Fieldbus networks are a form of LAN dedicated to data acquisition and the control of sensors and actuators in the process plant or factory floor. The fieldbus typically operates using low cost twisted pair cables and they tend to be optimised for exchange of short point-to-point status and command messages whereas the performance of an Ethernet is measured in terms of its capability to transfer large data blocks (but read on to see how the two are converging). Compared to legacy 4–20 mA installations the use of a fieldbus dramatically reduces the complexity of the control system and PLC/DCS requirements. Other benefits include reduced cabling and the elimination of junction boxes and control panels. As a direct result of this decrease in complexity, installation time is reduced and commissioning is more efficient as the diagnostic functions of the fieldbus identify connection errors.
The fieldbus as we know it today was first conceived as a simple digital replacement for 4-20 mA communication, its debut being accelerated through the introduction of smart field devices during the 1980s. While that same 4-20 mA analog communication was standardised as far back as the 1970s the so-called `Fieldbus War' still continues. One report suggests that literally hundreds of fieldbuses have been developed over the years while a current directory of fieldbus sites, last updated in 2002, lists almost 70 sites referring to variations on this theme, although some of these are very industry specific. (Note, however, should be made here that whereas CANbus was originally developed to simplify wiring in motor vehicles it is now used increasingly in factory automation as a result of its outstanding features. CAN was also the basis for the development of both DeviceNet [Allen Bradley] and Honeywell's SDS.) Two of the earliest offerings were FIP, which appeared during 1985, and Profibus that was first launched during 1986. While there are still proprietary fieldbus networks available, the trend is towards open networks. For a network to be able to claim that it is open means that devices from different manufacturers can be safely connected to the fieldbus and that devices from one manufacturer can be replaced with functionally equivalent devices from another manufacturer. Another requirement for open systems is that they must be freely available at reasonable cost and without licence restrictions.
While often listed with other fieldbus standards, HART (highway addressable remote transducer), which may still be the leading communication protocol in use today, is somewhat different in that it allows the transfer of digital data to the device using the existing 4-20 mA analog signal wires. HART appeared in the late 1980s and is an open protocol with more than 60 manufacturers supporting it, using it as the basis for the development of smart instrumentation. Unlike most other digital communication technologies the HART protocol provides a solution that is backward compatible with the installed base of 4-20 mA instrumentation. While this can be viewed as a major advantage, many experts consider HART as an intermediate step in the road to full fieldbus implementation.
The Instrument Society of America (ISA) through its SP50 committee was the original focal point for fieldbus development, ISA SP50 having been responsible for the 4-20 mA standard introduced during the '70s. (In 2000 the ISA changed its name to the Instrumentation, Systems, and Automation Society.)
As it is the IEC that has responsibility for the development of an international fieldbus standard these two groups have met regularly since 1984. The process of specifying the protocol remains unfinished, with infighting among existing protocols contributing to this as well as a decision to develop new technology instead of sanctioning an existing option. It must be said that it appears that much of the infighting was between control system suppliers and field device suppliers, the fieldbus being seen as an opportunity to realign the balance of power in the process industry in favour of the latter.
Delays in the standardisation process saw the rise of competitors such as the Interoperable Systems Project (ISP) and WorldFIP North America, which combined in the early 1990s to form the Fieldbus Foundation (FF). The Profibus User Organisation took the remaining technology from the ISP development effort and combined this with Profibus DP to establish the Profibus PA protocol. Profibus PA, behind which Siemens was the driving force, uses the standard IEC 1158-2 fieldbus physical layer as used also by the FF.
Acceptance of the major fieldbus standards is driven very strongly by geography. The FF (strongly supported by Fisher-Rosemount) has the support of powerful field device and control system suppliers in North America and Japan. Profibus on the other hand is very strong in Europe (and Africa) and in China. While some suppliers (eg, ABB) support both FF and Profibus it appears that these companies will offer FF products in North America and Profibus products in Europe.
The conundrum is that the drivers of FF such as Fisher-Rosemount, Honeywell, Foxboro and Yogogawa have no plans to introduce Profibus compatible devices in the near future while Siemens has no immediate plans to introduce FF-compatible products. While 10 years ago no serious design engineer would have suggested using the Ethernet for networking factory floor devices it is a worldwide standard and with the coming explosion of factory floor data traffic perhaps industrial Ethernet will indirectly lead to factory floor standardisation.
While there are those that lament the proliferation of fieldbus standards they should take note of the criticism that Microsoft receives as the dominant PC operating system supplier. All fieldbuses have advantages and disadvantages, and the correct choice must be carefully weighed up by the end user. In this regard, mention must perhaps be made of one other widely used Fieldbus, namely LONWorks, which has a major competitive advantage in that it works over greater distances and is extendible to many thousands of nodes. LONWorks, developed by Echelon in 1988, is seen by many as being an extension of the advantages of HART into a true fieldbus. The best analogy that I have found in the literature is the motor vehicle. You are unlikely to use your sedan to carry a load of sand from the local hardware store, but will a bakkie suffice, or do you really need that 5-ton truck?
Device manufacturers themselves have addressed the issue of the plethora of fieldbus protocols. As just one example, Moeller Electric, during 2001, released its XI/ON family of modular open I/O fieldbus technology. This product family provides fast communication via all the common fieldbus protocols, including Profibus DP, CANopen, InterBus and DeviceNet. Each I/O terminal station has a gateway that acts as the interface between the fieldbus and the I/O modules. One of the major objectives behind this release by Moeller is that the user's investment in both product and investment time is safe in the event of fieldbus protocol changes at a later date.
The 'Million Dollar' question is: are we soon going to see a single Fieldbus standard and the simple answer is no time soon. The prospect of a single fieldbus standard has in fact reduced over time with the absence of a true industry standard and a lack of openness and interoperability between devices from different manufacturers. What has in fact happened is that manufacturers have joined forces with specific fieldbus providers to share expertise, increase the level of integration and develop compatible products. This has required substantial investment and has thus made the emergence of a single standard more unlikely. Having said that there are only a few major players and Profibus for example was recently reported as having a 39% market share in Europe and 20% worldwide.
Profibus itself perhaps provides us with some insight into where the future will take us. During 2002 they announced the release of PROFInet, which instead of merely using twisted pairs operates on the standard Ethernet TCP/IP protocol. Any existing Profibus (or other fieldbus) network can interoperate with PROFInet using a Proxy. In fact in many applications (particularly those that do not need to be connected to the IT world) the Profibus network is still the recommended solution for connecting decentralised devices. The development of PROFInet was however driven by the increased requirement of users to be able to connect directly to an Ethernet.
While today, PROFInet can only offer the basic communication functionalities of Profibus it is intended to convert all existing Profibus application profiles over the next few years. While PROFInet is seen as a way forward into the future, Profibus in its existing form is expected to continue to be used for at least the next decade and even longer for the smaller standalone automation system where it will continue to be the most cost-effective solution.
However, maybe future standardisation on the Ethernet will see an end of the Fieldbus War.
Dr Maurice McDowell has many years' experience as a technical journalist, editor, business manager and research scientist. His third party analyses of world-class companies and processes, as well as his insight into industry and technology trends are well respected.
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