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Fieldbus is a generic-term which describes a new digital communications networkwhich will be used in industry to replace the existing 4 - 20mA analogue signal.The network is a digital, bi-directional, multidrop, serial-bus, communicationsnetwork used to link isolated field devices, such as controllers, transducers,

actuators and sensors. Each field device has low cost computing power installed init, making each device a smart device. Each device will be able to execute simplefunctions on its own such as diagnostic, control, and maintenance functions as

well as providing bi-directional communication capabilities. With these devicesnot only will the engineer be able to access the field devices, but they are also ableto communicate with other field devices. In essence fieldbus will replacecentralised control networks with distributed-control networks. Therefore fieldbusis much more than a replacement for the 4 - 20mA analogue standard.

The fieldbus technology promises to improve quality, reduce costs and boost

efficiency. These promises made by the fieldbus technology are derived partlyfrom the fact that information which a field device is required to transmit orreceive can be transmitted digitally. This is a great deal more accurate thantransmitting using analogue methods which were used previously. Each fielddevice is also a smart device and can carry out its own control, maintenance anddiagnostic functions. As a result it can report if there is a failure of the device ormanual calibration is required, this increases the efficiency of the system andreduces the amount of maintenance required.

Each field device will be more flexible as they will have computing power. One

fieldbus device could be used to replace a number of devices using the 4 - 20mAanalogue standard. Other major cost savings from using fieldbus are due to wiringand installation - the existing 4 - 20mA analogue signal standard requires eachdevice to have is own set of wires and its own connection point. Fieldbuseliminates this need so only a single twisted pair wiring scheme is required.

The International Debate

Although fieldbus technology has been around for the past 8 years it is still notwidely used. The reason for this delay is due to the lack of an international fieldbusprotocol standard which will ensure complete interchangeability andinteroperability between different suppliers. The major players in the fieldbusdebate areWorldFIPandISP. The completion of a fieldbus standard is notforecasted until 1997 meaning there is still a long time to wait. With consumersbecoming impatient many companies have decided to released there own systemswhich work off different standards.

The WorldFIP standard is based on the Factory Information Protocol (FIP). Itworks on a distributed database and time-service system and has a bus manager

which issues tokens on an accurate time basis which matches device requests. TheInterOperable System Project (ISP) is based on Profibus, which controls messages

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by using a token-passing method where a token circulates through all participatingstations and the station may talk while in possession of the token. The token iscirculated according to a preconfigured timing.

There is a technical report which defines the full functionality of the user layer intheISAs SP50version of the standard. The focus of the report is to define thecomplete distribution of data acquisition and control functions within the fielddevices. It is this layer on which a decision is needed to be reached to result in trueinterchangeability and interoperability. This would give users a free choice ofequipment instead of having to be locked into using equipment which uses one oftwo totally different standards. Currently both WorldFIP and ISP organisations areboth trying to implement as many high level control functions as possible in totheir new fieldbus products, Therefore we may be on the path to two differentstandards.

Fieldbus is no longer simply a communications standard but is a complete open,integrated measurement and control system which is looking to change the face ofprocess control forever.

Advantages of Fieldbus

The fieldbus has a multitude of advantages that the end users will benefit from.

The major advantage of the fieldbus, and the one that is most attractive to the

end user is its reduction in capital costs. The savings attained by the user stem

from three main areas, initial savings, maintenance savings, and savings due toimproved systems performance.

Initial Savings

One of the main features of the fieldbus is its significant reduction in wiring. Each

process cell requires only one wire to be run to the main cable, with a varying

number of cells available. The cost of installing field equipment in a fieldbus

system is thus significantly reduced. Installation costs are further reduced due to

the fact that the fieldbus it is a multi-drop rather than point-to-point system and

the multidrop network can offer a 5:1 reduction in field wiring expense.

The price of equipment is reduced significantly in a fieldbus system, with savingsof approximately $50 per field device possible.

The fieldbus system requires less labour to install than conventional bus systems,and saves money due to a reduction in materials needed for the installation.

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The simpler system design implies that fewer system drawings will be needed inorder to develop a fieldbus system. This also has the advantage that the simplerdesign will result in less complex and faster bus systems.

Maintenance Savings

The fact that the fieldbus system is less complex than conventional bus systems

implies that there will be less overall need for maintenance. The simplification of

systems means that the long term reliability of the bus system is increased.

With the fieldbus system, it is possible for the operators to easily see all of thedevices included in the system and to also easily interpret the interaction betweenthe individual devices. This will make discovering the source of any problems andcarrying out maintenance much simpler, and thus will reduce the overalldebugging time.

The debugging and maintenance of the system will also be enhanced due to thefact that fieldbus enables online diagnostics to be carried out on individual fielddevices. The online diagnostics include functions such as open wire detection andpredictive maintenance and simplify tasks such as device calibration.

Improved Systems Performance

Fieldbus allows the user increased flexibility in the design of the bus system. Some

algorithms and control procedures that with conventional bus systems must be

contained in control programs can now reside in the individual field devices,

reducing the overall size of the main control system. This reduces the overall

systems cost and makes future expansion a simpler prospect.

System performance is enhanced with the use of fieldbus technology due to thesimplification of the collection of information from field devices. Measurementand device values will be available to all field and control devices in engineering

units. This eliminates the need to convert raw data into the required units and willfree the control system for other more important tasks. The reduction ininformation complication will allow the development of better and more effectiveprocess control systems.

