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For many years, the field communication standard for process automation
equipment has been a milliamp (mA) analog current signal. The
milliamp current signal varies within a range of 4-2OmA in proportion
to the process variable being represented. Li typical applications
a signal of 4mA will correspond to the lower limit (0%) of the
calibrated range and 2OmA will correspond to the upper limit (100%)
of the calibrated range. Virtually all installed systems use this
international standard for communicating process variable information
between process automation equipment.
Communications Protocol extends this 4- 2OmA standard to enhance
communication with smart field instruments. The HART protocol
was designed specifically for use with intelligent measurement
and control instruments which traditionally communicate using
4-2OmA analog signals. HART preserves the 4- signal and enables
two way digital communications to occur without disturbing the
integrity of the 4-2OmA signal. Unlike other digital communication
technologies, the HART protocol maintains compatibility with existing
4-2OmA systems, and in doing so, provides users with a uniquely
backward compatible solution. HART Communication Protocol is well-established
as the existing industry standard for digitally enhanced 4- 2OmA
THE HART PROTOCOL
- AN OVERVIEW
HART is an
acronym for "Highway Addressable Remote Transducer".
The HART protocol makes use of the Bell 202 Frequency Shift Keying
(FSK) standard to superimpose digital communication signals at
a low level on top of the 4-2OmA. 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 4-2OmA signal and allows a host application
(master) to get two or more digital updates per second from a
field device. As the digital FSK signal is phase continuous, there
is no interference with the 4- 2OrnA signal.
HART is a
master/slave protocol which means that a field (slave) device
only speaks when spoken to by a master. The HART protocol can
be used in various modes for communicating information to/from
smart field in3truments and central control or monitor systems.
HART provides for up to two masters (primary and secondary). This
allows secondary masters such as handheld communicators to be
used without interfering with communications to/from the primary
master, i.e. control/monitoring system. The most commonly employed
HART communication mode is master/slave communication of digital
information simultaneous with transmission of the 4-2OmA signal.
The HART protocol permits all digital communication with field
devices in either point-to-point or multidrop network configuration.
There is an optional "burst" communication mode where
single slave device can continuously broadcast a standard HART
The HART protocol utilizes the OSI reference model. As is the
case for most of the communication systems on the field level,
the HART protocol implements only the Layers 1, 2 and 7 of the
OSI model. The layers 3 to 6 remain empty since their services
are either not required or provided by the application layer 7
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