough for field device configuration, diagnostics and secondary variable reading, which change slowly in process applications.
HART Communication Modes: Point-to-Point vs Multidrop
HART instruments can be configured to operate in one of two network modes:
| Feature | Point-to-Point Mode | Multidrop Mode |
|---|---|---|
| How it works | One HART device per loop. 4-20 mA carries the primary variable. Digital HART rides alongside. | Multiple HART devices (up to 15) on a single pair of wires. Each device has a unique poll address (1-15). |
| Analogue output | Active: 4-20 mA represents the process variable | Fixed at 4 mA. All communication is digital only. |
| Poll address | Address 0 (default for all HART devices) | Address 1 to 15 (each device has a unique address) |
| Typical use | The standard mode for almost all HART field instruments in process plants | Remote monitoring, asset management, where analogue control is not required |
| Number of devices per loop | One | Up to 15 |
| Update rate | 2 or more digital updates per second plus continuous 4-20 mA | Slower digital-only updates shared across all devices on the loop |
What Information Does HART Carry?
The HART digital signal carries a rich set of information that goes far beyond the single process variable available from the 4-20 mA signal alone. Understanding what is available helps you get far more value from your HART instruments.
The four process variables
Every HART-capable instrument can transmit up to four process variables digitally. These are labelled PV, SV, TV and QV. For a multivariable instrument like a Coriolis flow meter, all four carry different measurements. For a simple pressure transmitter, PV is pressure and the others may be secondary calculations.
| Variable | Full name | Example: Pressure transmitter | Example: Coriolis flow meter |
|---|---|---|---|
| PV | Primary Variable | Process pressure (also the 4-20 mA signal) | Mass flow rate (also the 4-20 mA signal) |
| SV | Secondary Variable | Sensor temperature (internal) | Density |
| TV | Third Variable | Calculated % of range | Fluid temperature |
| QV | Fourth Variable | Loop current (mA value) | Volumetric flow rate |
See our detailed article on process variables in HART transmitters: PV, SV, TV and QV explained for more on this.
Device status and diagnostics
Beyond process variables, HART carries continuous device status and diagnostic information. This is one of the most powerful and underused features of HART. Every time a HART communicator reads a device, it also receives:
- Device status byte: Reports whether the device is operating normally, has an active alarm, is in fixed-current mode, or has a hardware failure
- Additional device status: More detailed flags including loop current saturated, primary variable out of limits, non-primary variable out of limits
- Extended diagnostics (on HART 7 devices): Advanced diagnostics including sensor failure, electronics failure, configuration changed flag, and maintenance required warnings
Configuration data
Through a HART communicator or HART-capable DCS, you can read and write the transmitter's configuration including:
- Tag name and description
- Lower and upper range values (LRV and URV)
- Engineering units
- Damping value
- Transfer function (linear or square root)
- Sensor type and configuration (for temperature transmitters)
- Alarm and saturation limits
How to Connect a HART Communicator
Figure 2: The HART communicator connects in parallel across the loop wires at any convenient point. No loop break is needed. The communicator clips onto the loop at the DCS terminals, the junction box or the transmitter terminals.
- At the DCS/PLC input terminals: Convenient from the control room. Access requires opening the marshalling cabinet.
- At the field junction box: The most practical field location. Loop terminals are accessible without disturbing the transmitter.
- At the transmitter terminals: Direct connection. Useful when the transmitter is in a accessible location and you want to verify configuration at the device itself.
How to Use a HART Communicator: Step-by-Step
The procedure below uses the Emerson 475 Field Communicator as the reference, which is the most widely used HART communicator globally. Other brands (Beamex, Meriam) follow the same general steps. Full product documentation is available on the Emerson 475 Field Communicator product page.
- Connect the communicator to the loop Clip the two HART communicator leads (red and black) across the loop wires in parallel. Polarity does not matter for HART communication. Connect either way around. Then confirm the loop is powered (24V DC) and that 250 ohms or more of resistance is present in the loop. The communicator draws power from its internal battery only.
- Power on and select HART Press the power button on the 475. From the main menu, tap the HART application icon. The communicator will begin searching for a HART device on the loop. This takes 5 to 15 seconds. If it finds a device, it will display the device tag, model and PV reading.
- Read the online menu The communicator connects online and displays the Home screen showing: Tag name, PV (primary variable with engineering units), AO (analogue output in mA) and device status. Navigate using the arrow keys or touchscreen. The typical menu structure is: Home screen → 1 Device Setup → 2 Diagnostics → 3 Review.
- Read process variables and status From the Home screen you can see PV, SV, TV and QV (if available for your device), the 4-20 mA output value, and any active alarm flags. For a pressure transmitter this gives you pressure, sensor temperature and loop current all at once, far more than the DCS sees from the 4-20 mA signal alone.
- Check device configuration Navigate to Device Setup to view the current configuration: tag, range (LRV and URV), units, damping, transfer function (linear or square root) and alarm settings. If any of these are wrong, this is where you correct them. Always review configuration before performing any calibration.
- Perform sensor trim / calibration For calibration, navigate to Device Setup then Calibration (or Sensor Trim depending on the device manufacturer). Follow the on-screen prompts. The communicator will ask you to enter the reference value (actual temperature, actual pressure etc.) and apply the trim. See our full guide on how to calibrate a temperature transmitter and how to calibrate a differential pressure transmitter for detailed procedures.
- Read diagnostics Navigate to Diagnostics or Device Status to view detailed fault flags. Smart transmitters report specific faults: sensor open circuit, electronics failure, configuration changed, primary variable saturated, loop current fixed mode active, and more. Reading diagnostics regularly is one of the most valuable uses of a HART communicator.
