How HART rides on top of 4–20 mA
HART (Highway Addressable Remote Transducer) superimposes a small digital signal on the analog current. It uses frequency shift keying (FSK): a 1200 Hz tone means a binary 1 and 2200 Hz means 0, at about ±0.5 mA. Because the tone is a sine wave that averages to zero, the analog reading the DCS sees is not disturbed.
- Speed is slow, about 1200 bit/s: a typical request and reply takes a fraction of a second.
- The analog signal stays the primary control signal; HART adds configuration and diagnostics.
- Modern HART versions (6 and 7) add more device variables, diagnostics and wireless (WirelessHART).
Wiring and the 250 Ω resistor
The HART signal is sent as a current change and read as a voltage across a resistance in the loop. Without enough resistance there is no voltage to read, so most loops have a 250 Ω resistor (often inside the DCS input card). The allowed range is roughly 230–600 Ω including cable.
Masters and tools
HART is a master/slave protocol: the instrument only answers when asked. Two masters are allowed at once:
- Primary master: usually the DCS/PLC HART card or an asset management system.
- Secondary master: a handheld communicator or a laptop with a HART modem, used in the field.
Typical tasks: read tag and range, re-range (LRV/URV), change damping or units, sensor trim, analog output trim, loop test (fixed mA output), read diagnostics and alarms, and save the configuration for records.
Device variables: PV, SV, TV, QV
- PV (primary variable): the value sent on the 4–20 mA output, e.g. differential pressure.
- SV, TV, QV (secondary, tertiary, quaternary): extra values available only digitally, e.g. sensor temperature, static pressure, or a computed flow in a multivariable transmitter.
The DCS can read all four over HART, so one multivariable instrument can replace several separate ones.
Point-to-point, multidrop and burst mode
| Mode | How it works | Use |
|---|---|---|
| Point-to-point | One device per loop, polling address 0, analog signal active | Almost all control loops |
| Multidrop | Up to 15 devices on one pair, each with its own address (1–15); current fixed at about 4 mA each, digital only | Slow monitoring, tank farms, remote sites |
| Burst mode | The device sends data continuously without being asked | Faster digital updates to one listener |
A transmitter with a polling address other than 0 may fix its output at 4 mA. If a loop reads a constant 4 mA after maintenance, check the polling address and the "loop current mode" setting.
DD and DTM files
To show every menu of a specific instrument, the host needs that device's description file: a DD/EDD for handhelds and many DCS systems, or a DTM for FDT frames such as PACTware or FieldCare. Match the file to the device's manufacturer, model and device revision. Without it, you only get the generic HART commands (tag, range, PV). Download links are in the DTM & device software section of Study Materials.
Troubleshooting HART
| Symptom | Check |
|---|---|
| "No device found" | Loop resistance (≥ 230 Ω), loop powered, polling address (try 0–15), correct connection points |
| Communication drops out | Noise from VFD or power cables, shield earthed at both ends, long cable capacitance |
| Works at the field, not from the DCS | Barrier or isolator not HART-transparent, HART disabled on the I/O channel |
| Generic menus only | Missing or wrong-revision DD/DTM on the host |
| Output stuck at 4 mA | Multidrop address set, or loop test / fixed current left active |
Q&A: test yourself
Related: Loop Checking & Commissioning · DCS Communication Protocols · Transmitters & Signals