For many years, the field communication standard for process automation equipment has been a milliamp (mA) analog current signal. The milliamp current signal varies within a range of 4-20mA in proportion to the process variable being represented. In typical applications a signal of 4mA will correspond to the lower limit (0%) of the calibrated range and 20mA will correspond to the upper limit (100%) of the calibrated range. If the system is calibrated for 0-100 PSI, then an analog current signal of 12mA (50% of range) will correspond to a pressure of 50PSI. Virtually all installed systems use this international standard for communicating process variable information between process automation equipment.
HART Field Communications Protocol extends this 4-20mA standard to enhance communication with smart field instruments. The HART protocol was designed specifically for use with intelligent measurement and control instruments which traditionally communicate using 4-20mA analog signals. HART preserves the 4-20mA signal and enables two-way digital communications to occur without disturbing the integrity of the 4-20mA signal.
The HART protocol permits the process variable to continue to be transmitted by the 4-20mA analog signal and additional information pertaining to other variable, parameters, device configuration, calibration, and device diagnostics to be transmitted digitally at the same time. Thus, a wealth of additional information related to plant operation is available to central control or monitoring systems through HART communications.
Fig 1 HART digital communication signal superimposed
On the 4-20mA analog signal
Fig 2 HART uses FSK to encode digital information
On top of the 4-20mA analog signal
1.1WHAT IS HART:-
Stands for Highway Addresable Remote Transduce.
Was developed by Fisher-Rosemount to retrofit 4- 20mA current loop transducers with digital data communication.
It is a master-slave, bidirectional industrial field communication protocol developed in late 1980â„¢s used to communicate between smart field instruments and host systems.
Makes use of the Bell 202 Frequency shift keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4- 20mA
1.2 WHY HART
There are several reasons to have a host communicate with a field instrument. These include:
Reading the values of additional measurements provided by the device
Device Health and Status
And much more!
1.3 HART BENEFITS:
Information in HART enabled devices when fully used provides benefits in different phases including the planning and engineering phase, Installation and Commissioning Operations and Maintenance, and Asset Productivity
Key benefits of this unique open standard communication technology are:
4-20 mA compatibility with simultaneous digital information available
Easy to use and understand
Low risk-highly accurate and robust
Cost effective implementation for both users and suppliers
Available in a wide variety of device types
Supported by most industry device and systems suppliers
Fully interoperable and reliable
PERFORMANCE FEATURES & HCF USER ORGANIZATION
Introduced in 1989, this protocol has been proven successful in many industrial applications and enables bidirectional communication even in hazardous environments. HART allows the use of two masters: the engineering console in the control room and a second device for operation on site, eg. A PC laptop or a handheld terminal.
2.1 TYPICAL PERFORMANCE FEATURES:-
The most important performance features of the HART protocol include:
Proven in practice, simple design, easy to maintain and operate
Compatible with conventional analog instrumentation
Simultaneous analog and digital communication
Option of point-to-point or multipoint operation
Flexible data access via up to two master devices
Supports multivariable field devices
Sufficient response time of approx. 500ms
Open de-facto standard freely available to any manufacturer or user
2.2 HCF user organization:-
SUPPORT FOR IMPLEMENTATION AND OPERATION 
The HART protocol is an open communication protocol which interfaces the master device with the field device and can be implemented by manufacturer and freely employed by the user. The required technical support is provided by the HART Communication Foundation (HCF). This manufacturer-independent, not-for-profit organization encourages widespread use of the HART technology. HCF assumes the responsibility of coordinating and supporting the open protocol strandard and manages within this framework the device descriptions of all registered devices.
Fig 3: HART and HCF logo
THE HART PROTOCOL & PRINCIPLE
The HART Protocol - An Overview:
HART is an acronym for "Highway Addressable Remote Transducer". The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA as show in Figures 1 and 2. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. The HART protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.
HART is a master/slave protocol which means that a field (slave) device only speaks when spoken to by a master.
The HART protocol can be used in various modes for communicating information to/from smart field instruments and central control or monitoring systems. HART provides for up to two masters (primary and secondary) as show in Figure 3.
This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system.
The most commonly employed HART communication mode is master/slave communication of digital information simultaneous with transmission of the 4-20mA signal as shown in Figure 4.
Fig 4: The HART protocol enable two master devices to access information in slave devices
The HART Command Set is organised into three groups and provides read/write access to the wealth of additional information available in smart field instruments employing this technology
Fig 5 : master/slave communication (normal HART mode)
3.1 HART PRINCIPLES:-
The HART protocol makes use of the BELL 202 FREQUENCY SHIFT KEYING(FSK) standard to superimpose digital communication signals at low level on top of the 4-2mA
HART uses relatively low frequencies (1200 and 2200 Hz) and this restricts it to communicate at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device.
3.2 HART PROTOCOL:-
Hart communicates point-to-point, with a slave eg. Hand-held device, under the control of a master.
