page_title: PCI-X
section_title: Mixing of 32-bit and 64-bit PCI cards in different width slots
hierarchical_section_title: PCI-X / Mixing of 32-bit and 64-bit PCI cards in different width slots
caption_attribution_description: English: A semi-inserted PCI-X card in a 32 bit PCI slot, illustrating the extra notch necessary to remain backwards-compatible.
context_page_description: PCI-X, short for Peripheral Component Interconnect eXtended, is a computer bus and expansion card standard that enhances the 32-bit PCI local bus for higher bandwidth demanded mostly by servers and workstations. It uses a modified protocol to support higher clock speeds, but is otherwise similar in electrical implementation. PCI-X 2.0 added speeds up to 533 MHz, with a reduction in electrical signal levels.
  The slot is physically a 3.3 V PCI slot, with exactly the same size, location and pin assignments. The electrical specifications are compatible, but stricter. However, while most conventional PCI slots are the 85 mm long 32-bit version, most PCI-X devices use the 130 mm long 64-bit slot, to the point that 64-bit PCI connectors and PCI-X support are seen as synonymous.
  PCI-X is in fact fully specified for both 32- and 64-bit PCI connectors, and PCI-X 2.0 added a 16-bit variant for embedded applications.
  It has been replaced in modern designs by the similar-sounding PCI Express, with a completely different connector and a very different electrical design, having one or more narrow but fast serial connection lanes instead of a number of slower connections in parallel.
context_section_description: Most 32-bit PCI cards will function properly in 64-bit PCI-X slots, but the bus speed will be limited to the clock frequency of the slowest card, an inherent limitation of PCI's shared bus topology. For example, when a PCI 2.3 66-MHz card is installed into a PCI-X bus capable of 133 MHz, the entire bus backplane will be limited to 66 MHz. To get around this limitation, many motherboards have multiple PCI/PCI-X buses, with one bus intended for use with high-speed PCI-X peripherals, and the other bus intended for general-purpose peripherals.
  Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of speed. An example of this is the Adaptec 29160 64-bit SCSI interface card. However some 64-bit PCI-X cards do not work in standard 32-bit PCI slots. Even if it would work, installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging, which requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector.
caption: 64-bit PCI-X card partially inserted in 32-bit PCI slot, showing compatibility.
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