HART: digital communication on top of 4–20 mA

Two signals on one pair

HART (Highway Addressable Remote Transducer) keeps the 4–20 mA analog signal for control and adds a digital signal on the same wires: frequency-shift keying with 1200 Hz = 1 and 2200 Hz = 0 (Bell 202), 1200 bit/s. The FSK signal averages to zero, so the analog value is not disturbed.

Loop resistance

The digital signal is a current modulation that must be converted to a voltage: the loop needs a resistance of typically at least 250 Ω between the power supply and the transmitter. Many analog input cards have a lower input resistance; a 250 Ω resistor or a HART-enabled input is then required for a handheld communicator to work.

Masters

Both can be present at the same time.

Data you get

Commands (universal, common-practice, device-specific) read and write: the primary variable and up to three more dynamic variables (e.g. sensor temperature), units, ranges, tag, damping, diagnostics status, loop current. The device-specific part is described in a DD (Device Description) or DTM/FDI package.

Modes

• Point-to-point — normal 4–20 mA + HART, address 0.
• Multidrop — several devices on one pair with fixed 4 mA current each, only digital values (addresses 1–15 in HART 5, up to 63 in later revisions). Rare in modern plants.
• Burst mode — the device sends a selected command response periodically without polling.

HART-IP

HART messages over Ethernet (IANA port 5094) let asset-management systems reach devices through gateways and multiplexers.