PCI (Peripheral Component Interconnect) is an interconnection system between a microprocessor and attached devices in which expansion slots are spaced closely for high speed operation. Using PCI, a computer can support both new PCI cards while continuing to support Industry Standard Architecture (ISA) expansion cards, an older standard. Designed by Intel, the original PCI was similar to the VESA Local Bus. However, PCI 2.0 is no longer a local bus and is designed to be independent of microprocessor design. PCI is designed to be synchronized with the clock speed of the microprocessor.
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PCI is now installed on most new desktop computers, not only those based on Intel's Pentium processor but also those based on the PowerPC. PCI transmits 32 bits at a time in a 124-pin connection (the extra pins are for power supply and grounding) and 64 bits in a 188-pin connection in an expanded implementation. PCI uses all active paths to transmit both address and data signals, sending the address on one clock cycle and data on the next. Burst data can be sent starting with an address on the first cycle and a sequence of data transmissions on a certain number of successive cycles.
The PCI specifications define two different card lengths. The full-size PCI form factor is 312 millimeters long; short PCIs range from 119 to 167 millimeters in length to fit into smaller slots where space is an issue. Like the full-size PCI, the short PCI is a high-performance I/O bus that can be configured dynamically for use in devices with high bandwidth requirements. Most current PCI cards are half-sized or smaller. There are a number of variations of PCI, including CompactPCI, Mini PCI, Low-Profile PCI, concurrent PCI, and PCI-X.