HART Protocol.
HART, Highway Addressable Remote Transducer is a digital communications protocol that overlays a 1200-baud frequency-shift-keyed signal on top of the analog 4-20 mA loop current. The 4-20 mA value still represents the primary process variable. HART carries calibration data, secondary variables, device diagnostics, and configuration commands on the same two wires.
What HART Protocol means.
HART was developed by Rosemount in the mid-1980s and transferred to an industry consortium in 1986. The design intent was specific. Provide digital access to smart transmitters without replacing the 4-20 mA analog wiring infrastructure that already existed in thousands of plants. The FSK, frequency-shift-keyed digital signal at 1200 baud sits entirely above the low-frequency range of the 4-20 mA current signal, so both coexist on the same pair without mutual interference. The AI card reads the current as it always did. A HART modem, in a handheld communicator, an AI card, or a multiplexer reads the superimposed digital signal. HART 7, the current revision, supports up to 35 device variables per instrument, a WirelessHART extension for battery-powered devices, and improved security through device identification tokens. A key characteristic of HART is that it does not replace the analog signal. The 4-20 mA primary-variable measurement is still the real-time process value that the controller uses for regulatory control. HART is used for configuration, diagnostics, and secondary-variable access, not for replacing the analog signal path. This is a fundamental distinction from Foundation Fieldbus and Profibus PA, where the digital bus replaces the 4-20 mA entirely. The I/O list implication is that a HART-capable AI card reads one primary variable per channel over 4-20 mA, and that same card can also multiplex HART digital access to the same device for asset management without any change to the wiring or the control scheme.
Why HART won on legacy I/O
HART was the practical compromise between staying on familiar 4-20 mA wiring and getting digital instrument data. The plant didn't need to recable. The existing AI cards still read the analog signal. Smart-instrument tools, Emerson 475, 375 communicators, AMS Device Manager talk HART to the same wires for configuration and diagnostics. Almost every smart transmitter sold since the early 1990s supports HART, even on installations that never use the digital layer.
HART variants
HART 5, 6, and 7 are the dominant on-the-wire revisions, with HART 7 adding wireless support and improved security. WirelessHART runs on 2.4 GHz mesh networks with battery-powered field devices, useful for retrofit monitoring scope where running cable would dominate the project cost. HART-IP runs the protocol over Ethernet for digital-native installations. Most installed-base HART today is wired HART 5, 6, 7 over conventional 4-20 mA.
Primary variable, secondary variables, and device diagnostics
Every HART device exposes a primary variable, PV on the 4-20 mA signal and up to three secondary variables, SV, TV, QV over the digital channel. For a Coriolis meter, PV is mass flow, SV is fluid density, TV is fluid temperature. All four values are available from the same two wires. The digital channel also carries the device status byte, which reports self-diagnostics. Sensor degradation, process noise, simulation mode active, configuration parameters, tag, range, damping, units, and the device's calibration data including the last calibration date and the found-as and left-as readings. This information is what asset-management software like Emerson AMS, Honeywell Field Device Manager, or Yokogawa Plant Resource Manager reads to generate predictive maintenance reports without requiring physical field access.
HART in the I/O list and panel design
The I/O list must flag HART-capable instruments so the panel builder specifies the correct AI card variant. A standard AI card reads the 4-20 mA current only. A HART-capable AI card also passes the FSK digital signal to the DCS or PLC host, enabling asset-management functions from the control room. Some platforms, Emerson DeltaV, Honeywell Experion use dedicated HART interface modules. Others, Siemens S7 with ET 200SP integrate HART into standard AI cards with a firmware enable. For instruments where secondary variables are read in real time, not just via asset management, a HART multiplexer converts the multiplexed HART polling from a single Ethernet port to multiple analog pairs, and the secondary values appear as separate DCS tags.
What HART does not do
HART transmits one primary variable per instrument channel over analog, plus secondary variables digitally at 1200 baud. At 1200 baud, a HART digital poll-response cycle takes roughly 300 ms. This is adequate for diagnostics and configuration but not for fast regulatory control. The controller uses the 4-20 mA analog value for PID loops, not the HART digital readback, because the analog update rate is continuous while the HART digital channel updates once every few seconds depending on how many devices are polled. HART also does not provide a network. Each device has its own pair back to the control room. The wiring saving of Foundation Fieldbus, multiple devices per segment does not apply to HART. For projects where wiring reduction is the priority, Profibus PA, Foundation Fieldbus, or HART over WirelessHART mesh are the alternatives.
Common questions
Is HART a fieldbus.
Does the I/O list need to flag HART vs non-HART instruments.
What is a HART communicator and who uses it.
Does HART work with SIS-classified instruments.
Can HART secondary variables reduce the number of instruments on the I/O list.
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