With fieldbus technology, two-way communication between field devices and thecontrol system is made possible. System performance is enhanced due to the abilityto communicate directly between two field devices rather than via the controlsystem. This also enables several related field devices to be combined into onedevice.

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With fieldbus technology, field instruments can be calibrated, initialised, operatedand repaired faster than most conventional analog instrumentation. this leads to anoverall reduction in time required to operate the fieldbus system.

Other Advantages

As well as the cost advantages that fieldbus technology embodies, there are many

other miscellaneous advantages that are included in the fieldbus package.

Although it is a major challenge trying to develop a single worldwide protocol forprocess control, there are currently only two real protocols for fieldbus, being ISPand WorldFIP and while there is still two protocols rather than a world standard, itis better than a possible many. Work is being done to merge the two protocols intoa standard which will be a major advantage. The fact that eventually all fieldbus

equipment will be standardised will mean that expansion of a system or addition offield devices will be extremely simple, requiring no interfaces or converters.

The fieldbus protocol involves only four layers and a set of management services.Fieldbus has the advantage that the user should not have to be concerned with theData Link layer or the Application layer. As far as the end user is concerned, theyshould simply work. The user will only be required to have a limited knowledge ofthe management services, because some of the information generated by them willbe needed if a problem occurs in the system. In fact, it should only be necessary forthe user to concern themselves with the Physical and User layers.

Fieldbus Organisations

This section establishes who have been the major instigators of fieldbus

development over the past several years. A brief summary of the developing

standard is also covered.

The major players in the fieldbus area were previously dominated by two majorgroups:

WorldFIP(World Factory Instrumentation Protocol) ISP(Interoperable Systems Project)

However, recently, these two groups have joined together to form theFieldbus

Foundation(FF). The Fieldbus Foundation and another organisation known as

Profibus-ISP are now competing for market dominance.

Two standards bodies known as the IEC (International ElectrotechnicalCommission) and the ISA (Industry Society of America ) are currently working onan international standard known as SP50. This standard will hopefully allow the

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manufacturers of fieldbus equipment all around the world to produce compatibleinstruments for industrial applications. WorldFIP, ISP and FF have pledged thatthey will eventually evolve their products to meet the standard when it arrives.However, when the standard finally does arrive, users of existing non-conforming

equipment will run the risk of having obsolete equipment or having to purchasenew systems at an excessive cost.

At the time of writing, information regarding actual market share for the FieldbusFoundation and Profibus-ISP was not available, but Process EngineeringsInstrumentation Supplement for 1994, predicts that the Fieldbus Foundation willtake the greater market share.

World Factory Instrumentation Protocol

The World Factory Instrumentation Protocol (WorldFIP) was developed from anearlier French National Standard known as NFC 46-600, or more commonly as FIP.

It is a consortium of companies producing field bus instruments that use a

messaging system. Time critical options are supposedly guaranteed in a WorldFIP

implementation. WorldFIP plans to add a device description tool, known as the

WorldFIP Device Builder. The Device Builder will automatically inform the control

system what features and parameters each instrument connected to the bus has.

Interestingly , WorldFIP is divisional in nature with a UK, European and NorthAmerican division. Each division is motivated by similar goals and similarimplementations, but each operates almost autonomously from the others.

Some of the major members of WorldFIP include:

Honeywell (Arizona) Bailey Controls (Ohio) Cegelec (Paris) Allen Bradley Corporation (Ohio) Telemecanique (Paris) Ronan Engineering Co. (California) Square D Electricite de France (France) Elf (France)

Interoperable Systems Project

The Interoperable Systems Project (ISP) implementation is based on the German

National Standard DIN STD19245, also known as Process Field Bus, or Profibus.Profibus is similar to the token passing network commonly implemented on many

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networks today. The ISP extension to Profibus is the Device Description Language

(DDL). DDL allows an instrument added to the bus system to communicate to a

master control what its functions and capabilities are.

Some of the major members of ISP include:

Siemens (Germany) The Rosemount Group (Minnesota) Fisher Controls, Inc. (Texas) Foxoboro Co. (Massachusetts) ABB Co. (Sweden) Yokogawa Electric Corporation (Tokyo)

Fieldbus Foundation

On a positive note ISP and WorldFIP (North American division) have been working

together since late 1993 on a possible merger of their technology. A single

solution has been what industry has needed for a long time, so in June of 1994,

the Fieldbus Foundation (FF) was set up between ISP and WorldFIP (NA).

However, at least 1 to 2 years of delay is expected before a complete product can

be produced.

Profibus-ISP

Effectively a breakaway group of the Profibus and ISP organisations, this group

effectively announced to the world that they will have their own fieldbus

communications system ready in approximately June/July 1994. Profibus-ISP is

derived from the Profibus and ISP products, and hence has the features of both

with some small additions.

At the time of writing, little information on Profibus-ISP and the FieldbusFoundation was available.

IEC/ISA SP50

The ISA/IEC are developing a standard with the working name of SP50. The

standard will follow the ISO/OSI seven layer model for data communications with

an additional eighth layer which focuses on the product interoperablility.

Current progress on the SP50 is as follows

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