ML-OI-AX-HART – Option board

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Multi-Drop HART input board

The ML-417 data logger can be equipped with an ML-OI-AX-HART input board to be able to connect HART compatible sensors/indicators.

The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation open protocol. Its most notable advantage is that it communicates over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog-only host systems. Although its dating back to the mid-1980s, the HART protocol is still one of the most popular protocols used in process and instrumentation systems.

The ML-OI-AX-HART board can be used in both point-to-point (single sensor) as in multi-drop (multiple sensors) mode. The board has a 12V excitation and an internal load resistor in the ground loop, it requires just two wires to both power and communicate with a sensor.

Before you get started

Some useful installation notes

The 12V wire should be connected to the “Loop +” input of a sensor and the returning wire between the “Loop -” output of a sensor and the “Loop +” input of the ML-OI-AX-HART board. In case of connecting multiple sensors (multi-drop), just connect all sensors in parallel.

The input board supports up to 8 channels, where each channel can be linked to the address of a connected sensor and to one of the values given by it (the current loop, primary, secondary, tertiary or quaternary value). The board has a configuration feature to scan for connected sensor ID’s.

Note: When a sensor is powered from the board’s or ML-417 power switch, don’t forget to specify a sufficient power-up time, which can be tens of seconds depending on the used sensor.


  • Protocol: HART
  • Topology: Point-to-Point & Multi-Drop
  • Power switch: 12V
  • Load resistor: Internal (270 Ohm)
  • Input channels: 8
  • Sensors: Up to 8
  • Values: Current loop, primary, secondary, tertiary or quaternary

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