PCI-X is often confused with PCI-Express, commonly abbreviated as PCI-E or PCIe. While they are both high-speed computer buses for internal peripherals, they differ in many ways. The first is that PCI-X is a parallel interface that is directly backward compatible with all but the oldest (5-volt) standard PCI devices. PCIe is a serial bus that offers no compatibility with older buses. PCI-X and standard PCI buses may run on a PCIe bridge, similar to the way ISA buses ran on standard PCI buses in some computers. This should not be confused with compatibility. PCIe also matches PCI-X and even PCI-X 2.0 in maximum bandwidth. PCIe x1 offers 250 MB/s in both directions, and currently supports up to an x32 standard at 8 GB/s.
PCI-X has a number of technological and economical disadvantages from PCI-Express. The 64-bit parallel interface requires inherently difficult trace routing, because as with all parallel interfaces, the signals from the bus must arrive simultaneously or within a very short window, and noise from adjacent slots may cause interference. The serial interface of PCIe suffers fewer such problems and therefore requires less complex and less expensive designs. PCI-X buses, like standard PCI, are half-duplex bidirectional whereas PCIe buses are full-duplex bidirectional. PCI-X buses run only as fast as the slowest device whereas PCIe devices are able to independently negotiate the bus speed. Also, PCI-X slots are longer than PCIe 1x through PCIe 16x, which makes it impossible to make short cards for PCI-X. PCI-X slots thus take quite a bit of space on motherboards, which can be a problem for ATX and smaller form factors.
