Highway Addressable Remote Transducer (HART) is a bi-directional form of digital communication used in the process automation industry. HART employs Frequency-Shift Keying (FSK) technology to transmit field instrument data to an asset management system used to analyze and react to changing conditions of the device or process. The digital HART communication signal is produced using 1,200 and 2,200 Hz frequencies (corresponding to 1 and 0) and superimposed on the analog 4-20 mA control signal without interrupting or interfering with the analog signal.
As this data is transmitted, a HART multiplexer or HART loop converter is used to extract the digital information and make it available for an intelligent asset management system. The following illustration shows the typical 4-20 mA control loop in combination with the extracted HART data.