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Dated: Oct. 01, 2013
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“HART” is an acronym that is used for the name Highway Addressable Remote Transducer. In order to superimpose digital communication signals at a low level on top of the 4-20mA, this Protocol uses the Bell 202 Frequency Shift Keying (FSK) standard. This allows the two-way field communication to happen and allows for more information next to just the normal process variable to be communicated to/from a smart field instrument. The HART Protocol enables a host application (master) to have two or even more digital updates per one second from a smart field device and it can communicate at 1200 bps without any interruption in the 4-20mA signal. Because the digital FSK signal is phase continuous, there is no interruption in the 4-20mA signal.
HART technology is a protocol that is referred to as a master/slave protocol, and that means that a smart field (slave) device can only speak when spoken to by the master. The HART Protocol can be used in a lot of modes like as multidrop or point-to-point for communicating information from or to smart field instruments and a central control or monitoring system.
HART Communication happens between two devices that are HART-enabled, usually a control or monitoring system and a smart field device. Communication happens by using standard wiring and using the usual wiring and termination practices.
The HART Protocol gives two communication channels that are simultaneous: a digital signal and a 4-20mA analog signal. The 4-20mA signal can communicate the primary measured value by using the 4-20mA current loop - it is the fastest and probably the most reliable industry standard. Additional device information can be communicated by using a digital signal which must be superimposed on the analog signal.
The digital signal has information within itself that comes from the device which includes the device status, diagnostics, additional measured or calculated values, and more. The two communication channels, when they are together, give a low-costing and pretty robust complete field communication solution which is really easy to use and it can be configured without hassle.
The HART Protocol can provide for a maximum of two masters (the primary and secondary master). With this, the secondary masters can be used without interrupting in communications that come from or go to the primary master, i.e. control/monitoring system.
It allows all digital communication that have field devices in either point-to-point or multidrop network configurations. Also, an optional "burst" communication mode exists in which a single slave device can broadcast a standard HART reply message over and over again. Higher update rates can happen because of this mode.
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