- Send (write) any changes to the device Any configuration or calibration changes made through the communicator are held in the communicator's memory until you send them to the device. Always look for a SEND button or prompt after making changes. If you close the communicator without sending, the changes are lost and the device is unchanged.
- Verify and disconnect After any changes, return to the Home screen and verify the PV, mA output and tag are correct. Disconnect the communicator leads. The loop continues operating normally throughout.
The HART Loop Resistor: Why It Is Essential
The 250 ohm minimum loop resistance requirement is one of the most commonly misunderstood aspects of HART. Here is a clear explanation.
The HART FSK signal is a low-level voltage signal (approximately 0.5V peak-to-peak). For the communicator to generate and detect this voltage reliably, it needs a minimum impedance in the loop to work against. This is Ohm's law: V = I × R. With 250 ohms in the loop and a typical HART signal current of about 2 mA peak, the resulting voltage across the resistor is approximately 0.5V, which is enough for reliable HART detection.
Without 250 ohms, the voltage signal effectively collapses because the impedance is too low. The communicator and the transmitter cannot decode each other's signals. Read our detailed article on the HART loop resistor in HART communication for further explanation.
| Situation | Resistor needed? | Notes |
|---|---|---|
| Loop connected to DCS input card | Usually no | Most DCS analogue input cards have 250 ohms built in. Check your DCS input card datasheet to confirm. |
| Loop connected to a simple ammeter (bench calibration) | Yes, add 250 ohm in series | An ammeter has near-zero resistance. You must add an external 250 ohm resistor in series to enable HART communication. |
| Loop with a HART multiplexer | Usually no | HART multiplexers include the required impedance. Check manufacturer specification. |
| Long cable run with high cable resistance | No additional resistor | Cable resistance contributes to loop resistance. At some point, cable resistance alone may exceed 250 ohms. Check total loop resistance. |
| Zener barrier in intrinsically safe loop | Usually no | Most IS barriers include sufficient series resistance for HART. Some older barriers may need a supplementary resistor. |
HART Communication Problems: Troubleshooting Table
| Problem | Likely cause | Fix |
|---|---|---|
| "No Device Found" / communicator cannot connect | Less than 250 ohms in loop. No loop power. Wrong poll address. Device not HART-capable. | Verify 250 ohm minimum resistance. Check 24V power at transmitter. Try polling addresses 0-15 (poll all). Confirm device is HART-enabled (check datasheet). |
| Intermittent HART communication drops | High electrical noise on the loop (from VFDs, contactors, poor cable shielding). Loop resistance too close to 250 ohm minimum. | Check cable shielding and grounding. Increase loop resistance slightly above 250 ohms. Route HART cables away from power cables. Add HART filter at DCS input if required. |
| Communicator connects but device description is unknown | HART Device Description (DD) file for that specific device model is not loaded on the communicator. | Update the communicator DD library. Connect the communicator to a PC and download the latest DD files from the manufacturer. The device will still communicate in generic mode but not all functions will be accessible. |
| Configuration changes written but device returns to old settings | Write protection enabled on device. Changes sent but device rejected them (range outside limits). Soft write protection via HART lock. | Check device write protect switch (physical or software). Verify the configuration values are within the device's specified limits. Unlock write protection through the communicator menu. |
| DCS shows correct reading but HART communicator shows different value | Device is in simulation or test mode. Communicator is reading digital value while DCS is reading analogue. Offset in output trim. | Check whether fixed-current or simulation mode is active on the device. Verify output trim is correct (digital and analogue should match). Exit simulation mode. |
Further Reading and External Resources
- FieldComm Group: HART Technology Explained. The official source from the organisation that owns and governs the HART standard. Authoritative reference on protocol specifications.
- Wikipedia: HART Protocol. Comprehensive technical overview including protocol layers, packet structure and standards references.
- RealPars: What Is HART Protocol?. Clear, well-illustrated beginner explanation of HART from a respected industrial automation education platform.
- Inst Tools: What Is HART Protocol?. Detailed technical reference on HART from one of the most trusted instrumentation knowledge bases.
Frequently Asked Questions: HART Protocol
- Process Variables in HART Transmitters: PV, SV, TV, QV Explained
- HART Loop Resistor: Why It Is Needed in HART Communication
- How to Calibrate a Temperature Transmitter: Step-by-Step Procedure
- How to Calibrate a Differential Pressure Transmitter
- Signals in Instrumentation: AI, AO, DI, DO Explained
- Communication Protocols in PLC: Types Explained
- What Is a Smart Transmitter? Features and Benefits
- NAMUR NE43 Standard: Signal Range and Fault Detection Explained
What we learn today
- HART (Highway Addressable Remote Transducer) is a hybrid analogue and digital protocol that communicates digitally over the same 4-20 mA wires without disturbing the analogue signal.
- HART uses Frequency Shift Keying (FSK): 1,200 Hz = binary 1 and 2,200 Hz = binary 0, superimposed on the DC current loop as a symmetrical AC sine wave that averages to zero.
- A minimum of 250 ohms resistance must be present in the loop for HART communication to work. DCS input cards typically provide this; add an external resistor only when calibrating on a bench with an ammeter.
- HART carries far more than just the process variable: secondary measurements (SV, TV, QV), device status, diagnostics, and full configuration data are all available digitally.
- Point-to-point mode (address 0) is the standard for almost all HART installations. Multidrop mode (up to 15 devices per loop) is used for remote monitoring applications where analogue control is not required.
- The HART communicator connects in parallel across the loop wires without breaking the loop. The 4-20 mA signal continues normally during HART communication.
- Always inform the control room and put loops in manual before using fixed-current mode, sensor trim or simulation functions that freeze the transmitter output.
- HART is installed in over 40 million devices worldwide and remains the most widely deployed digital field communication protocol in the process industry today.
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