PCI-X Interfaces

PCI-X v1.0, a high performance addendum to the PCI Local Bus specification co-developed by IBM, Hewlett-Packard, and Compaq – normally competitors in the PC server market – was unanimously approved by the Peripheral Component Interconnect Special Interest Group (PCI SIG) in the autumn of 1999. Fully backward compatible with standard PCI, PCI-X was seen as an immediate solution to the increased I/O requirements for high-bandwidth enterprise applications such as Gigabit Ethernet, Fibre Channel, Ultra3 SCSI and high-performance graphics.

PCI-X not only increases the speed of the PCI bus but also the number of high-speed slots. With the current design, PCI slots run at 33MHz and one slot can run at 66 MHz. PCI-X doubles the current performance of standard PCI, supporting one 64-bit slot at 133MHz, for an aggregate throughput of 1 GBps. The new specification also features an enhanced protocol to increase the efficiency of data transfer and to simplify electrical timing requirements, an important factor at higher clock frequencies.

For all its performance gains, PCI-X was positioned as an interim technology while the same three vendors develop a more long-term I/O bus architecture, referred to as Future I/O. While of potential use throughout the entire computer industry, the initial application of PCI-X was expected to be in server and workstation products, embedded systems and data communication environments.

The symbolism of a cartel of manufacturers making architectural changes to the PC server without consulting Intel is seen as being a significant development. At the heart of the dispute is who gets control over future server I/O technology. The PCI-X faction – already wary of Intel’s growing dominance in the hardware business – hoped to wrest some control by developing and defining the next generation of I/O standards, which they hope Intel will eventually support. Whether this would succeed – or merely generate a standards war – was a moot point since the immediate effect was merely to provoke Intel into leading another group of vendors in the development of rival I/O technology, which they referred to as Next Generation I/O (NGIO).

In 2002 PCI-X 2.0 emerged, initially doubling and ultimately promising to quadruple the speed of PCI-X. Its longevity contributed to the path to PCI’s eventual successor being a bumpy one.