For questions regarding PCI-X (Peripheral Component Interconnect Extended) buses and adapters, not PCI-Express.
PCI-X is an enhancement to the 64-bit PCI bus. PCI-X devices operate at one of the following clock speeds: 66 Mhz, 133 Mhz, 266 Mhz, or 533 Mhz. PCI-X is backwards compatible and can run at slower speeds. Therefore, you can plug a PCI-X adapter into a PCI slot and it runs at a potentially slower PCI speed.
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