The short answer
HART
HART, the Highway Addressable Remote Transducer protocol, superimposes a small frequency-shift-keyed digital signal on a standard 4-20 mA loop, using 1200 Hz and 2200 Hz tones that average to zero so the analog value is undisturbed. It lets a handheld communicator or a HART-capable input card read the device identity, secondary variables, diagnostics, and configuration while the loop keeps carrying the primary variable as current.
Key points
- The analog 4-20 mA signal is unchanged; HART adds a small audio-frequency signal on top of it.
- A loop needs roughly 250 ohms of resistance for HART to work. Most input cards provide it; a bare milliammeter does not.
- HART gives identity, secondary variables, diagnostics, and configuration through a handheld or a HART-capable input card.
- Multidrop puts several devices on one pair at a fixed 4 mA each, trading the analog signal for the digital.
- Capacitance, filtering, and some isolators pass 4-20 mA and block HART.
What HART is
HART is a digital protocol designed in the 1980s to run over the wiring that already existed for 4-20 mA instruments. It uses frequency-shift keying based on the Bell 202 modem standard: a 1200 Hz tone represents a 1 and a 2200 Hz tone a 0, superimposed on the loop current at an amplitude of about half a milliamp. The tones average to zero over any interval an analog input cares about, so the primary variable in the current is not disturbed. The data rate is 1200 bits per second, which sounds slow and is entirely adequate for what HART carries.
What it carries is everything about the device that the current cannot: the tag name, manufacturer, model, and serial number; the primary variable as a number with units, which is the same value the current represents; secondary, tertiary, and quaternary variables such as sensor temperature or a second measurement; device status and diagnostics; and read and write access to the configuration, including range, damping, units, and calibration trims.
How it is used
- Handheld communicator
- A field communicator clipped across the loop at any point, in the field or at the panel, to configure, range, trim, and diagnose the transmitter. This is the most common use, and for many plants the only one.
- HART-capable analog input
- An input card or a HART multiplexer that reads the digital data continuously while measuring the current. The controller then has the secondary variables and the device status as tags, and an asset management package can read the configuration of every device from the control room.
- Loop-powered indicators and isolators
- Devices in the loop that display the HART primary variable in engineering units without needing their own calibration, or that repeat the current and pass the HART signal through.
- Multidrop
- Several devices on one pair, fifteen in older revisions and more under HART 7, each at a fixed 4 mA and addressed digitally. The analog signal is given up, so multidrop is for slow measurements where update rate does not matter. Rare in water and wastewater.
- Burst mode
- A device configured to transmit its variables continuously without being polled, which speeds up reading by a host that supports it.
The wiring conditions
HART is a voltage signal developed across the loop resistance by the modulated current. It needs that resistance to exist, and it needs the path to pass audio frequencies.
| Condition | Requirement | What goes wrong |
|---|---|---|
| Loop resistance | At least about 250 ohms total (the specification minimum is 230), and no more than about 1100 ohms | Below the minimum the HART voltage is too small to be detected. Most input cards have a 250 ohm sense resistor and satisfy this; a transmitter on a bench with only a power supply and a milliammeter does not. |
| Power supply | Enough voltage for the transmitter after the loop drop, and a supply that does not filter out the tones | A supply with heavy output capacitance can shunt the HART signal. Size the supply for the transmitter lift-off voltage plus the drop across the loop resistance at 22 mA. |
| Cable | Twisted shielded pair, with the shield grounded at one end | Long unshielded runs pick up noise at audio frequencies; capacitance on very long runs attenuates the signal. |
| Filtering and isolation | Any isolator, barrier, surge device, or filter in the loop must be HART-compatible | Devices designed only to pass DC will pass the current and remove the HART signal. The loop reads correctly and the handheld cannot connect. |
| Analog input filtering | Input filtering on the card must not respond to the tones | Usually fine; some old cards show a slight flicker when HART is active. |
What HART changes about maintenance
Before HART, ranging a transmitter meant potentiometers and a calibrator. With HART, range, damping, and units are set from the communicator, and a sensor trim corrects the measurement against a reference. That convenience has a hazard: a HART change is invisible to the control system unless the input card reads HART. A transmitter re-ranged in the field without the controller scaling being changed reads wrong in a way that looks plausible, and the loop check that would catch it is skipped because the change was small.
HART diagnostics are the other change. A transmitter that knows its sensor is failing, its electronics temperature is too high, or its configuration was changed can say so. If the input card reads HART, that status is a tag the controller can alarm on. If it does not, the information is available to whoever next connects a handheld, which may be years.
HART and the controller
A HART-capable input card exposes more than the primary variable. Typical uses at a water plant are a second measurement from one device, such as sensor temperature from a pH loop or static pressure from a differential pressure flow transmitter; device status for alarming a failing instrument before it fails; and identification so that the asset database can be built from what is actually installed. Read HART variables at a slow rate, on the order of once a second per device, because the protocol is slow and polling many devices on a multiplexer takes time. The control loop runs on the current, not on the HART value.
Frequently asked questions
- Does HART affect the accuracy of the 4-20 mA signal?
- No, when the input is designed for it. The tones average to zero over a few milliseconds and the input filtering removes them. On an old or very fast input, a small flicker of a few counts can appear while HART is communicating; it disappears when the handheld is disconnected.
- Can I use HART on a loop with a safety barrier?
- If the barrier is rated for HART. Many are; some passive barriers with capacitive filtering attenuate the signal. Check the barrier data sheet for HART compatibility and the loop resistance including the barrier.
- What is the difference between HART revisions?
- HART 5 and 6 are the installed base; HART 7 added extended addressing, more device variables, and time stamping, and is what current devices support. Handhelds and hosts are backward compatible. WirelessHART carries the same commands over a mesh radio network and is a different physical layer entirely.
- Is HART a replacement for a fieldbus?
- It fills the same need at lower complexity: identity, diagnostics, and configuration over existing wiring. It does not replace the analog signal for control, and it does not provide the multi-variable speed of a true digital fieldbus. For a utility, HART on 4-20 mA is usually the right amount of digital.
Related topics
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- 4-20 mA Current LoopsWhy the industry standardized on current instead of voltage, how a two-wire loop is powered, what the live zero buys you, and how to read a loop with a meter.
- How to Test a 4-20 mA LoopProve a current loop end to end with a meter and a calibrator, without interrupting a process you were not authorized to interrupt.
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- Ground Loops in InstrumentationTwo grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
- Pulse SignalsPulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
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