HART Technology Explained
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 technician's hand-held device or laptop to a plant's process control, asset management, safety or other system using any control platform. Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Instrumentation grade wiring and standard termination 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.
How HART Works
“HART” is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes uses Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument.
The HART Protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal. The HART Protocol 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 low-cost and very robust complete field communication solution that is easy to use and configure.
To learn more about how HART works, take our new self-paced online course (free of charge for a limited time): HART Protocol - Technology Overview.
Check out these resources, including brochures, white papers and books.