The HART protocol allows field instruments to communicate digitally by superimposing an FSK signal on top of the traditional 4-20mA analog signal. It operates at 1.2kbps using 1,200Hz and 2,200Hz frequencies to represent binary 1s and 0s. HART can be used in point-to-point or multidrop modes and supports both master-slave and burst communication modes. The physical layer defines cable and connector specifications while higher layers ensure reliable data transfer and define supported commands. HART is globally accepted as it preserves existing control strategies and infrastructure.

1. HART – HIGWAY ADDRESSABLE REMOTE
TRANSDUCER PROTOCOL
Presented by,
R.Vasanthan -14E113
K.Sathish -14E100
B.Ramanathan -14E89
T.Vijaya Baskar -14E120
B.Vignesh Karthick -14E117

2. Terminologies
 Primary master ---- control system
 Secondary master ---- A hand-held communicator or a maintenance
computer.
 Slave – Transmitter, sensor, transducer, attenuator

3. MODBUS Vs PROFIBUS
PROFIBUS MODBUS
• Profibus can run on multi-master mode using
RS-485
• Profibus can't operate on Ethernet
• Profibus is a very robust protocol that was
designed to automate entire plants.
• For situations where there are more points,
where different vendors are involved, or where
there is a hazardous environment
• Modbus can run only on single master mode
using ASCII,RTI
• Modbus can run on Ethernet (TCP/IP)
• Modbus is a very simple, easy to use, modem-
friendly protocol
• When connecting a controller in a point-to-
point configuration, or if there is only one
remote site

4. 1

5. How it Came
 Earlier all installed systems in process control industries used
4-20 mA international standard for communicating process
variable information between process automation
equipments.
 HART Field Communications Protocol extends this 4-20mA
standard to enhance communication with smart field
instruments, without disturbing the 4-20 mA line.

6. Why HART is globally acceptable?
 Is supported by all of the major vendors of process field instrument
 Preserves present control strategies by allowing traditional 4-20
mA signals to co-exist with digital communication on existing two-
wire loops
 Is compatible with traditional analog devices
 Can support cabling savings through use of multidrop networks
 Reduces operation costs, through improved management and
utilization of smart instrument networks.

7. HART Vs. Fieldbus
HART is compatible with existing wiring in older
plants.
Hart is understood, accepted and appreciated by
virtually everyone involved.
Fieldbus costs a lot more to implement, especially for
existing equipment.
Lot of instruments not radially available with
Fieldbus.

8. FREQUENCY SHIFT KEYING
 It is based on the Bell 202 telephone communication standard and
operates using the frequency shift keying (FSK) principle.
 The digital signal is made up of two frequencies— 1,200 Hz and 2,200 Hz
representing bits 1 and 0, respectively.
 Sine waves of these two frequencies are superimposed on the direct
current (dc) analog signal cables to provide simultaneous analog and
digital communications.
 It has a response time of approximately 2–3 data updates per second
without interrupting the analog signal.
 Masters are connected parallel to field devices, So devices can be easily
connected or disconnected and the current loop remains uninterrupted.

9. Frequency Shift Keying

10. HART communication protocol
Super-impose sine
wave on conventional
4/20 mA signal
LOGIC 1 1,200 Hz
LOGIC 0  2,200 Hz
Communication
speed (baud rate)
= 1.2 kbps
(1,200 baud)

11. -0,5 mA
+0,5 mA
Time
FSK 1,200 Hz 2,200 Hz
LOGIC “0”“1”
4/20 mA
DC current
1/1,200 sec
HART uses Frequency Shift Keying to encode
digital information on top of the 4/20 mA analog signal

12. -0,5 mA
+0,5 mA
Time
4/20 mA
DC current
1/1,200 sec
A
B
The current modulation for “A” is positive cycle
and “B” is negative cycle. The resultant current or
average current is zero. This meaning the FSK
current modulation will not disturb the
4/20 mA DC current.

13. HART Signal Levels
 HART protocol specifies that master device transmit a voltage signal whereas
slave device transmits a current signal.
 The current signal is converted in to corresponding voltage by a loop load
resistor(250 ohms), so all devices use voltage sensitive receiver circuits.
 The wave shape is sinusoidal, but trapezoidal waveform is acceptable. A
square wave is not acceptable.

14. COMMUNICATION MODES
 Master Slave Mode (Poll---Response Mode)
Maximum communication speed --- 2 messages/sec.
Configuration ---- Point-to-point or Multi-point
Master sends command (request) , slave responds (replies)
Request
Reply

16.  Burst Mode ( Broadcast Mode)
Slave transmits messages continuously (e.g. values of a measurand).
The communication speed --- 3 message/sec.
A slave device repeatedly sends a data message.
Used for fast updation of the value of a measured variable
Continuous Reply

17. Different HART networks
 Point-to-point Mode
 Both analog and digital communications are supported.
 4–20 mA signal is used to communicate one process variable.
 Additional process variables, configuration parameters, and other device
data are transferred digitally using the HART protocol

18.  Multidrop Mode
 Only digital communication is supported
 Requires only a single pair of wires
 15 slaves can be connected in parallel

19. Physical layer
 It defines physical and mechanical properties of connectors and cables.
 Also it defines signal characteristics.
 It uses asynchronous transmission.
 Masters & slaves are connected in parallel.
 Twisted pair cable with single shielded cable length of 3000 m.

20. Data link Layer
 Responsible for reliable transfer of message over
network.
 Organizes data and control byte into frames, adds
error detection codes to data stream.
 Assures symmetric access to communication channel
by both master and slave.
 Each HART transaction consists of master command
& slave response.
HART telegram
• Short frame format uses 1byte
• Long frame format uses 5bytes in
address field of telegram
• Telegram contains min. of 10 control
bytes & max. of 25 bytes.

21. Application layer
 Defines commands ,responses, data type, and
status reporting.
 HART commands enable a master device to
send instruction to field devices to return data
 Field devices respond by sending
acknowledgement telegram which contains data
or requested status.
COMMANDS
Universal commands
Common practice
commands
Device specific

22. References
 [1]. Introduction to HART
[Online]. Available FTP: http://en.hartcomm.org/
 [2]. BASICS OF HART PROTOCOL
[Online]. Available FTP :http://www.pacontrol.com/download/hart-protocol.pdf
 [3]. HART Communication
[Online]. Available FTP : http://www.samson.de/pdf_en/l452en.pdf
 [4]. HART INTRODUCTION
[Online]. Available FTP : http://www.slideshare.net/hart-37730657
 [5] ABSTRACT ON HART
[Online]. Available FTP : http://ieeexplore.ieee.org/hart
 [6] Ieee Spectrum Magazine, edition : September 2000.

23. THANK YOU