HART (Highway Addressable Remote Transducer) is a digital communication protocol superimposed on the 4-20 mA analogue loop using FSK modulation, without disturbing the current signal. It carries secondary variables, diagnostics and configuration from field instruments (pressure, temperature, level and flow transmitters) over the wiring already installed.
Where HART shows up on the plant floor
HART turns up wherever process instrumentation is wired in 4-20 mA: chemicals and pharma, food and beverage, paper, cement, ceramics and water treatment. Most smart transmitters sold in recent decades are HART-capable even when the plant only uses the analogue reading, and the instrument technician’s handheld calibrator is itself a portable HART master. The physical layer is based on Bell 202: an FSK carrier at 1200 Hz for a logical one and 2200 Hz for a zero, running at 1200 bit/s with a ±0.5 mA amplitude whose mean is zero, so the analogue loop keeps carrying the primary variable untouched. The standard is maintained by the FieldComm Group, and HART 7 introduced WirelessHART, standardised as IEC 62591 over IEEE 802.15.4 radio at 2.4 GHz with a mesh topology, TDMA and channel hopping.
HART versus plain 4-20 mA and digital fieldbuses
A 4-20 mA loop carries one variable, continuously and essentially instantly: it is the process channel. HART carries everything else (secondary variable, sensor temperature, device status, tag, range, calibration history) slowly and by polling. They do not compete: they share the same pair of wires and do different jobs. The usual confusion is with fully digital fieldbuses (PROFIBUS PA, FOUNDATION Fieldbus), which drop the analogue current, put several instruments on one segment and offer far more bandwidth, but demand a different cabling topology and different I/O cards. HART won on installed base precisely because it forces no change: it reuses the existing point-to-point loop. There is also multidrop mode, where several devices share a loop fixed at 4 mA and all communication becomes digital, at the cost of losing the analogue reading and its speed. The figure usually quoted is up to 15 devices, which matches the 1-15 polling address range of revision 5 and earlier; revision 6 widened that range to 63, though loop current budget and polling time normally set the practical ceiling first.
Getting HART data into a data layer
The usual problem is not that the data is missing but that it never leaves the instrument: only the current reaches the PLC or DCS, and the diagnostics stay inside the transmitter. Getting them out requires something that demodulates the FSK signal. The common routes are HART-capable analogue input cards in the controller itself, a HART multiplexer in the marshalling cabinet that scans the loops and republishes them over RS-485/Modbus or Ethernet, a HART-IP gateway, or, for the wireless variant, the WirelessHART gateway, which typically exposes values over Modbus TCP, HART-IP or OPC-UA.
Three constraints shape the design. First, speed: at 1200 bit/s under master-slave discipline, a loop sustains roughly two to three transactions per second at best, and a multiplexer sweeping dozens of loops in sequence refreshes each device in seconds or minutes. HART is therefore an asset and diagnostics channel, not a real-time process channel. Second, arbitration: the protocol allows two simultaneous masters, typically the control system and the handheld communicator, so a third reader has to occupy one of those slots rather than simply join in. Third, electrical: communication requires a loop resistance between 230 and 1100 ohms (the customary 250 ohms sits in that range); without it there is no digital communication, whatever software you install.
Reading itself requires no line stop and no change to the controller program: HART universal commands are read commands and do not alter the instrument configuration. What does need planning is the physical work, because fitting a multiplexer means working inside the marshalling cabinet of live loops, often tied to safety functions. Write access should also be explicitly restricted: the same channel that reads diagnostics can reconfigure the transmitter range. Once the information is out, the sensible pattern is to normalise it at the edge and publish it northbound once, rather than having every consumer speak HART on its own. That connection engineering is the subject of sensor connectivity.
Related terms
HART sits in the field layer, below the PLC and the SCADA. Its path towards information systems normally runs through Modbus or an industrial gateway that translates it into OPC-UA. The diagnostics it exposes are raw material for predictive maintenance, and WirelessHART is one of the most established industrial IoT technologies in process industries.