PCI-X (Peripheral Component Interconnect Extended) is a computer bus and expansion card standard designed to supersede PCI. It is a double-wide version of PCI, running at up to four times the clock speed, but is otherwise similar in electrical implementation and uses the same protocol. It has itself been replaced in modern designs by the similar-sounding PCI Express, which features a very different logical design, most notably being a "narrow but fast" serial connection instead of a "wide but slow" parallel connection.
PCI-X was needed as some devices, most notably Gigabit Ethernet cards, Fibre Channel and Ultra320 SCSI controllers, and cluster interconnects could, by themselves, saturate the full bandwidth (only 133 MB/s) of the PCI bus. The first solution was to run the 33-MHz PCI bus at double the speed, 66 MHz, effectively doubling the throughput to 266 MB/s. However, machines with multiple high-bandwidth devices still needed more headroom, so additional pins were added to the slot, going from 120 to 184, to form a 64-bit variety. This initially only ran at 33 MHz, basically giving the same maximum throughput of 266 MB/s. Though combined 64-bit, 66-MHz ports had also been implemented, these extensions had been only loosely supported as optional parts of the PCI 2.x standards. Device compatibility beyond the basic 133 MB/s continued to be difficult.
Developers eventually used the combined 64-bit and 66-MHz extension as a foundation, and anticipating future demand, established 66-MHz and 133-MHz variants to raise the maximum bandwidths to 527 MB/s and 1064 MB/s, respectively. The joint result was submitted as PCI-X to the PCI Special Interest Group (Special Interest Group of the Association for Computing Machinery). Subsequent approval made it an open standard adoptable by all computer developers. The PCI SIG controls technical support, training and compliance testing for PCI-X. IBM, Intel, Microelectronics and Mylex were to develop supporting chipsets. 3Com and Adaptec were to develop compatible peripherals. To accelerate PCI-X adoption by the industry, Compaq offered PCI-X development tools at their web site. All major chip makers generally now have or have had some variant of PCI-X in their product lines.
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.
All PCI-X cards or slots have a 64-bit implementation and vary as follows:
Despite the various theoretical advantages of PCI-X 2.0 and its backward compatibility with PCI-X and PCI devices, it has not been implemented on a large scale (as of 2008). This lack of implementation is primarily because hardware vendors have chosen to integrate PCI-Express instead.
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.