HART

What is HART?

HART (Highway Addressable Remote Transducer) is a hybrid communication protocol that superimposes a digital signal on an existing analogue 4-20 mA current loop, so that process instruments transmit their measured value as an analogue signal while exchanging configuration, status and diagnostic information digitally. No extra cabling is needed: the digital data travels over the same pair of wires. This has made HART the most widely used communication protocol for field instruments in process automation, from pressure transmitters and flow meters to control valves. The protocol is backwards compatible: a DCS or PLC that only reads the analogue signal is unaffected by the digital communication.

🕰️ Where does HART come from?

Rosemount developed HART in the mid-1980s (introduced in 1986) for its smart pressure transmitters. In the late 1980s the protocol was opened up to other manufacturers, and in 1993 Rosemount transferred the rights to the independent HART Communication Foundation. In 2015 that foundation merged with the Fieldbus Foundation to form the FieldComm Group, which now manages both HART and Foundation Fieldbus.

Internationally, HART is defined as Communication Profile Family 9 in IEC 61784-1, based on Type 20 of the fieldbus standard IEC 61158. According to ARC Advisory Group, around 32 million field devices used HART at the end of 2010; by 2019 its share had grown to more than 60% of the process instruments installed worldwide.

🔧 How does HART work technically?

Using frequency shift keying (FSK, based on the Bell 202 modem standard), HART superimposes a small alternating current of about ±0.5 mA on the DC current of the loop. A frequency of 1,200 Hz represents a logical 1, and 2,200 Hz a logical 0. Because the average of this sine wave is zero, the analogue measured value does not change: the control system still sees a plain 4 to 20 mA.

• Master-slave - the field device (slave) only responds to requests from a master. Two masters are allowed per loop: a primary master (the DCS, the PLC or an asset management system) and a secondary master (usually a handheld communicator).
• Burst mode - optionally, the device repeatedly sends the same response without being asked, for example the process value. This gives about three to four updates per second instead of two to three.
• Point-to-point - the usual arrangement: one instrument per loop, the analogue measured value plus digital extras.
• Multidrop - several instruments on one wire pair, each with its own polling address. The loop current is then fixed at about 4 mA and all values are transmitted digitally. HART 5 supported addresses 1 to 15; from HART 6 onwards, 63 addresses are possible.

Every HART device supports three kinds of commands: universal commands that every device must implement (such as Command 0 to read the unique identifier and Command 3 to read the dynamic variables and loop current), common practice commands for frequently used functions, and device-specific commands defined by the manufacturer. A multivariable transmitter provides up to four dynamic variables: PV, SV, TV and QV (primary to quaternary variable), for example pressure, temperature and calculated flow.

📏 What are typical values for HART?

A common mistake during commissioning is insufficient loop resistance. Without enough resistance, the HART signal does not produce a measurable voltage and the handheld cannot communicate, even though the analogue signal works normally. A HART-compatible analogue input card or a separate 250 Ω resistor solves this.

🏭 What is HART used for in practice?

Asset management software reads the HART data through HART-compatible I/O cards or through separate HART multiplexers connected in parallel to the existing loops. To handle each device type correctly, the software uses device descriptions: DD files in EDDL (IEC 61804-3), DTMs based on FDT (IEC 62453) or FDI packages (IEC 62769). In the chemical and pharmaceutical industries, HART is also an important data source for NAMUR Open Architecture, which unlocks diagnostic information through a second channel alongside the control system.

🔄 How does HART compare with fieldbuses and Ethernet-APL?

HART sits between purely analogue and fully digital communication. True fieldbuses such as PROFIBUS PA and Foundation Fieldbus H1 transmit everything digitally over one cable shared by several devices, at 31.25 kbit/s. Ethernet-APL goes further with 10 Mbit/s over two wires, suitable for hazardous areas.

With HART-IP, the same HART command protocol runs over TCP/IP and UDP, for example between a multiplexer and asset management software, or directly to an Ethernet-APL instrument. Existing knowledge of commands and device descriptions therefore remains usable. For brownfield plants, HART is often the logical intermediate step: the existing wiring stays in place, yet the diagnostic information becomes available.

🔐 How secure is HART?

Classic HART over the current loop has no authentication or encryption. Anyone with physical access to the loop, or who reaches a system that can forward HART commands, can change instrument settings such as measuring range or damping. The risk therefore lies mainly in the systems that bring HART onto the network: multiplexers, asset management servers and engineering workstations.

• Write protection - enable the hardware or software write protection on instruments that are not adjusted regularly, especially in a SIS with SIL requirements.
• Segmentation - place asset management systems and HART multiplexers in a separate zone in line with IEC 62443 and the Purdue Model, separated from the office network through network segmentation.
• Secure HART-IP - since revision 7.7, the specification requires TLS or DTLS (version 1.2 or later), client access levels and audit logs via syslog. Implementations themselves can also be vulnerable: in 2020, CISA reported a critical buffer overflow (CVE-2020-16209) in the HART-IP Developer Kit and hipserver.
• Monitoring - record in an asset inventory which instruments can be reached via HART, and log configuration changes through monitoring.

For organisations in energy, water and chemicals in the Netherlands, this is not optional. The Dutch Cybersecurity Act (Cyberbeveiligingswet, the national implementation of NIS2), in force since 15 August 2026, requires essential and important entities to manage the risks in their OT environment, and the configuration of field instruments is part of that.

❓ Frequently asked questions

What does the abbreviation HART stand for?

HART stands for Highway Addressable Remote Transducer. The name refers to making remote measuring transducers addressable over a shared connection. The protocol is managed by the FieldComm Group.

Can HART be used for real-time control?

The digital HART channel, at 1,200 bit/s and two to four updates per second, is too slow for real-time control. In a normal point-to-point arrangement, control therefore runs over the analogue 4-20 mA signal, and HART is used for configuration, status and diagnostics. Only in multidrop mode is the measured value transmitted digitally, and then for slow processes such as tank levels.

Do I need special cabling for HART?

No, HART works over the ordinary twisted, shielded cable of a 4-20 mA loop. The loop does need enough resistance, at least about 230 Ω and usually 250 Ω, and the I/O cards and any intrinsic safety barriers must pass HART signals.

What is the difference between HART and WirelessHART?

HART communicates over the wired 4-20 mA loop, whereas WirelessHART uses the same command protocol wirelessly through an IEEE 802.15.4 mesh network with mandatory AES-128 encryption. WirelessHART is part of HART 7 and is standardised internationally as IEC 62591. It is mainly used for additional measuring points where cabling is too expensive or impractical.

What is HART-IP?

HART-IP is the variant of HART that runs over Ethernet and IP networks, using TCP and UDP port 5094. HART-IP connects multiplexers, gateways and Ethernet-APL instruments to asset management software. Since HART revision 7.7, TLS or DTLS encryption and audit logging are mandatory for HART-IP devices.

Will Ethernet-APL replace HART?

Ethernet-APL is increasingly chosen for new builds, but HART will remain in use for decades because tens of millions of installed instruments support it. HART-IP over Ethernet-APL also keeps existing knowledge of HART commands and device descriptions usable. In existing plants, loops usually stay on 4-20 mA with HART until the end of their service life.

📌 In summary

HART is the hybrid protocol that carries digital configuration and diagnostic information over an existing 4-20 mA loop, without disturbing the analogue measurement signal.

Used by more than half of all process instruments worldwide, HART is the quiet standard of the process industry and a bridge between traditional industrial automation and data-driven maintenance. If you use the diagnostic information, you get better asset data and fewer failures. If you bring HART onto the network through multiplexers and HART-IP, you must secure that access as part of your OT environment.