Abbreviation for Peripheral Component Interconnect Extended. PCI-X is an extension and improvement upon the PCI bus technology that’s been common in Mac and PC computers for years. It increases the speed that data can move on a bus within a computer from a maximum of around 66 MHz to 133 MHz. With standard PCI design, one 64-bit bus runs at 66 MHz and additional buses move 32 bits at 66 MHz or 64 bits at 33 MHz. The maximum amount of data exchanged between the processor and peripherals using standard PCI design is 532 MB per second. With PCI-X, one 64-bit bus runs at 133 MHz with the rest running at 66 MHz, allowing for a data exchange of 1.06 GB per second. PCI-X is backwards compatible, meaning that you can, for example, install a PCI-X card in a standard PCI slot, but expect a decrease in speed to 33 MHz. You can also use both PCI and PCI-X cards on the same bus, but the bus speed will run at the speed of the slowest card. PCI-X is also designed to be more fault tolerant than PCI. For example, PCI-X is able to reinitialize a faulty card or take it offline before computer failure occurs.