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HART (Highway Addressable Remote Transducer)

The very known 4-20 mA signal is a tried, tested and widely used standard for transmitting sensor data to the Control system. However only limited amount of information is sent by a 4-20 mA signal.
HART (Highway Addressable Remote Transducer) protocol enhances these operations by transmitting digital data along with the 4-20 ma signal – without disturbing the signal. HART also permits two-way communications. It also has a digital modes that allows instrument to be connected to a single cable, cutting installation costs drastically.
HART makes use of the Bell 202 frequency shift keying (FSK) standard to superimpose low-level digital signals on the 4-20 mA circuit enabling more information than just the process variable to communicate between transmitters and receivers.
 
HART and The Conventional Process Loop

HART is best understood by looking at how it evolved from a conventional process loop. Figure  is a simplified diagram of the familiar analog current loop. The process transmitter signals by varying the amount of current flowing through itself. The controller detects this current variation by measuring the voltage across the current sense resistor. The loop current varies from 4 to 20 mA at frequencies usually under 10 Hz.
analog current loop
Figure is the same thing with HART added. Both ends of the loop now include a modem and a "receive amplifier."  The receive amplifier has a relatively high input impedance so that it doesn't load the current loop. The process transmitter also has an AC-coupled current source, and the controller an AC-coupled voltage source. The switch in series with the voltage source
(Xmit Volt Source) in the HART controller is normally open. In the HART Controller the added components can be connected either across the current loop conductors, as shown, or across the current sense resistor. From an AC standpoint, the result is the same, since the Pwr Supply is effectively a short circuit. Notice that all of the added components are AC-coupled, so that they do not affect the analog signal. The receive amplifier is often considered part of the modem and would usually not be shown separately. We did it this way to indicate how (across which nodes) the receive signal voltage is derived. In either the Controller or the Transmitter, the receive signal voltage is just the AC voltage across the current loop conductors
HART added analog loop
superimposing a voltage of about 500 mV p-p across the loop conductors. This is seen at the
process transmitter terminals and is sent to its receive amplifier and demodulator.Figure 4.2 implies that a Master transmits as voltage source, while a Slave transmits as a current source. This is historically true. It is also historically true that the lowest impedance in the network -- the one that dominates the current-to-voltage conversion -- was the current sense resistor. Now, with some restrictions, either device can have either a low or high impedance. And the current sense resistor doesn't necessarily dominate.As mentioned earlier the Hart protocol operates using the FSK principal. The digital data is made up from two frequencies viz. 1200 Hz and 2200 Hz representing bits 1 and 0 respectively. Sinusoidal waves of these frequencies are superimposed on the DC analog signal cables to give simultaneous analog and digital communications.

Features of HART protocol
  • Field proven concept that is easy to understand and use
  • Compatible with existing 4-20 mA systems
  • Simultaneous point-to-point 4-20 mA and digital communication
  • Alternative multi-drop mode
  • Measured variables, tag no., range and span settings, device information,
  • diagnostics and simple messages transmitted
  • Digital response time of 500 msec; burst mode response of 300 msec
  • Open architecture; freely available to any vendor and every user
HART Protocol Structure
HART follows the basic Open Systems Interconnection (OSI) reference model, developed by the International Organization for Standard (ISO). The HART protocol uses a reduced OSI model, implementing only layers 1,2 and 7.  
  
HART OSI Reference model
Layer No.
Layer
Layer Function
HART Function
7
Application
Provides formatted
data
HART instruction
6
Presentation
Converts data

5
Session
Handles the dialog

4
Transport
Secures the trans-
port connection

3
Network
Establishes network
connection

2
Link
Establishes the data
link connection
HART protocol
regulations
1
Physical
Connects the equip-
ment
Bell 202
HART's protocol command set is organized into three groups. Universal commands are implemented by all HART devices and provide interoperability across products from different manufacturers. Universal commands include: manufacturer and device type; primary variable and units; current output and percent of range; four predefined dynamic variables; eight-character tag, 16 character descriptor, and date; and several more.
HART's common-practice command set is used in many HART field devices,but not all, and include such functions as: writable transmitter ranges; ability to set zero and span; perform self-test; and more. Device-specific commands are the third set and are unique to a particular field device. Functions of the device-specific command set include: start, stop, or clear totalizer; select primary variable; PID set point, and tuning parameter manipulation.
Using HART transmitters in intrinsically safe installations requires special isolated (active)intrinsic barriers capable of passing the digital FSK data while maintaining safety on the 4-20 mA circuit.

Specification
HART
Governing Standard(s)
FSK modulation on
4-20mA signal
Communication Methods
Master/Slave
Communication Speed
1200 bits/s
Max. Data Size
8 bits
Max. Stations
15 Slaves
Maximum Cable Length
2000 mtr.

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