P-08 — HART

Smart digital data riding a 4-20 mA loop.

Estimated 13 minutes.

The brilliant compromise

In the late 1980s, plants had millions of 4-20 mA loops in the ground. Replacing them with a fieldbus was unthinkable. HART solved this by adding a tiny digital signal ON TOP of the analog current — same wires, same DCS card, but now you can talk to the device.

Highway Addressable Remote Transducer

That is what HART stands for. "Highway" = the existing 4-20 mA loop. "Addressable" = each device has a poll address. "Remote Transducer" = the smart sensor in the field. The name dates from 1986 — it is the most widely deployed industrial protocol on Earth, with over 40 million devices in service.

How it actually works

HART superimposes a low-amplitude (±0.5 mA) FSK signal on the 4-20 mA loop. Bell 202 modulation: 1200 Hz = "1", 2200 Hz = "0". The signal is AC and averages to zero, so the DCS still reads the analog PV exactly as before. A HART master just AC-couples and listens.

Master-slave, half-duplex

A loop has at most two masters: the primary (usually the DCS or asset-management host) and a secondary (a handheld communicator clipped onto the loop for maintenance). Only one talks at a time. The slave (the field device) only answers when polled — never volunteers.

How fast?

A HART transaction (request + response) takes roughly how long?

  • A few microseconds
  • About 500 ms
  • Several seconds

See the signals coexist

Move the loop current and toggle the FSK overlay. Notice how the digital wiggle never shifts the DC level the analog DCS reads.

Why FSK and not amplitude?

Why does HART use frequency-shift keying (1200/2200 Hz) instead of just toggling the current up and down?

  • FSK is faster than amplitude modulation
  • FSK averages to zero current — the analog PV reading is undisturbed
  • Amplitude modulation is patented
  • FSK uses less power

The DCS does not even notice

A traditional DCS analog input has a low-pass filter on the input — it only cares about the slow DC level. The 1200/2200 Hz wiggle is filtered out as noise. So a HART transmitter is a drop-in replacement for any 4-20 mA transmitter, even on a controller built decades before HART existed.

Four variables, one device

A modern HART transmitter exposes up to four dynamic variables: PV, SV, TV, QV. The PV is mapped to the 4-20 mA loop. The other three are digital-only — free upgrades a smart device gives you over a dumb 4-20 mA sensor.

Inspect PV / SV / TV / QV

Click each variable to see what it represents and how it travels.

Where does the SV go?

A pressure transmitter exposes pressure as PV and sensor temperature as SV. The DCS analog card sees:

  • Both pressure and temperature interleaved
  • Only the pressure (PV) as 4-20 mA
  • Nothing until the HART master is configured

Two ways to wire

HART runs in two topologies: point-to-point (one device per loop, analog PV + digital extras) and multidrop (up to 15 devices on one pair, all digital, all parked at 4 mA). Multidrop trades speed and analog PV for cabling savings.

Compare the topologies

Toggle each mode to see when you would pick one over the other.

Why park multidrop devices at 4 mA?

In multidrop, every transmitter draws exactly 4 mA from the loop power supply. Why?

  • 4 mA is the minimum to power the device
  • So the loop current stays constant — there is no analog PV to encode
  • It marks the device as "online" to the master

DD and EDDL: the device descriptor

Every HART device ships with a Device Description (DD) — a text file that tells host tools (AMS, PDM, handheld) what variables, parameters, methods and menus the device offers. EDDL extended this with graphics so the same descriptor renders trend charts and gauges in any compliant host.

DD vs EDDL

EDDL is a superset of DD — same idea, plus rendering hints.

FieldComm Group: the registry

DDs and EDDLs are registered with the FieldComm Group (the same body that owns HART, FOUNDATION Fieldbus, and PROFIBUS PA). Vendors submit their DD; FieldComm signs it; AMS and PDM hosts download the signed file. Same idea as IODD for IO-Link, but older and more curated.

Where does the DD live?

You install a brand-new flowmeter. Your asset-management software does not know how to display its diagnostic menu. What do you do?

  • Reflash the DCS firmware
  • Import the device's DD/EDDL file into the host tool
  • Re-poll all devices on the loop
  • Switch the loop to multidrop mode

WirelessHART: same data, no wires

WirelessHART (IEC 62591) wraps the same HART command set in a 2.4 GHz mesh radio network. Devices form a self-healing mesh; gateways aggregate to the host. Same DDs work. It is the standard way to instrument tank farms and remote skids without trenching cable.

When wireless wins

A new tank farm has 40 transmitters spread across 200 m. Cabling cost would be huge. Which HART variant is the natural fit?

  • Multidrop on a single 4-20 mA pair
  • WirelessHART mesh with a single gateway
  • Point-to-point — one cable per device

Final challenge

A site has 120 legacy 4-20 mA temperature transmitters. Management wants per-device diagnostics and a second variable (sensor health) added — without rewiring. The cheapest credible path is:

  • Replace every loop with Profinet — full digital
  • Swap the transmitters for HART-capable models, add a HART multiplexer at the marshalling cabinet, keep all wiring
  • Add a second pair of wires to every device for digital data
  • Convert all loops to multidrop

What you learned

  • HART overlays a Bell 202 FSK signal on a 4-20 mA loop — analog PV undisturbed.
  • Master-slave, ~2 transactions/sec per device. Up to two masters per loop.
  • PV rides the current; SV / TV / QV are digital extras read by a HART host.
  • Two topologies: point-to-point (analog + digital) and multidrop (15 devices, digital only at 4 mA).
  • DD / EDDL files describe the device so any compliant host can render its menus.
  • WirelessHART carries the same commands over a 2.4 GHz mesh.
  • HART is the bridge: it modernises 4-20 mA installations without rewiring.

HART complete

  • You can explain why FSK does not corrupt the analog reading.
  • You know when to choose point-to-point, multidrop, or wireless.
  • You can navigate a device using its DD/EDDL.