PCI-X is an expansion card standard designed to supersede PCI.
Although they are commonly confused, PCI-X and PCI-Express are not the same. PCI-X (Peripheral Component Interconnect Extended) is a computer bus technology (the "data pipes" between parts of a computer) that increases the speed that data can move within a computer from 66 MHz to 133 MHz. The technology was developed jointly by IBM, HP, and Compaq. PCI-X doubles the speed and amount of data exchanged between the computer processor and peripherals. With the current PCI design, one 64-bit bus runs at 66 MHz and additional buses move 32 bits at 66 MHz or 64 bits at 33 MHz. The maximum amount of data exchanged between the processor and peripherals using the current PCI design is 532 Mb per second. With PCI-X, one 64-bit bus runs at 133 MHz with the rest running at 66 MHz, allowing for a data exchange of 1.06 Gb per second. PCI-X is backwards-compatible, meaning that you can, for example, install a PCI-X card in a standard PCI slot but expect a decrease in speed to 33 MHz. You can also use both PCI and PCI-X cards on the same bus but the bus speed will run at the speed of the slowest card. PCI-X is more fault tolerant than PCI. For example, PCI-X is able to reinitialize a faulty card or take it offline before computer failure occurs.
IBM, HP, and Compaq designed PCI-X for servers to increase performance for high bandwidth devices such as Gigabit Ethernet cards, Fibre Channel, Ultra3 SCSI, and processors that are interconnected as a cluster. Compaq, IBM, and HP submitted PCI-X to the PCI Special Interest Group (Special Interest Group of the Association for Computing Machinery) in 1998. PCI SIG approved PCI-X, and it is now an open standard that can be adapted and used by all computer developers. PCI SIG controls technical support, training and compliance testing for PCI-X. IBM, Intel, Microelectronics and Mylex plan to develop chipsets to support PCI-X. 3Com and Adaptec intend to develop PCI-X peripherals.
To accelerate PCI-X adoption by the industry, Compaq offers PCI-X development tools at their Web site.
PCI-X (Peripheral Component Interconnect eXtended) is a computer expansion card standard designed to supersede the PCI (Peripheral Component Interconnect) standard. Formally PCI-X is the technology jointly developed by IBM, HP, and Compaq that uses a 64-bit extension of the PCI bus running at frequencies between 66 and 133 Mhz.
PCI-X was developed in an attempt to codify individual extensions to the standard PCI bus. PCI-X was needed as some devices, most notably Gigabit Ethernet cards, Fibre Channel and Ultra3 SCSI controllers, as well as cluster interconnects could saturate the full bandwidth (only 133MB/s) of the PCI bus themselves. 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 intially only ran at 33 Mhz basically giving the same maximum throughput of 266MB/s. Combined 64-bit 66 Mhz ports also showed up. However, these extensions were only loosely supported as optional part of the PCI 2.x standards. Device compatibility beyond the traditional 133 MB/s was still difficult.
In 1998 Compaq, IBM, and HP combined the 64-bit extension with the 66 Mhz extension, and predicting future demand developed 100 MHz and 133 Mhz variants to raise the possible bandwidth to 798 MB/s and 1064 MB/s respectively. They submitted the joint result to the PCI Special Interest Group (PCI SIG) as PCI-X. PCI SIG approved PCI-X, and it is now an open standard that can be adapted and used by all computer developers. PCI SIG controls technical support, training and compliance testing for PCI-X. All major chip makes generally now have some variant of PCI-X in their product lines.
PCI-X is somewhat backwards compatible with a standard PCI bus and its early extensions. 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 Volt only. The PCI-X bus is not compatible with 5 Volt cards. However, generally most newer PCI and faster devices will run in a PCI-X slot, however, they will limit the speed of the entire bus. For example a PCI 2.3 device running at 32-bit and 66 Mhz in a PCI-X 133 bus will limit the total throughput of the bus to 266 MB/s. To get around this limitation, many motherboard have separate PCI-X channels, allowing for better backwards compatibility, and higher total bandwidth. PCI-X is also more designed to be more fault tolerant than PCI. For example, PCI-X has provisions to reinitialize or deactivate a faulty card before a total system failure occurs.
To accelerate PCI-X adoption by the industry, Compaq offers PCI-X development tools at their Web site.
PCI-X 2.0
In 2003 PCI SIG ratified PCI-X 2.0 which adds a 266 MHz and 533 MHz variant of the PCI-X bus which gives roughly 2.15 GB/s and 4.3 GB/s throughput respectively. Additional protocol revisions are designed to help system reliability as well as add ECC, (Error Correcting Code) to the bus to avoid resends. To deal with one of the most common complaints of the PCI-X form factor, the 184 pin connector, 16-bit ports were developed to allow PCI-X to be used in devices with tight space constraints. Similar to PCI-Express, P2P functions were added to allow for devices on the bus to talk to each other without loading the CPU or bus controller.
Despite the numerous theoretical advantages of PCI-X 2.0, and its backwards compatibility with PCI-X and PCI devices, it has not been implemented on a large scale as on 2006. The main reason being the preference of hardware vendors to integrate PCI-Express instead.
