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The need for speed

From Raceway to a switched serial fabric. By Graham Pitcher.

There's nothing wrong with the VME backplane as such. However, if you're looking to send high speed data between a number of boards in a card cage, then you are likely to run into problems. The reason – apart from it being a 30year old concept – is that VME is a bus based system. And that, despite several upgrades to the specification, has limited the amount of data that can be transferred.

Despite VME's bus structure, it retains widespread support because of its inherent mechanical and electrical stability. Legacy is the watchword when developing enhancements for VME systems.

Over the years, a number of attempts have been made to unblock the data transfer bottleneck whilst retaining support for the basic VME architecture. One of these attempts is Raceway, first developed by Mercury Computer Systems in 1994.

Raceway, in its first instantiation, was a high speed synchronous backplace fabric capable of delivering 32bit data between VME boards at rates of up to 160Mbyte/s. What was important about the development was the fact that this high speed data transfer was independent of the VME bus. So users, in theory, had the best of both worlds: the VME architecture and higher speed data transfer.

Rodger Hosking, vice president of board manufacturer Pentek, expanded: "The idea of Raceway was to allow a flexible backplane fabric to connect boards in a VME environment such that each board in the cage could talk to any other through a crossbar switch, which would be configured by the message itself. At the same time, you could have simultaneous traffic between other pairs of boards in the system. It was fundamentally different to a bus architecture, where only one pair of boards at a time could use the bus."

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Graham Pitcher

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