HART Loop Resistance Calculator
Instrument techs and controls engineers use this when a HART handheld or multiplexer will not communicate on a loop. It sums the series resistance in the loop and compares it against the resistance window HART communication commonly needs, so you can tell whether the loop resistance is the problem.
HART digital communication rides on the 4-20 mA loop and needs a minimum series resistance to develop a readable signal, but too much resistance eats the voltage budget. This calculator adds the sense resistor, wire, and any other series resistance, then checks the total against the roughly 230 to 1100 ohm window HART communication commonly needs.
The Calculator
Result
Common HART window is about 230 to 1100 ohms.
The Formula
Series resistances add:
The result is then compared against the common HART loop-resistance window of roughly 230 to 1100 ohms.
Worked example. A loop with a 250 ohm sense resistor, 20 ohm of wire, and no barrier totals 250 + 20 + 0 = 270 ohm, which sits inside the 230 to 1100 ohm window, so loop resistance should not block HART communication. Drop the sense resistor to 100 ohm and the total is 120 ohm, below the window, and a handheld may fail to communicate.
Assumptions and Limits
- The 230 to 1100 ohm figure is a commonly cited communication window; the exact usable range depends on the specific master, transmitter, cable, and supply voltage. Treat it as guidance, not a hard limit.
- Passing the resistance check does not guarantee communication. The supply must also have enough voltage for the loop at the resistance you have (see the loop voltage headroom calculator), and cable capacitance and noise can still interfere.
- Wire resistance is round-trip; double a one-way figure before entering it.
- Results are engineering estimates. Confirm the resistance requirements against the device and master documentation.
FAQ
Why does HART need a minimum resistance?
The digital signal is an AC current modulation on the loop. A series resistance turns that current variation into a voltage the master can read; with too little resistance, the signal voltage is too small to decode reliably.
Can there be too much resistance?
Yes. High resistance consumes supply voltage, which can starve the transmitter and also attenuate the signal. That is why the window has an upper bound as well as a lower one.
My resistance is in range but HART still fails. Why?
Resistance is only one requirement. Check the loop voltage budget, look for excessive cable capacitance or length, and rule out noise sources. A resistance in the window is necessary but not sufficient for reliable communication.
Source
The HART physical layer and loop requirements are maintained by the FieldComm Group (fieldcommgroup.org).
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