HART – HIGWAY ADDRESSABLE REMOTE TRANSDUCER PROTOCOL

Presented by,R.Vasanthan -14E113K.Sathish -14E100B.Ramanathan -14E89 T.Vijaya Baskar -14E120B.Vignesh Karthick -14E117

Terminologies Primary master ---- control system Secondary master ---- A hand-held communicator or a maintenance

computer. Slave – Transmitter, sensor, transducer, attenuator

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

1

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.

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.

HART Vs. FieldbusHART 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.

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.

Frequency Shift Keying

HART communication protocol

Super-impose sine wave on conventional4/20 mA signal

LOGIC 1 1,200 HzLOGIC 0 2,200 Hz

Communicationspeed (baud rate)= 1.2 kbps(1,200 baud)

-0,5 mA

+0,5 mA

TimeFSK 1,200 Hz 2,200 HzLOGIC “0”“1”

4/20 mADC current

1/1,200 sec

HART uses Frequency Shift Keying to encodedigital information on top of the 4/20 mA analog signal

-0,5 mA

+0,5 mA

Time

4/20 mADC current

1/1,200 sec

AB

The current modulation for “A” is positive cycleand “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.

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.

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

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

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

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

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.

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.

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.

COM

MAN

DS

Universal commands

Common practice commands

Device specific

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. 

THANK YOU