In industrial automation, the 4–20 mA current loop is the most widely adopted signaling method. Even with the rise of fully digital fieldbuses, the HART (Highway Addressable Remote Transducer) protocol continues to dominate by leveraging this analog foundation.

But why was 4 mA chosen as the baseline instead of a more intuitive 0 mA? The answer lies in the fundamental requirements of safety, power management, and the physics of the Bell 202 FSK standard.

1. The “Live Zero” and NAMUR NE43 Compliance

The most critical reason for starting the signal at 4 mA is the “Live Zero” concept. This allows the control system (PLC/DCS) to distinguish between a zero-scale process reading and a hardware failure.

• Fault Diagnostics: In a 0–20 mA system, 0 mA could mean either a zero-scale measurement or a broken wire. In a 4–20 mA loop, any current below the 4 mA threshold (typically <3.6 mA) indicates a fault.

• Industry Standards: Most modern HART devices follow the NAMUR NE43 recommendation. In this standard, specific current levels (e.g., ≤3.6 mA or ≥21 mA) are reserved for sensor failure diagnostics, ensuring that a wire break is never confused with a valid process value.

2. Power Budget for Loop-Powered Devices

Most HART transmitters are loop-powered (two-wire devices). This means the instrument must draw its operating power from the same two wires that carry the signal.

• Minimum Operating Current: The 4 mA baseline provides a constant power budget. Even when the process variable is at 0%, the transmitter has 4 mA of current available to power its internal microprocessor, sensor element, and FSK modem.

• Efficiency: Without this 4 mA “floor,” a two-wire device would lose power and shut down as the signal approached 0 mA, making continuous measurement and communication impossible.

3. Maintaining Symmetrical Modulation (Bell 202 FSK)

HART communication relies on the Bell 202 FSK standard, superimposing a constant-amplitude, zero-average AC signal onto the DC current loop.

• Current Modulation: Contrary to popular belief, HART is a current-modulated protocol. It superimposes a ±0.5 mA AC signal on top of the analog DC value.

• The Importance of Bias: Because the FSK signal is a symmetrical AC waveform, it requires a sufficient DC bias to maintain its shape. If the loop current approaches 0 mA, the FSK signal cannot maintain its symmetrical modulation, leading to signal distortion or total loss of communication.

• Measurement Integrity: Since the FSK signal has a zero-average value, it does not affect the accuracy of the 4–20 mA analog measurement—a key feature of HART.

4. Why a 250 Ω Resistor is Required

Since HART communication is based on current modulation, a load resistor is required to convert these current fluctuations into a measurable voltage for communication devices (like a HART handheld or gateway).

• The 1–5 V Standard: A 250-ohm resistor converts the 4–20 mA signal into a 1–5 V voltage drop, which is the optimal range for most analog input circuits.

• Impedance for Detection: More importantly, it ensures sufficient loop impedance (typically ≥230 Ω) for the high-frequency FSK signal to be properly detected by the HART modem.

Deep Dive: For a detailed breakdown of the math and physics behind this, see our technical guide: Why is a 250-ohm resistor needed for HART protocol?

Conclusion

The 4–20 mA standard is not just a legacy habit; it is a sophisticated engineering choice. By utilizing a “Live Zero,” the HART protocol ensures robust fault detection, provides a reliable power budget for field devices, and maintains the integrity of digital FSK modulation.

Understanding these principles is essential when designing reliable HART networks, selecting field devices, or troubleshooting signal issues.

Explore more practical guides on HART, Foundation Fieldbus, and PROFIBUS at Microcyber’s Technical Blog.