Introducing HART

Original instructions

HART

The Highway Addressable Remote Transducer () protocol provides digital communication to microprocessor-based analog process control instruments.

HART uses the Bell 202 frequency-shift-keying () standard to superimpose a digital signal on top of the 4-20mA current loop analog signal:

the analog signal communicates the primary measured process variable value
the digital signal communicates additional instrument information including instrument status, additional process variables, configuration data, and diagnostics

The digital signal shifts between a frequency of 1200 Hz (representing a binary 1) and a frequency of 2200 Hz (representing a binary 0):

These digital signal frequencies are higher than the typical analog signaling frequency range of 0...10 Hz. The digital signal is typically isolated using a passive with a cut-off frequency in the range of 400...800 Hz. The analog signal is likewise isolated using a passive .

The separation in frequency between HART and analog signaling allows both signals to coexist on the same . Because the HART digital signal is phase continuous, the HART digital signal:

does not interfere with the 4-20 mA signal, and
allows the analog process to continue operating during HART digital communication

Half-duplex Communication Protocol

HART communication is in design, which means that a HART-compliant instrument does not simultaneously transmit and receive.

Master - Slave Protocol

HART is a master-slave protocol. A responds only when commanded by a . Examples of HART-compliant instruments include:

HART masters:
- asset management software (AMS) running on a PC
- a HART interface module, for example, the HART modem feature of both the BMEAHI0812 input module and the BMEAHO0412 output module when communicating with a HART process control instrument (such as a HART-compliant sensor or actuator)
- a hand-held device temporarily attached to the network
HART slaves:
- a HART process control instrument
- a HART interface module, for example, the HART modem feature of both the BMEAHI0812 input module and the BMEAHO0412 output module when operating as a slave with asset management software (AMS) or a HART server

Highway Addressable Remote Transducer protocol: A bi-directional communication protocol for transmitting—across analog wires—digital information between intelligent field devices and a host control/monitoring system. For more information, refer to the HART Communication Foundation web site: www.hartcomm.org.

A standard defining the operation of frequency shift keying: a frequency modulation scheme that transmits digital information by means of discrete frequency changes in a carrier wave.

A frequency-based filter that permits transmissions only above a pre-set frequency threshold to pass. In HART, the frequency threshold is typically set in range of 400...800 Hz: transmissions above this threshold—HART digital signals—are allowed to pass through; transmissions beneath this threshold are filtered.

A frequency-based filter that permits transmissions only below a pre-set frequency threshold to pass. In HART, the frequency threshold is typically set in range of 25 Hz: transmissions below this threshold—analog signals—are allowed to pass through; transmissions above this threshold—HART digital signals—are filtered.

An analog electrical signaling scheme, that allows a device to be monitored or controlled over a pair of conductors. Only one current level can exist in a current loop at any point in time. A digital signal can be added to the analog current loop using the HART protocol, enabling additional communication with the analog device.

A system of communication between 2 networked devices that provides for transmissions in both directions, but in only one direction at a time.

A HART compliant smart field device, which responds via the HART protocol only when commanded to do so by a HART master.

A HART host application, typically resident in a PC. For example, asset management software.

A modem that serves as the pass through device between one or more HART master devices, and multiple HART slave devices.