HART 250 Ohm Resistor: Why It Is Required and How to Size It

A HART 250 ohm resistor provides communication impedance that converts the digital current modulation into a detectable voltage. Many device manuals specify at least 250 ohms, while some HART interface guidance uses other limits. Count the existing input, cable and barrier impedances and follow the particular device and host manuals; do not add a resistor automatically.

How the HART signal is detected

HART uses current modulation of approximately +/-0.5 mA with 1200 Hz and 2200 Hz tones. Across a purely resistive 250 ohm load, a 0.5 mA peak produces 125 mV peak; across 50 ohms it produces 25 mV peak. These calculations illustrate signal conversion, not a universal receiver pass/fail threshold. Actual communication depends on impedance at the HART frequencies, noise, cable capacitance and the transmitter and host requirements.

Communication impedance and voltage headroom

The total series load is limited by the voltage that must remain at the transmitter at the highest possible loop current.

R_max = (V_supply - V_min_device - other fixed voltage drops) / I_max. Include all series resistance, the supply tolerance, maximum possible current and manufacturer communication limits.

With a 24 V supply and a 12 V minimum device voltage:

R_max = (24 − 12) / 0.022 ≈ 545 Ω

In this simplified 24 V example, a 250 Ω load drops 5.5 V at 22 mA, leaving 18.5 V before cable and other component drops are included.

Use the cable manufacturer's out-and-back resistance at the design temperature. The 36 ohm cable load in the table is an illustrative assumption, not a cable-length specification; include it only once in the total resistance budget.

Five wiring cases

1. DCS or PLC analogue input with built-in 250 Ω

Check the actual input impedance and whether the input is HART-transparent. If adequate communication load is already present, an added resistor is unnecessary. Additional series resistance reduces voltage margin; a parallel resistor across the measurement input can divert current and corrupt the reading.

2. Input below the specified HART communication load

For a low-impedance input, add only the series resistance needed by the approved loop drawing, after allowing for existing loads, supply voltage and the maximum possible current. Follow the required safe connection points for the HART modem.

3. Handheld communicator

Connect a compatible communicator in parallel at an approved HART connection point. Depending on the loop and manual, that may be the transmitter terminals, a load resistor, the input terminals or dedicated test sockets. Keep the required communication impedance in the loop and follow hazardous-area connection rules.

4. Transmitter on a bench supply

For a bench test, check the transmitter, supply and modem instructions for the required load and connection points. If the setup requires a 250 ohm series resistor and none is already provided, add it before testing communication. Some interfaces include a switchable load; do not count it twice. Verify terminal voltage at the highest permitted loop current.

5. Multidrop

In multidrop operation, each device needs a unique address and a suitable loop-current mode. Older revisions commonly tie a nonzero address to fixed current, while newer implementations can configure loop-current mode separately. Do not assume the analogue output remains a usable process signal.

Symptoms of a missing or wrong resistor

Checking the installed load

1. After the required electrical and process precautions, measure the voltage specifically across the known passive load resistor, not across the entire powered loop.
2. Divide that resistor voltage by the measured current. At 12 mA and 3.0 V across the same passive load, its resistance is 250 ohms. This does not measure total HART-band impedance or include every loop voltage drop.
3. Alternatively, measure resistance with the loop de-energised and the transmitter disconnected.
4. Compare the result, cable capacitance and available terminal voltage with the actual transmitter and host requirements; a DC resistance test alone is not a complete HART diagnosis.

Power rating, tolerance and installation

• Power rating. At 22 mA, 250 ohms dissipates about 0.121 W. Select a suitably rated component with temperature and enclosure derating, tolerance and the required safety or installation approval.
• Tolerance. Follow the specified communication-load range and allow for temperature drift. If the resistor is a separate series load, current remains regulated only while the transmitter has enough voltage. If it is also the input shunt used to convert voltage to current, its resistance and calibration directly affect the measured value.
• Mounting. Provide a labelled, accessible test point in accordance with the loop drawing and electrical safety requirements.
• Use a fixed resistor of the specified value and rating. An adjustable component can change the communication load and complicate fault diagnosis.

HART, NAMUR NE43 and the fault current

HART communication and NE43 fault signaling are different requirements. A device may use low or high fault current outside its normal signal and saturation range. The configured values must come from its manual and settings; a high-fault output is not universally 22 mA. The calculations above use 22 mA only as an example. Recalculate with the largest current permitted by the actual device.

Coordinate saturation, failure-current thresholds, diagnostics and alarm actions. A high-failure output is intended to signal a device fault; do not tune the DCS merely to suppress that alarm.

HART revision differences that affect wiring

Check communication load against the actual device and host manuals rather than inferring it from the HART revision. Basic communication and access to device-specific parameters are separate checks. Confirm the supported revision and device-description or FDI package, then test the functions required for commissioning and maintenance.

Using the available HART diagnostics

Where supported, read the device status, loop current and terminal voltage over HART. Diagnostic availability depends on the model and host; these checks complement, rather than replace, the required electrical tests.

• Terminal voltage below the specified minimum: check supply voltage, all series drops, wiring and the load budget before blaming cable length alone.
• Loop current drifting with a stable process: compare local value, digital value and measured current; inspect connections, moisture, supply and output configuration as appropriate.
• A saturation indication means the analog output has reached its configured limit. Compare the measured variable, output range and diagnostics to distinguish a real overrange condition from an incorrect setup.

Record these diagnostics alongside the local indication and PLC value to help identify where further electrical checks are needed.

FAQ

Q1. Can HART work without a resistor?

An added resistor is unnecessary when the approved loop already provides the required communication impedance. Check the input card, barriers and any internal modem resistor before adding a component. Enough DC resistance alone does not prove the loop passes the HART signal at its communication frequencies.

Q2. Why 250 Ω and not 230 Ω?

The specified minimum depends on the device and host documentation. A common 250 ohm load may meet a particular manual, but a different published interface limit is not permission to ignore that manual. Check both the communication requirement and the maximum load allowed by the power budget.

Q3. Does the resistor affect the 4–20 mA reading?

An independent series resistor should not change the measured current while the transmitter has adequate voltage to regulate it. Excessive load can prevent that regulation. A resistor placed in parallel with the input can divert current, and an input shunt used to calculate current requires the correct resistance and calibration.

Q4. Where do I connect a handheld communicator?

Connect in parallel only at the locations allowed by the loop drawing and equipment manuals, with the required communication load still in circuit. Transmitter terminals or dedicated test sockets may be suitable. In a classified area, use approved equipment and connection procedures; an electrically plausible connection is not sufficient.

Q5. Can I install the resistor on the negative side?

Use the position specified by the approved loop drawing. The resistor has no polarity, but input referencing, barriers, grounding and available test points can constrain where it belongs. Do not move it between positive and negative conductors solely because the DC series circuit appears equivalent.

Q6. Does a HART modem need the same resistance?

A USB modem still needs a compatible HART loop, but its connection arrangement and any built-in resistor must be checked. Some adapters provide a switchable load; others do not. Count that load once and verify transmitter terminal voltage before concluding that the modem or device is faulty.

For a HART loop review, provide the loop drawing, transmitter and input-card models, supply voltage and all existing loads. Ask our support engineers

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