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 to improved 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 savings of 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.

The simpler system design implies that fewer system drawings will be needed in order to develop a fieldbus system. This also has the advantage that the simpler design 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 the devices included in the system and to also easily interpret the interaction between the individual devices. This will make discovering the source of any problems and carrying out maintenance much simpler, and thus will reduce the overall debugging time.

The debugging and maintenance of the system will also be enhanced due to the fact that fieldbus enables online diagnostics to be carried out on individual field devices. The online diagnostics include functions such as open wire detection and predictive 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 the simplification of the collection of information from field devices. Measurement and 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 will free the control system for other more important tasks. The reduction in information complication will allow the development of better and more effective process control systems.

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

With fieldbus technology, field instruments can be calibrated, initialised, operated and repaired faster than most conventional analog instrumentation. this leads to an overall 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 for process control, there are currently only two real protocols for fieldbus, being ISP and WorldFIP and while there is still two protocols rather than a world standard, it is better than a possible many. Work is being done to merge the two protocols into a 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 of field 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 the Data Link layer or the Application layer. As far as the end user is concerned, they should simply work. The user will only be required to have a limited knowledge of the management services, because some of the information generated by them will be needed if a problem occurs in the system. In fact, it should only be necessary for the 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 major groups:

1. WorldFIP (World Factory Instrumentation Protocol)
2. ISP (Interoperable Systems Project)

However, recently, these two groups have joined together to form the Fieldbus 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 Electrotechnical Commission) and the ISA (Industry Society of America ) are currently working on an international standard known as SP50. This standard will hopefully allow the manufacturers of fieldbus equipment all around the world to produce compatible instruments for industrial applications. WorldFIP, ISP and FF have pledged that they 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 purchase new systems at an excessive cost.

At the time of writing, information regarding actual market share for the Fieldbus Foundation and Profibus-ISP was not available, but Process Engineering’s Instrumentation Supplement for 1994, predicts that the Fieldbus Foundation will take the greater market share.

World Factory Instrumentation Protocol
The World Factory Instrumentation Protocol (WorldFIP) was developed from an earlier 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 North American division. Each division is motivated by similar goals and similar implementations, 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 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 Fieldbus Foundation 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

1.  Physical - Completed. Specification includes

• 31.25 kbit/sec, 1 Mbit/sec and 2.5 Mbit/sec data transfer rates.

• Requirements for fieldbus component parts.

• Media and network configuration requirements for data integrity and interoperability between devices.

2. Data Link - Balloting 1995. Draft standard by 1996.
3. Application - Balloting in 1995.
4. User - In Committee now. Balloting late 1995.
5. System & Network Management - expect to be completed mid 1996.

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