PCI-X

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 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 specified for both 32- and 64-bit PCI connectors, and PCI-X 2.0 added a 16-bit variant for embedded applications. PCI-X has been replaced in modern designs by the similar-sounding PCI Express (PCIe), with a different physical connector and a different electrical design, having one or more serial lanes instead of a number of slower parallel connections. In PCI, a transaction that cannot be completed immediately is postponed by either the target or the initiator issuing retry-cycles, during which no other agents can use the PCI bus. Since PCI lacks a split-response mechanism to permit the target to return data at a later time, the bus remains occupied by the target issuing retry-cycles until the read data is ready.