PCI devices and drivers for Windows don't come in just one standard variety. There are at least three major implementations of PCI devices: PCI, PCI-E and PCI-X. As the newer varieties of PCI busses become more widely implemented, it's worth knowing how older and newer PCI devices can co-exist, if at all.

Most of us are familiar with standard 32-bit PCI hardware: The interface connector on the card consists of one large and one small tab, with the smaller tab to the rear. There are also 64-bit PCI cards, which have a third tab at the rear that is larger than the second one but smaller than the first.

Motherboards that support 64-bit PCI technology have slots to support the 64-bit configuration, and will have three slots instead of two. The 64-bit cards can be placed in conventional 32-bit slots -- the third tab will hang over the edge of the socket -- but they won't run at full speed. Likewise, some 32-bit cards can also be placed in 64-bit sockets, and will run as expected.

PCI-E: new kid on the bus
PCI-E and PCI-X are often confused with each other, mostly because the names are similar. That's where their likenesses end, however, as the two are not interchangeable. PCI-X is essentially the 64-bit implementation of PCI, with a much faster data rate and peak transfer speed.

PCI-E, on the other hand, is a completely new bus design, not an extension of the existing PCI standard. PCI-E uses a serial architecture instead of PCI's parallel data model,

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and it can run at much higher speeds. PCI-E sockets cannot accept traditional 32- or 64-bit PCI hardware, and vice versa. (There are motherboards that sport both PCI and PCI-E slots, however.)

More and more current and future PC designs are being built around the PCI-E architecture for a variety of reasons. For one, PCI-E is transparent to software developers (it's as easy to program for as the existing PCI architecture) and can co-exist on the same motherboard with older PCI implementations.

Knowing the differences between the various implementations of PCI can help you and your company avoid mistakes in planning for and purchasing hardware.

Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!

Fast Guide: Peripheral Component Interconnect (PCI)
 A primer on PCI devices
  Four ways to troubleshoot PCI-X
  Three reasons to choose PCIe over PCI-X technology

This was first published in December 2005

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