In industrial automation, the HART (Highway Addressable Remote Transducer) protocol is a widely adopted communication standard. It cleverly superimposes digital signals onto the traditional 4-20mA analog current loop, enabling compatible transmission of both analog and digital data.
Engineers often ask a common question when building HART communication loops: Why must a 250 ohm resistor be connected in series? This article explains the underlying principle in detail and illustrates it with real-world application examples.
Basic Principles of the HART Protocol
HART uses Frequency Shift Keying (FSK) technology to overlay a low-level AC signal on the 4-20mA DC current:
- Logic “1” is represented by a 1200 Hz sine wave
- Logic “0” is represented by a 2200 Hz sine wave
The digital signal typically has a current amplitude of 1 mA peak-to-peak (p-p), meaning ±0.5 mA.
The Core Function of the 250 Ohm Resistor
HART receivers (such as handheld communicators or host systems) demodulate digital information by detecting voltage signals, not current directly. A load resistor is therefore required to convert the superimposed AC current into a detectable voltage.
- Formula: Voltage = Current × Resistance
- 1 mA p-p current × 250 Ω = 250 mV p-p voltage
This 250 mV p-p voltage is sufficient for reliable detection by the HART modem. Per HART protocol specifications, loop resistance should generally fall between 230–1100 Ω. The value of 250 Ω is the most common and recommended industry standard—it ensures stable communication without consuming excessive loop voltage.
Additional Benefit: Easy Analog Signal Conversion
Beyond digital communication, the 250 Ω resistor offers a practical advantage for the analog signal:
- 4 mA × 250 Ω = 1 V
- 20 mA × 250 Ω = 5 V
This neatly converts the 4-20 mA current into a 1-5 V voltage signal, matching the input standard used by many legacy control systems.
Typical Placement Locations for the Resistor
In real-world installations, the 250 Ω resistor is commonly placed at one of these points:
- Inside the control room power supply or I/O card
- In field junction boxes or intrinsic safety barriers
- Across the test terminals for temporary handheld communicator connection
If total loop impedance drops below 230 Ω, the HART digital signal voltage becomes too low, resulting in unreliable or failed communication.
Summary
Though simple in appearance, the 250 ohm resistor is essential for reliable HART communication. It provides adequate voltage amplitude for the digital overlay while preserving full compatibility with the analog 4-20 mA signal.
When designing HART loops, always ensure sufficient load resistance (minimum 250 Ω) to prevent communication problems.
Microcyber is a professional provider of industrial communication solutions, offering a complete HART product portfolio that includes HART modems, protocol gateways, temperature and pressure transmitters, and more. If you encounter issues with resistor configuration or HART communication, please contact us for technical support.
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