HART Communicator and the 250 Ohm Resistor: Why the Loop Needs It

Almost every HART question from a field technician eventually lands on the same resistor: why do I need a 250 ohm resistor, where does it go, and what happens if I leave it out? The answer is not a ritual. It is a consequence of how HART physically encodes its digital signal on top of the analogue 4–20 mA loop.

How HART sends data on a 4–20 mA loop

HART is a frequency-shift-keyed (FSK) signal. It superimposes a ±0.5 mA sine wave on top of the DC loop current, using two frequencies: 1200 Hz for a logical "1" and 2200 Hz for a logical "0". Because the signal is an AC current riding on the DC loop, the loop current average stays at the 4–20 mA process value — the PLC sees the same process variable, and the communicator sees the digital data at the same time.

To turn that small AC current into a readable AC voltage, the loop must present some resistance. A current pushed through a resistance produces a voltage, and the communicator reads that voltage. With no resistance in the loop, the AC voltage that carries the HART data collapses toward zero, and the communicator sees noise instead of data.

The 250 ohm rule, explained properly

A 250 ohm series load is a common HART wiring provision. Required AC impedance and maximum load come from the transmitter, communicator and associated-equipment manuals; some devices specify a 230 ohm minimum and others 250 ohms. With an ideal ±0.5 mA signal, 250 ohms develops approximately ±125 mV. It also consumes DC voltage headroom, so an extra resistor is not free.

Where the resistor actually goes

A conventional external load is placed in series with the current loop. A communicator connects in parallel at the approved communication points, commonly across the transmitter terminals or an appropriate load. It is not inserted as a series ammeter. Follow the loop drawing and tool manual, including any hazardous-area restrictions.

At the transmitter end. With sufficient load already between the connection point and the supply, a communicator can normally connect across the approved transmitter terminals. Do not break an operating loop simply to add a second resistor. Any required wiring change needs the site’s controlled work procedure.

At the DCS/PLC marshalling side. Check the card’s actual input impedance and whether barriers, isolators or filters pass HART. A suitable existing input load may satisfy the requirement. Use the designated communication points rather than assuming every analogue input is HART-transparent.

Built into the communicator. Some tools provide a switchable load or loop-power function in specific wiring modes. Others are only parallel modems and provide no series resistance. Use the tool’s connection diagram; clipping an ordinary modem across a transmitter does not automatically insert a resistor into the loop.

Why it works at the panel but not at the field

If HART works at the panel but not in the field, the shared series load has not disappeared merely because the communicator moved. Check approved terminals, barriers or isolators, AC bypass paths, cable capacitance, noise, polarity where applicable and tool settings. Avoid the assumption that each physical end needs its own 250 ohm resistor.

Before adding resistance, measure the actual loop conditions and inspect the complete circuit. Correct the missing load or unsuitable signal path only after identifying it, with the process in the approved condition for any interruption.

The load limit you must not ignore

The resistor is not free. Every 250 ohm resistor drops voltage equal to the loop current times the resistance: about 1 V at 4 mA, rising to 5 V at 20 mA. Calculate the worst case at the configured maximum output or fault current, not only 20 mA. The transmitter needs its minimum terminal voltage (typically 10.5–12 V for a 4–20 mA HART transmitter), and the supply must cover the transmitter, the resistor, and the cable resistance. On a long loop with a marginal 24 V supply, adding a 250 ohm resistor at the wrong point can drop the transmitter below its minimum and stop the whole loop.

Check the arithmetic before adding resistance: supply voltage minus (transmitter minimum voltage plus cable drop plus resistor drop) must stay positive with margin.

Questions technicians ask

Q1. Do I need the resistor if I am using a Bluetooth or USB modem?

The connection method does not change the loop’s impedance requirement. Check whether that exact tool supplies a load in the selected mode; do not assume that most USB or Bluetooth modems contain one.

Q2. Can I use a value other than 250 ohms?

Sometimes. The usable range is constrained by the instrument’s HART impedance requirement, cable capacitance, associated equipment and remaining supply voltage. Use the documented limits instead of assuming that any 230–600 ohm value works in every loop.

Q3. Why does my communicator work on one channel but not the next?

Compare the input load, HART path through isolators and filters, wiring, power margin, device address and tool configuration. A non-HART input can still support a local communicator in a suitable loop; a HART-enabled card is needed when the controller itself must exchange HART data.

Q4. What if the loop has no resistor and I cannot break the wire?

First check for a suitable existing load and approved connection point. A simple parallel communicator cannot create a missing series load without a supported connection arrangement. Schedule a controlled change or use the manufacturer’s approved non-disruptive tool mode, if available.

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Technical references

• HART terminal and loop-wiring reference