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Electronic Design

A Romp Through The Bus Structures

VME, PCI, CompactPCI, PCI-X, InfiniBand. Some are and some aren't familiar terms, yet all are acronyms for popular bus standards. Each of these standards lends itself to multiple sourcing and a range of available solutions, as well as time-to-market advantages. But from a design standpoint, one very crucial, strategic benefit overshadows all others. Each bus system enables decoupling peripheral design from system design.

The grande dame of the bus world introduced some 30 years ago, VME has been well received in industrial and commercial applications. Today, more than 300 companies manufacture over 3000 off-the-shelf VME products. For further information about VME, visit

Then, in the early '90s, Peripheral Component Interconnect (PCI) arrived. Running at 33 MHz, this synchronous PCI local bus transfers data at 32- or 64-bit bus widths, thereby moving data at up to 264 Mbytes/s (at 64 bits). In 1995, the PCI local bus architecture was souped up to 66 MHz, raising the transfer rate to 528 Mbytes/s (at 64 bits).

Building upon PCI, CompactPCI first appeared in 1994. It brought rugged Eurocard packaging to PCI, including 2-mm metric pin and socket connectors, plus the ability for cards to be front-loaded into a rack-mount system. It was initiated by Ziatech Corp., San Luis Obispo, Calif., under the auspices of the PCI Industrial Computers Manufacturers Group (PICMG). Compact-PCI meets the needs of those building a rack of equipment in which PCI performance is adequate and where the various functions will be derived from off-the-shelf components and subsystems. For further information on CompactPCI, check out

Incidentally, those seeking to raise clock speeds and/or add slots to a CompactPCI system may want to examine the advances made at the EnSigna Lab, at Fairchild Semiconductor, South Portland, Maine. Go to [email protected].

Finally, there's PCI-X. Introduced late last year, it evolved from PCI, adding higher frequency operation, split transactions, and other enhancements aimed at improving bandwidth. PCI-X is in the very early stages. Its architecture builds upon PCI bus performance, extending it to a higher level for faster data-transfer rates. So, PCI-X is able to support 32-bit and 64-bit operations at frequencies up to 133 MHz, enabling data-handling capabilities of over 1 Gbyte/s. Presently, its appeal is to server and high-end workstation manufacturers, principally those controlling their own system architecture, such as Compaq Computer. For further information on PCI and PCI-X, go to

Removing and replacing boards with power up is called "hot-plugability" in the PCI-X world (and Hot Swap in the CompactPCI world). With regard to CompactPCI, Hot Swap has been a standard for three years now. As for PCI-X, the PCI Special Interest Group (PCI SIG) is the industry organization chartered with the development and management of the PCI Local Bus specification, including PCI-X. It reports that its PCI Hot Plug Group is striving hard to complete the Hot Plug standard that will support the PCI-X specification.

So what about InfiniBand? It's on the way. In a fine white paper entitled "InfiniBand Architecture: Bridge over Troubled Waters," David Pendery and Jonathan Eunice explain how InfiniBand is expected to replace PCI with a high-bandwidth, multiple gigabyte-per-second, switched-network technology. This approach has already been used in the larger servers. With specifications expected to appear this summer, first products will emerge in 2001. Hit for the full story.

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