How HART Works

HART communication occurs between two HART-enabled devices, typically a field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices.

HART provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA current loop - the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal. The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc. Together, the two communication channels provide a complete field communication solution that is easy to use and configure, is low-cost and is very robust.

HART device communicate via commands. The command set includes three classes: universal, common practice, and device specific.

Universal	All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations. For example: read primary variable and units, read manufacturer and device type, read current output and percent of range and read sensor serial number and limits.
Common Practice	Common practice commands provide functions implemented by many, but not necessarily all, HART communication devices. The HART specifications recommend devices to support these commands when applicable. Examples of common practice commands are: read a selection of up to four dynamic variables, write damping time constant, write transmitter range, set fixed output current and perform self-test.
Device Specific	Device specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on device-specific commands is available from device manufacturers or in the Field Device Specification document. Examples of device specific commands: read or write sensor type, start, stop or clear totalizer, read or write alarm relay set point, etc.