The analog and digital information can be communicated simultaneously in this mode. The following are the description of two main HART operation modes. HART can be used in various modes such as point-to-point or multi-drop for communicating information to/from smart field instruments and central control or monitoring systems. HART is a master/slave protocol which provides for up to two masters (primary and secondary) and the secondary master such as handheld can be used to monitor/control the informatoin of HART bus. Therefore, the HART communication including analog and digital information provides a low-cost and very robust complete field communication solution that is easy to use and configure. The digital signal can be used for additional device information including device status, diagnostics, additional measured or calculated values, etc. The 4-20mA signal communicates the primary measured value - the fastest and most reliable industry standard. HART communicates at 1200 bps without interrupting and interference with the 4-20mA signal and allows a host application (master) to send/receive digital information from a smart field device. The HART (Highway Addressable Remote Transducer) protocol uses the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals on the 4-20 mA loop current shown as below figure. Voltage Attenuator and Current Transformer.