Fig 6: HART frame format
3.3 HART FRAME FORMAT:-
FIELD NAME LENGTH(BYTES) PURPOSE
Preamble 5-20 Synchronization and carrier defects
Start Byte 1 Specifies master number
Adress 1-5 Specifies slave, specifies master and indicates burst mode
Command 1 Numerical value for the command to be executed
Number of data bytes 1 Indicates size of the data field
Status Master(0)slave(2) Execution and health reply
Data 0-253 Data associated with the command
Check sum 1 XOR of all bytes from start byte to last byte of data
4.1THE HART COMMANDS:-
Must be implemented by all HART devices and provide interoperability across the large and growing base of products from different suppiliers of HART technology, like:
Read primary variables and units
Read manufacturer and device type
Read current output and percent of range
Read sensor serial number and limits
COMMON PRACTICE COMMAND(optional):
Common practice commands provide functions implememnted by many, but not necessarily all,HART communication devices, like:
Read a section of up to four dynamic variables
Write damping time constant
Write transmitter range
Set fixed output current and perform self-test
DEVICE SPECIFIC COMMAND(user-defined):
Represent functions that are unique to each field device. These commands across setup and calibration information, as well as information about the construction of the device, like:
Read or write sensor type
Start ,stop or clear totalizer
Read or write alam relay set point
4.2 HART COMMAND SUMMARY:-
4.3 EXAMPLES OF COMMANDS:-
COMMAN PRACTICE COMMANDS:
CONNECTING HART DEVICES
HOST & FIELD DEVICES:
Devices which support the HART protocol are grouped into master (host) and slave (field) devices. Master devices include handheld terminals as well as PC-based work places, eg. In the control room. HART slave devices, on the other hand, include sensors, transmitters and various actuators. The variety ranges from two-wire and four-wire devices to intrinsically safe versions for use in hazardous environments.
COMMUNICATION OVER ANALOG SIGNAL LINE:
The HART data is superimposed on the 4 to 20 mA signal via a FSK modem. This enables the devices to communicate digitally using the HART protocol, while analog signal transmission takes place at the same time.
FSK MODEM:: LINK TO ANALOG SIGNAL:
Field devices and compact handheld terminals have an integrated FSK modem, whereas PC stations have a serial interface to connect the modem externally. Fig shows a typical connection scheme of a HART host device and a HART field device. HART communication is often used for such simple point-to-point connections. Nevertheless, many more connection variants are possible.
Fig 7: Connection of HART master devices
In extended systems, the number of accessible devices can be increased by using a multiplexer. In addition to that , HART enables the networking of devices to suit special applications. Network variants include multidrop, fsk bus and networks for split-range operation.
5.1 NUMBER OF PARTICIPANTS & ADDRESSING:-
The HART communication shown in fig 2 is referred to as point-to-point connection, ie. The HART master device is connected to exactly one HART field device (POINT-TO-POINT COMMUNICATION). This connection variant requires that the device address of the field device be always set to zero since the operating program uses this address to establish communication. (DEVICE ADDRESS ZERO)
Fig 8: point to point node
Fig 3 shows the use of a multiplexer system, which enables a large number of HART devices to be connected in a network. The user selects a particular current loop for communication via the operating program. As long as the communication takes place, the multiplexer connects the current loop to the host. Due to the cascaded multiplexer structure, the host can communicate
With many (>1000) devices, all with the address zero.(multiplexer selects current loops at shifted time intervals)
Fig 9: HART communication via multiplexer
The HART protocol was originally designed for transmitters .The multidrop mode was also developed for them. In multimode operation the devices, exchange their data and measured values only via the HART protocol. The analog current signal serves just to energize the two-wire devices, providing a direct current of 4mA.(multidrop mode for transmitters)
In multidrop mode, up to 15 field devices are connected in parallel to single wire pair fig 4 . The host distinguishes the field devices by their preset addresses which range from 1 to 15.
HART COMMUNICATION LAYERS
Fig 10: HART protocol implementing the OSI model
The HART protocol utilizes the OSI reference model. As is the case for most of
the communication systems on the field level, the HART protocol implements
implements the OSI-
only the layers 1, 2 and 7 of the OSI model. The layers 3 to 6 remain empty
layers 1, 2 and 7
since their services are either not required or provided by the application
layer 7 (see Fig. 9).
Fig 11: Device access
DDL DEVICE DESCRIPTION
FIG 12: HART handheld devices
ADVANTAGES & DISADVANTAGES
The HART protocol provides users with the best solution and migration path for capturing the benefits of enhanced communication with smart instrumentation. No other communication technology can match the base of support or wide range of products that are available with HART today. The technology is easy to use and HART-compatible products are available from major instrumentation suppliers to address virtually all process measurement and control applications.
The emergence of fieldbus will not displace HART in either existing or new production facilities. HART provides users with many of the same benefits while retaining the compatibility and familiarity of existing 4-20 mA systems. HART allows the cost saving benefits of remote communication, flexible/accurate digital data transmission, field device diagnostics, and powerful multiparameter instruments to be captured without replacing entire systems.
Connection to current and future plant networks is assured by the digital communication capability and large installed base (more than 5,000,000 installations and growing rapidly).
Support of the HART Communication Foundation ensures that the technology will continue to evolve for serving the needs of smart instrumentation today and tomorrow.
please read http://www.seminarprojects.org/t-hart-hi...transducer
for getting more information about HART (Highway Addressable Remote Transducer) technology