HART (Highway Addressable Remote Transducer) Protocol is an open standard used globally to send and receive digital information using analog wiring between smart devices and control systems. With over 30 million devices installed, it is the most popular protocol used in the field. HART was first available in the late 1980s and quickly gained in popularity mainly due to its ability to continue to support the older 4-20 mA analog protocol while adding the significant benefits of digital smart instrumentation.
The HART Protocol is based on the Bell 202 Standard to superimpose digital information on the conventional 4-20 mA analog signal. Maintained by an independent organization, the HART Communication Foundation, the HART protocol is an industry standard developed to define the communications protocol between intelligent field devices and a control system.
The HART Protocol defines physical connection technology as well as commands used by applications. There are three classes of HART commands: 1) Universal, 2) Common Practice, and 3) Device Specific.
- Universal commands are required to be implemented by all HART devices. They are primarily used by a controller to identify a field device and read process data.
- Common Practice commands define functions that are generally applicable only to field devices. These include commands to change the range, select engineering units and perform self-tests.
- The third set of commands, Device Specific, are different for each device. Device Specific commands implement unique configuration and adjustment functions. It is important to note that while devices from different manufacturers that externally implement similar functionality—for example, differential pressure measurement—may have completely different hardware and a similarly different Device Specific command set.