HART is a bi-directional communication protocol that provides data access between intelligent field instruments and host systems. A host can be any software application from a technician's hand-held configurator to a plant's process control, safety or SCADA system and enterprise asset management or data historian applications. Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Standard 4-20mA wiring practices assure reliable communication.
HART provides two simultaneous communication channels, one analog, the other digital:
A 4-20mA signal communicates the primary measured value (PV) as an analog value of current using the wiring that provides power to the instrument. The host system then converts the current value to a physical value according to parameters defined by HART Software. For example, 7 mA = 80 degrees F.
Digital device information is communicated by encoding a digital signal, generally using a technique known as Frequency Shift Keying on the same 4-20mA wiring used for analog communications. The digital signal contains information from the device including PV, device status, diagnostics, and additional measured or calculated values, etc.
Together, the two communication channels provide a complete field communications solution that is easy to design, simple to use, low cost and extremely reliable.