Express, commonly abbreviated as PCI-E or PCIe, although the cards themselves are totally incompatible and look different. While they are both high-speed computer buses for internal peripherals, they differ in many ways. The first is that PCI-X is a 64-bit parallel interface that is backward compatible with 32-bit PCI devices. PCIe is a serial point-to-point connection with a different physical interface that was designed to supersede both PCI and PCI-X. 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. PCIe also matches PCI-X and even PCI-X 2.0 in maximum bandwidth. PCIe 1.0 x1 offers 250 MB/s in each direction (lane), and up to 16 lanes (x16) are currently supported each direction, in full-duplex, giving a maximum of 4 GB/s bandwidth in each direction. PCI-X 2.0 offers (at its maximum 64-bit 533-MHz variant) a maximum bandwidth of 4,266 MB/s (~4.3 GB/s), although only in half-duplex. PCI-X has technological and economical disadvantages compared to PCI Express. The 64-bit parallel interface requires 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 does not require such complex and 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 take quite a bit of space on motherboards, which can be a problem for ATX and smaller form factors. See also PCI configuration space List of interface bit rates References Further reading PCI Bus Demystified; 2nd Ed; Doug Abbott; 250 pages; 2004; . PCI-X System Architecture; 1st Ed; Tom Shanley; 752 pages; 2000; . PCI & PCI-X Hardware and Software Architecture & Design; 5th Ed; Ed Solari; 1140 pages; 2001; . Ray Weiss, (6/9/2000) "PCI-X Exposed", EE Times External links Good diagrams and text on how to recognize the difference between 5 volt and 3.3 volt PCI (and PCI-X) slots. Peripheral Component Interconnect Motherboard expansion slot Open standards