The HART Protocol was developed in the late 1980's and transferred to the HART Foundation in the early 1990's. Since then it has been updated several times. When the protocol is updated, it is updated in a way that ensures backward compatibility with previous versions. The current version of the HART Protocol is revision 7.3. The "7" denotes the major revision level and the "3" denotes the minor revision level.
The HART Protocol implements layers 1,2, 3, 4 and 7 of the Open System Interconnection (OSI) 7-layer protocol model:
The HART Physical Layer is based on the Bell 202 standard, using frequency shift keying (FSK) to communicate at 1200 bps. The signal frequencies representing bit values of 0 and 1 are 2200 and 1200Hz respectively. This signal is superimposed at a low level on the 4-to-20mA analog measurement signal without causing any interference with the analog signal.
The HART Data Link Layer defines a master-slave protocol - in normal use, a field device only replies when it is spoken to. There can be two masters, for example, a control system as a primary master and a handheld HART communicator as a secodary master. Timing rules define when each master may initiate a communication transaction. Up to 15 or more slave devices can be connected to a single multidrop cable pair.
The Network Layer provides routing, end-to-end security, and transport services. It manages "sessions" for end-to-end communication with correspondent devices.
The Transport Layer: The Data-Link Layer ensures communications are successfully propagated from one device to another. The Transport Layer can be used to ensure end-end communication is successful.
The Application Layer defines the commands, responses, data types and status reporting supported by the Protocol. In the Application Layer, the public commands of the protocol are divided into four major groups: