PCI, PCI-X, PCI Express – Oh boy!

Lately I bought an old pc to use as a server and needed a network card for it. I didn’t think it be such a hassle but because of multiple PCI specs finding a card wasn’t easy. Theres been alot of confusion about pci cards and what card to get for your computer – PCI cards come in alot of different types and versions. I’ve done a good amount of research on this (if there are any discrepancies, please let me know) and hopefully this post will help clear things up.


Standard PCI cards (sometimes called pci 1.0) have a 32 bit width slot, and operate at 33 MHz. Originally they started as 5 volt cards but 3.3 volt cards began to be made that use a different slot.

PCI 2.1 came a few years later that added the Universal PCI card spec that allowed cards to be used in both 3.3 and 5v slots, and upped the bus to 66 MHz. Also they created a pci 64 bit width slot for high end cards (gigabit networking,…). This meant that there could be one of 4 different slots in your computer: 5v 32bit, 3.3v 32bit, 3.3v 64bit, 5v 64bit (see graphic below). This meant you either had to buy an exact card for the slot or a universal card (which most manufactures began to build).

The PCI bus 2.3 spec came along and nix’d 5v adapters (cards). PCI 2.3 was adaptable though and supported 3.3v cards and universal pci cards.


PCI-X or PCI eXtended was built mainly for high end use. It has a bus speed of 66 or 133 MHz and only used the 64 bit 3.3v slot. It is fully backward compatible though with the existing PCI architecture: 33/66 MHz PCI adapters (cards) can be used in PCI-X slots and PCI-X adapters can be used in PCI slots. PCI-X 2.0 came along and really upped the bus speed to either 266 MHz or 533 MHz, but was still fully backwards compatible.

Which Card to Get?

Well really you can get any universal card and have it work. Carnildo helped me see things the easy way:

The rule of thumb for PCI and PCI-X cards is that if it fits in the slot, it’ll work. The bus and cards will negotiate the fastest, widest connection that all of them can use, so a 133MHz 64-bit card in a standard PCI slot will transfer data as if it were a 33MHz 32-bit PCI card.

Also keep in mind that, “The slowest board dictates the maximum speed on a particular bus!”

PCI Express?

PCI Express uses an entirely different architecture, different slot sizes, and is incompatible with with PCI or PCI-X. It’s expected to coexist with PCI-X and not replace it.