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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.
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