HART Protocol: What It Is and How It Is Used

Short answer

HART is a hybrid protocol that superimposes digital communication on the 4-20 mA analog signal over the same wire pair. The process value keeps arriving through the current, while configuration, diagnostics and additional variables travel over the digital layer. It can operate point-to-point or in multidrop.

What HART is

HART (Highway Addressable Remote Transducer) is a hybrid industrial communication protocol: it transmits digital information over the same wire pair that carries the 4 to 20 mA analog signal. It was developed in the 1980s and released as an open protocol in 1986. Its practical value is that an instrument already installed with its analog loop can send more data without changing the wiring: device identification, secondary variables, diagnostic status and configuration parameters.

In OSI model terms, HART defines the application layer (layer 7) and relies, on the current loop, on the Bell 202 standard for the physical layer. To understand the analog part, see 4-20 mA signal: how it works.

How analog and digital coexist

The digital signal is modulated by frequency-shift keying (FSK), per Bell 202: a logic one is represented by 1200 Hz and a zero by 2200 Hz, at 1200 bits per second. That oscillation has a small amplitude relative to the loop, on the order of half a milliamp, and its average value is zero. That is why it does not change the average current an analog receiver sees, and the receiver keeps reading the process value as normal.

The dialog is master-slave: a master, which can be a handheld communicator, a modem connected to a computer or the HART card of a control system, sends a command and the instrument answers. The instrument only speaks when the master asks it, except in burst mode, where it transmits periodically on its own.

Two connection modes

Mode	Instruments per pair	Loop current	Polling address
Point-to-point	One	Varies between 4 and 20 mA, carries the process variable	0
Multidrop	Several	Fixed at a minimum value per instrument (4 mA)	Different for each one

Point-to-point mode is the most common use. The current is still the main signal and digital communication is used to configure and diagnose. The polling address is 0.

Multidrop mode: several instruments share the pair and each one’s current is fixed at 4 mA, so it no longer carries the process variable; all the data travels over the digital layer. In recent protocol versions, polling addresses run from 0 to 63; in earlier versions they went up to 15. Multidrop saves wiring when there are many points with slow variables, such as tank levels or temperatures, but each query takes time, so it is not suitable for fast control loops.

Illustrative example. Five temperature transmitters in a tank farm are connected in multidrop on a single pair. Each one sits at 4 mA, so the total loop current is 20 mA and does not change as temperatures vary. The system reads each instrument by its address, one after another, at 1200 bits per second; if each command and response exchange takes a few tenths of a second, the complete reading of the five takes one or two seconds. That is enough for tank temperatures; for a flow loop it would be too slow.

What you can do with HART

The most used functions are:

Remote configuration. Change the range, the unit, the damping or the tag of a transmitter from the control room or with a communicator, without opening the instrument housing.
Diagnostics. Read the internal status of the instrument: sensor failure, saturation, deviation from the range, configuration alerts.
Additional variables. A transmitter can deliver other secondary variables besides the primary one; for example, the internal temperature or the pressure that accompanies a flow.
Loop verification. Put the instrument in fixed-current mode to check the response of the whole loop up to the control system.
Instrument inventory. Read the identification, manufacturer, serial number and revision of each instrument, useful in plant audits.
Device description. The instrument comes with a file that describes its commands and parameters, and allows third-party tools to be used with devices from different manufacturers.

This set is the reason many projects buy HART instruments even though the control system only reads the analog signal: the digital capability stays available for maintenance.

What HART requires of the loop

Minimum loop resistance. For the modem or communicator to “see” the signal, the loop must have a minimum total resistance, stated in the instrument manual, which is normally met by the analog card input resistor.
Maximum resistance. The voltage budget of two-wire and four-wire transmitters still applies; the analog card must not introduce capacitance that attenuates the signal.
Barrier and isolator compatibility. Intrinsic safety barriers and signal isolators must let the HART signal through; some models block it.
Cable length and type. The digital signal tolerates typical instrumentation cables, but cable capacitance limits the length on very long loops.

