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Instrumentation·7 min read·

HART: digital data on the same wires as the 4-20 mA

How a digital signal rides on an analogue loop, what it gives you that the current does not, and why most installations never use it.

Short answer

HART superimposes a low level frequency shift keyed digital signal on a standard 4 to 20 mA current loop. The analogue value keeps working exactly as before, and the digital layer carries device identity, configuration, diagnostics and up to three additional process variables. Most installed HART devices are never read digitally, because doing so needs a HART capable input card or a multiplexer.

4-20 mA scaled to engineering unitsbroken loop4 mA20 mA0 bar10 bar3277 / 553016384 / 27648raw counts: AB / Siemens

A 4 to 20 milliamp loop carries one number. HART is what happens when somebody works out how to send more without changing the wiring.

How it works

The digital signal is frequency shift keyed: 1200 Hz for one, 2200 Hz for zero, at about half a milliamp peak to peak, superimposed on the loop current.

The average of that alternating signal is zero, so the DC current, which is the process value, is untouched. An ordinary analogue input filters the audio frequency away and never knows it was there.

That backward compatibility is why HART won. A HART transmitter drops into an existing analogue loop and works. Everything digital is optional and additive.

What the digital layer carries

Identity. Tag, manufacturer, model, serial number. A loop check that confirms the right instrument is on the right loop, from the control room, rather than by walking the plant.

Configuration. Range, units, damping, transfer function. Readable and writable, so a range change is done from a handheld or from the asset management system rather than with a screwdriver in the rain.

Diagnostics. The genuinely valuable part. Sensor failure, electronics failure, out of range, and on modern devices statistical measures that indicate a plugged impulse line or a drifting sensor.

Additional variables. Up to three beyond the primary. A Coriolis meter can report mass flow on the current and density and temperature digitally.

Why most installations never use it

Because reading it needs hardware that has to be specified at the design stage.

A HART capable analogue input card costs more than a plain one. A multiplexer across a rack of conventional loops is an extra panel item. Neither gets specified unless somebody asks for it, and by the time anybody wants the diagnostics, the cards are installed.

The result is plants full of instruments with self diagnostics that nobody reads.

Multidrop, and why it is rare

HART can run several devices on one pair, with each fixed at 4 mA and all values passed digitally.

It is rarely used, for a good reason: the analogue signal is gone, so the loop no longer works with a conventional input, and the update rate across several devices is slow, a couple of seconds. Where digital is wanted properly, a fieldbus or an Ethernet protocol does it better.

HART-IP and wireless

Two later additions worth knowing exist.

HART-IP carries the same commands over Ethernet, which is how a modern gateway presents a rack of HART instruments to a control system.

WirelessHART is a mesh network of battery powered devices using the same command set, for measurements where running cable is disproportionate. Useful for monitoring rather than control, given the update rates and the battery life.

The practical advice

If you are specifying a new system, put HART capable inputs on the analogue cards, or a multiplexer on the loops. The additional cost is small at build and impossible to retrofit cheaply.

Then actually connect them to something that reads the diagnostics, because a HART capable card feeding nothing is the same as a plain one.

Common questions

Does HART interfere with the 4-20 mA signal?
No. The digital signal is a small alternating current whose average is zero, superimposed on the loop. The analogue reading is unaffected, which is the whole design intent: a HART transmitter works perfectly on a conventional analogue input that knows nothing about it.
Do I need special input cards for HART?
To read the digital layer, yes: a HART capable analogue input, or a multiplexer connected across the loops. To read the 4 to 20 mA, no. This is why so many HART instruments are installed and never interrogated: nobody specified the card.
What can HART tell me that the current cannot?
The device tag and serial number, its calibration and range, its own diagnostics including sensor failure and electronics faults, and additional variables. A pressure transmitter can report its internal temperature and whether the sensor is drifting, weeks before the current reading goes wrong.

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