Special issue: Backplane bus standards

Special issue: Backplane Bus Standards With the advance to ever more powerful microprocessors and modular, multiprocessor systems and the consequent need for communication links between components, backplane bus standards are growing in importance. For microsystem designers, a reliable bus standard simplifies design procedures, enabling them to concentrate on the functionality of individual components with the assurance that they will work together correctly at the hardware level. Looking at the fast-changing field of backplane bus standards, this special issue is intended as an 'update' on today's major bus schemes, focussing particularly on recent developments, the present status and capabilities of these buses, and illustrating their application in advanced systems design. It is not intended to be a comprehensive guide to all bus standards, nor does it attempt to trace in detail past developments. As an indication of the rapid change that affects bus development an earlier'backplane bus standards' issue of this journal (Vol 6 No 9, November 1982) covered, among others, Multibus I, Versabus and Eurobus, all now largely superseded by more recent bus designs. Today, bus schemes proliferate -- from 8- and 16- to 32-bit systems -- both manufacturer-independent standards and manufacturers' de facto industry standards. For advanced systems, manufacturer-independent standards such as the 32-bit buses Futurebus and Fastbus, discussed here by Roger Edwards and Dave Gustavson, have been developed by working parties sponsored by major institutions and thus benefit from the expertise and wide experience of the members of these groups; also, they do not tie the designer to any one manufacturer's products. Futurebus has still to receive final approval as an official I EEE standard, but its advanced design features have ensured that already it finds applications. In this issue, Simon Peyton Jones highlights its use in a fifthgeneration computer being developed at University College London, UK. Applications such as this push bus performance to its limits. It thus becomes necessary to consider the physical constraints of bus systems. Treatment of buses as transmission lines and the problems of driving buses are addressed by Rob Wilson, who presents some solutions, now implemented in silicon, for Futurebus. Another manufacturer-independent bus, the 16-bit M3, was completed in 1982 under the auspices of the Computer Science Program of the Italian Research Council in response to the then inadequate capabilities of buses to fully support multiprocessor architectures. Dante Del Corso describes how the current status of

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the bus evolved and comments on its relation to other bus standards. Manufacturer-developed 16- and 32-bit buses such as VME and Multibus II, on the other hand, have the advantage of wide product availability and support. Many companies promoting buses in fact see bus standards as a strong marketing and competitive tool, prompting market research company Frost & Sullivan to predict that no one standard will dominate the market this decade. And the potential worldwide market for buses is vast: F & S predicts that one of the leading buses, VMEbus, will reach a market value of $950 million by 1989 from today's level of $75 million, for example. Performance of the two most widely used manufacturer-supported buses can be compared in the papers by Steve Heath, who discusses some design applications of VMEbus, and Simon Muchmore, who describes the message passing facility of Multibus II. More direct conclusions on the merits of all the 32bit buses can be drawn from Paul Borrill's detailed comparisons of VME, Futurebus, Multibus II, Nubus and Fastbus. We also cover some of the earliest buses to be developed--Q-bus, the STD bus and $100. These buses continue to survive, having been regularly upgraded to meet new system requirements whilst maintaining compatibility with existing products. Steve Dawes, Dave Batchelor and George W George illustrate how these buses have evolved to meet the needs of a changing marketplace. It is clear that with so many bus schemes available, individual bias or preference for or against particular buses, for whatever reasons, is inevitable. To place this in perspective, Mike Rowe of the UK Independent Broadcasting Authority draws on his practical experience of bus systems to give an 'independent' user's view of backplane buses. The IBA maintains almost 1500 television and radio transmitters across the UK, and its equipment is required to be both reliable and maintainable. Mike's study of the bus-based microprocessor systems that best fit these requirements recommends combinations of three buses for optimum performance.

Acknowledgements An expert editorial panel formed to coordinate this special issue provided many suggestions which have helped shape its content. We would like to acknowledge the valuable contributions of the members of this panel -- Roger Edwards, Hugh Field-Richards, Bob Squirrell and Anthony Winter -- and also Paul Borrill for his advice.

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