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A double edged development

Breathing new life into VME. By Graham Pitcher.

In an industry where the life expectancy for a technology seems only a bit longer than the products which it enables, the VMEbus is a continuing success story.

Unveiled 20 years ago, the Versa Module Europa bus – to use its full name – was designed as a flexible environment, capable of supporting a variety of computing intensive tasks. And the bus has indeed been successful; many similar initiatives from the early 1980s have long since fallen by the wayside. But needs change and so, too, has the VMEbus.

When launched, the VMEbus supported a maximum theoretical data rate of 40Mbyte/s across the backplane. This was, of course, perfectly adequate at the time. According to John Rynearson, technical director of VITA – the VMEbus International Trade Association – VMEbus has its origins in Motorola's Versabus architecture; itself derived from protocols developed to support Motorola's 68000 microcontroller.

Two types of data transfer were possible: single cycle transfers, with an address and a data transfer; and block transfers, with an address and a sequence of data transfers. Transfers were asynchronous and conducted on the basis of a master/slave handshake. Although, in theory, the bus could transfer 32bit wide data at 40Mbyte/s, reality, says Rynearson, depended upon the physics of the backplane and speed of the logic used to interface to the backplane.

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Graham Pitcher

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