HART and its relationship with other protocols

HART coexists with the other industrial communication media. Many controllers and asset management systems read the HART digital layer and convert it to Modbus or industrial Ethernet. To compare with buses that already have the full digital layer, see Modbus RTU and TCP. There is also a wireless variant, WirelessHART, which uses the same application commands over a mesh network and is applied where running cable is expensive.

Data security

A HART instrument that can be configured remotely is also an instrument that someone can change remotely. A good practice is to limit physical and logical access to HART modems and multiplexers, restrict write permissions and log configuration changes. The general principles of segmentation and access control are covered in cybersecurity for substations and plants.

Protocol revisions and compatibility

HART has evolved through revisions, and each one adds commands and capabilities, such as extended addressing and longer device identifiers. A new instrument is usually backward compatible with an old system for the basics, but some diagnostic functions only appear if the control system or the asset manager recognizes the instrument revision and has its description file. Before buying or replacing a device, confirm the HART revision handled by the controller input card and whether the plant asset manager has the file for the model.

Another point that is easy to overlook is the difference between what the instrument knows and what the control system shows. A transmitter can report a configuration alert or a sensor warning over the digital layer for months without anyone seeing it, because the card only reads the current. Enabling the device status reading and bringing a maintenance alarm into the system is a low-cost improvement that uses a capability already paid for.

Common mistakes

Trying to communicate on a loop that lacks the minimum resistance, or that has a barrier that blocks HART.
Connecting the modem at the wrong place in the loop, for example across a low-impedance supply that shorts the digital signal.
Leaving the instrument in fixed-current mode after a test, so the system keeps reading a frozen value.
Setting up multidrop with duplicate addresses.
Changing instrument parameters without recording it, so the instrument range no longer matches the control system.
Assuming every HART instrument has the same set of functions: it depends on the manufacturer and the revision.
Using multidrop for fast-changing variables.
Not archiving the device description files for each model.

Checklist

Protocol revision and device description files available.
Total loop resistance within the minimum and maximum range.
Barriers and isolators compatible with HART.
In multidrop: unique addresses, fixed current verified and acceptable polling time.
Instrument range equal to the one recorded in the control system.
Write access restricted and changes logged.
Instrument returned to normal operation after each test.
Inventory of tags, serial numbers and revisions up to date.

How we help

The HART digital layer is little used when nobody has the instrument file or knows which loop has enough resistance. In design, our engineering design and studies service defines which instruments carry communication and how it integrates with the control system, remotely and worldwide. In the field, testing and commissioning verifies the loops, the configured range and the communication at each point before the plant is handed over, in Colombia or elsewhere on request.

References

Wikipedia, “Highway Addressable Remote Transducer Protocol” and “Bell 202 modem” (CC BY-SA 4.0), accessed 2026-11-05.
IEC 61784, fieldbus communication profiles (cited by title).
FieldComm Group, the organization that maintains the HART specification.
Kuphaldt, T. R., Lessons in Industrial Instrumentation, chapter on digital data acquisition and networks (CC BY 4.0).

Frequently asked questions

Does HART interfere with the 4-20 mA analog signal?

Not appreciably. The digital signal is a small oscillation with zero average that rides on the current. An analog receiver with a filter ignores it and keeps reading the process value, while a HART master reads the digital part.

What do I need to communicate with a HART transmitter?

A HART modem or communicator connected to the loop, with enough series resistance for the digital signal to be seen, and the right software or communicator for the instrument. The PLC or control system can also do it if its analog card supports HART.

What is multidrop mode?

It is an arrangement in which several HART instruments share the same wire pair. Each one gets a unique address and the current of each loop is fixed at a minimum value, so process information travels only over the digital part and not over the current.

Is HART the same as WirelessHART?

No. Wired HART superimposes the digital signal on the 4-20 mA loop. WirelessHART is a wireless variant of the same application command set, using a mesh network instead of wires. Each one needs its own infrastructure.