PCI-X revised the PCI standard by doubling the maximum clock speed (from 66 MHz to 133 MHz) and hence the amount of data exchanged between the computer processor and peripherals. Standard PCI supports up to 64 bits at 66 MHz (though anything above 32 bits at 33 MHz is only seen in high-end systems) and additional bus standards move 32 bits at 66 MHz or 64 bits at 33 MHz. The theoretical maximum amount of data exchanged between the processor and peripherals with PCI-X is 1.06 GB/s, compared to 532 MB/s with standard PCI. PCI-X also improves the fault tolerance of PCI allowing, for example, faulty cards to be reinitialized or taken offline.
PCI-X is generally backward-compatible with most cards based on the PCI 2.x or later standard, meaning that, a PCI-X card can be installed in a PCI slot, provided it has the correct voltage keying for the slot and (if inserting into a 32-bit slot) nothing obstructs the overhanging part of the edge connector. Originally the PCI bus was a 5-volt bus. Later, in PCI Revision 2.x, the PCI bus was a dual-voltage interconnect. In 3.0 this was changed to 3.3 volts only. The PCI-X bus is not compatible with the older 5-volt cards but newer 3.3-volt PCI cards will work in a PCI-X slot. Apart from this, PCI and PCI-X cards can generally be intermixed on a PCI-X bus, but the speed will be limited to the speed of the slowest card. For example, a PCI 2.3 device running at 32 bits and 66 MHz on a PCI-X 133-MHz bus will limit the total throughput of the bus to 266 MB/s. To get around this limitation and the voltage compatibility issue, many motherboards have separate PCI-X channels that can be dedicated to different PCI hardware families if needed, allowing for better backward compatibility while maintaining higher total system bandwidth.
