• Access to all instrument parameters & diagnostics
• Supports multivariable instruments
• On-line device status
• Simultaneous analog & digital communication
• Compatible with existing 4-20 mA equipment & wiring
• Fully open de facto standard
• Common Command and data structure
• Enhanced by Device Description Language
HART® Field Communications Protocol is widely recognized as
the industry standard for digitally enhanced 4-20 mA smart
instrument communication. Use of the technology is growing
rapidly, and today virtually all major global
instrumentation suppliers offer products with HART
The HART protocol provides a uniquely backward compatible
solution for smart instrument communication as both 4- 20 mA
analog and digital communication signals are transmitted
simultaneously on the same wiring. HART provides many
benefits promised by fieldbus, while retaining the
compatibility and familiarity of existing 4-20 mA systems.
This paper provides a brief overview of the HART protocol
and the benefits achievable with this important technology.
Per instrument cost savings benefits of $300-500 in initial
installation/commissioning and $100-200 per year in ongoing
maintenance/operations are commonly reported.
Analog + Digital Communication
For many years, the field communication standard used by
process automation equipment has been a milliampere (mA)
analog current signal. In most applications the milli ampere
signal varies within a range of 4-20 mA in proportion to the
process variable being represented. Virtually all installed
plant instrumentation systems use this international
standard to communicate process variable information.
Simultaneous Analog + Digital Communication
information on top of the 4-20 mA analog signal
HART Field Communications Protocol extends the 4-20 mA
standard to enhance communication with intelligent
measurement and control instrumentation. A major step in the
evolution of process control, the HART protocol is fostering
significant innovation in process instrumentation
capabilities. The enhanced communication characteristics of
this important technology are reflected in the protocol
name, HART which stands for “Highway Addressable Remote
The HART protocol enables two-way digital communication with
smart instruments without disturbing the 4-20 mA analog
signal. Both the 4-20 mA analog and HART digital
communication signals can be transmitted simultaneously over
the same wiring. Primary variable and control signal
information is carried by the 4-20 mA (if desired), while
additional measurements, process parameters, device
configuration, calibration, and diagnostics information is
accessible through the HART protocol over the same wires at
the same time. Unlike other “open” digital communication
technologies for process instrumentation, HART is compatible
with existing systems.
The HART Technology
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 4-20 mA. Since the
digital FSK signal is phase continuous, it does not
interfere with the 4-20 mA signal.
A logical “1” is represented by a frequency of 1200 Hz and a
logical “0” is represented by a frequency of 2200 Hz as
shown in Figures 1 and 2.
The HART FSK signaling enables two-way digital communication
and makes it possible for additional information beyond just
the normal process variable to be communicated to or from a
smart field instrument. The HART protocol communicates at
1200 bits per second without interrupting the 4-20 mA signal
and allows a host application (master) to get two or more
digital updates per second from a field device.
Figure 2 - HART digital communication signal superimposed on the
4-20 mA analog signal
HART is principally a master/slave protocol which means that
a field (slave) device speaks only when spoken to by a
master. Two masters (primary and secondary) can communicate
with slave devices in a HART network. Secondary masters,
such as handheld communicators, can be connected almost
anywhere on the network and communicate with field devices
without disturbing communication with the primary master. A
primary master is typically a DCS, PLC, or computer based
central control or monitoring system. A typical installation
with two masters is shown in Figure 3.
The HART protocol can be used in various modes for
communicating information to/from smart field instruments
and central control or monitoring equipment. Digital
master/slave communication simultaneous with the 4-20 mA
analog signal is the most common. This mode, allows digital
information from the slave device to be updated twice per
second in the master. The 4-20 mA analog signal is
continuous and can still carry the primary variable for control.
The majority of smart field devices installed worldwide
today are HART-enabled. But some new in the automation
field may need a refresher on this powerful technology.
Simply put, the HART (Highway Addressable Remote
Transducer) Protocol is the global standard for sending and
receiving digital information across analog wires between
smart devices and control or monitoring system.
More specifically, 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.