"HART" is an abbreviation for High Speed Addressable Remote Sensor. The HART protocol uses the Bell 202 Frequency Shift Keying (FSK) standard to superimpose low-level digital communication signals on 4 - 20mA.

This technology realizes two-way field communication and makes it possible to transmit more information than general process variables with intelligent field instruments. The HART protocol communicates at a rate of 1200 bps without affecting the 4 - 20mA signal, and allows a host application (master device) to obtain two or more digital updates per second from an intelligent field device. Since the digital FSK signal is phase continuous, it will not interfere with the 4 - 20mA signal.

HART technology is a master/slave protocol, which means that intelligent field (slave) devices send signals only when the master device sends signals. HART protocol can be used in many modes, such as point-to-point or multipoint mode, to transmit information between intelligent field instruments and central control or monitoring systems.

HART communication occurs between two devices with HART functions, usually between intelligent field devices and control or monitoring systems. Standard instrument cables are used for communication, and standard wiring and terminal processing methods are used.

The HART protocol provides two synchronous communication channels: 4 - 20mA analog signal and a digital signal. 4 - 20mA signal uses 4 - 20mA current loop – it is the fastest and most reliable industry standard to transmit the main measured values (in the case of field instruments). In addition, HART uses digital signals superimposed on analog signals to transmit other device information.

Digital signals contain information from equipment, including equipment status, diagnosis, additional measurement or calculated value, etc. The combination of these two communication channels provides a low-cost, highly reliable and complete on-site communication solution that is easy to use and configure.

The HART protocol can have up to two primary devices (first primary and second primary). This makes it possible to use a second master device, such as a handheld communicator, without interfering with the communication of the first master device, such as a control/monitoring system.

HART protocol allows all digital communication with field devices, which can be configured in point-to-point or multipoint mode:

Configuration of multipoint mode

There is also an optional "burst" communication mode where a single slave can continuously broadcast standard HART reply messages. This optional "burst" communication mode is likely to adopt a higher update rate and use a configuration that is usually limited to point-to-point.