Developed by Intel in 1992, PCI was primarily intended to replace MCA and EISA as the standard expansion bus on high-end server PCs. In the desktop market, PCI was slower to replace VESA Local Bus (VLB) until 1994 when it appeared in second-generation Pentium PCs. By 1996, VLB was largely replaced as PC motherboard manufacturers migrated in mass to PCI for 486 series computers.
Some of the key features include:
- 33.33 MHz clock with synchronous transfers
- peak transfer rate of 133 MB/s for 32-bit bus width (33.33 MHz × 32 bits ÷ 8 bits/byte = 133 MB/s)
- peak transfer rate of 266 MB/s for 64-bit bus width
- 32-bit or 64-bit bus width
- 32-bit address space (4 gigabytes)
- 32-bit I/O port space
- 256-byte configuration space
- 5-volt signaling
PCI SIG released the original 2.3 revision as an evolutionary change to the PCI Local Bus Specification. Revision 2.3 makes a significant step in migrating the PCI bus from the original 5.0 volt signaling, to a lower 3.3 volt signaling bus. Revision 2.3 supports the 5V and 3.3V keyed system board connectors (as did revision 2.2) but revision 2.3 supports only the 3.3V and Universal keyed add-in cards. Later revisions of PCI added new features and performance improvements, including a 66 MHz and 133 MHz PCI-X, and the adaptation of PCI signaling to other form factors.
PCI-X 2.0 builds on the foundation of PCI and PCI-X while offering bandwidths 4 times higher than PCI-X 1.0b without increasing pin-count. Targeted at server and workstation applications in the areas of Fibre Channel, RAID, networking, InfiniBand™ and other high-bandwidth technologies, PCI and PCI-X powers many of the world’s PC-based server environments.
The migration to PCI-X 2.0 is simplified by the fact that it is both hardware and software compatible with PCI-X 1.0b and PCI. PCI-X 2.0 design and implementation are also made easy because many elements of PCI-X 1.0b are retained. There are also hundreds of products currently available that can seamlessly connect with PCI-X 2.0.
Both PCI-X 1.0b and PCI-X 2.0 are backward compatible with some PCI standards. With the introduction of the PCI Express standard in 2004, motherboard manufacturers have included progressively fewer PCI expansion slots in favor of the new standard. Although it is still common to see both interfaces implemented side-by-side, more device and system vendors are moving to PCI Express in the future.