Peripheral Component Interconnect

The Peripheral Component Interconnect standard (in practice almost always shortened to PCI) specifies a computer bus for attaching peripheral devices to a computer motherboard. These devices can take any one of the following forms:

• An integrated circuit fitted onto the motherboard itself, called a planar device in the PCI specification.
• An expansion card that fits in sockets.

The PCI bus is common in modern PCs, where it has displaced ISA and VESA Local Bus as the standard expansion bus, but it also appears in many other computer types. The bus will eventually be succeeded by PCI Express and other technologies, which have already started to appear in new computers.
The PCI specification covers the physical size of the bus (including wire spacing), electrical characteristics, bus timing, and protocols. The specification can be purchased from the PCI Special Interest Group (PCISIG).

History

Work on PCI began at Intel circa 1990. PCI 1.0, which was merely a component-level specification, was released June 22, 1992. PCI 2.0, which was the first to establish standards for the connector and motherboard slot, was released on April 30, 1993.
PCI was immediately put to use in servers, replacing MCA and EISA as the server expansion bus of choice. In mainstream PCs, PCI was slower to replace VESA Local Bus (VLB), and did not gain significant market penetration until late 1994 in second-generation Pentium PCs. By 1996 VLB was all but extinct, and manufacturers had adopted PCI even for 486 computers. EISA continued to be used alongside PCI through 2000. Apple Computer adopted PCI for professional Power Macintosh computers (replacing NuBus) in mid-1995, and the consumer Performa product line (replacing LC PDS) in mid-1996.
Later revisions of PCI added new features and performance improvements, including a 66 MHz 3.3 V standard and 133 MHz PCI-X, and the adaptation of PCI signaling to other form factors. With the introduction of the serial 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, traditional PCI is likely to slowly die out in coming years.

Configuration

PCI devices are plug and play. The system firmware examines each device's PCI Configuration Space and allocates resources. Each device can request up to six areas of memory space or I/O port space. They can also have an option ROM that can contain executable x86 or PA-RISC code, Open Firmware or an EFI driver.
Interrupts are assigned to the device by firmware rather than being configured by the use of jumpers on the card as was common with ISA devices. While PCI devices are required to have level-triggered interrupts so they can share interrupt numbers, system software will normally try to assign unique interrupts to each device to improve performance. PCI devices must have special hardware in order to support sharing an interrupt port as it needs a way to tell if an interrupt is for itself or for a separate device sharing the IO port.

Conventional PCI bus specifications

• 33.33 MHz clock with synchronous transfers
• peak transfer rate of 133 MB per second for 32-bit bus width (33.33 MHz × 32 bits ÷ 1 byte/8 bits = 127 MB/s) [1 Megabyte = 2^20]
• 32-bit or 64-bit bus width
• 32-bit address space (4 gigabytes)
• 32-bit port space (now deprecated)
• 256-byte configuration space
• 3.3 or 5-volt signaling
• reflected-wave switching

Conventional PCI variants

• PCI 2.2 allows for 66 MHz signalling (requires 3.3 volt signalling) (peak transfer rate of 503 MB/s)
• PCI 2.3 permitted use of 3.3 volt and universal keying, but did not support 5 volt keyed add in cards.
• PCI 3.0 is the final official standard of the bus, completely removing 5 volt support.
• PCI-X changes the protocol slightly and increases the data rate to 133 MHz (peak transfer rate of 1014 MB/s)
• PCI-X 2.0 specifies a 266 MHz rate (peak transfer rate of 2035 MB/s) and also 533 MHz rate, expands the configuration space to 4096 bytes, adds a 16-bit bus variant and allows for 1.5 volt signalling
• Mini PCI is a new form factor of PCI 2.2 for use mainly inside laptops
• Cardbus is a PCMCIA form factor for 32-bit, 33 MHz PCI
• Compact PCI, uses Eurocard-sized modules plugged into a PCI backplane.
• PC/104-Plus is an industrial bus that utilizes the PCI signal lines with different connectors.
• Advanced Telecommunications Computing Architecture (ATCA or AdvancedTCA) is a next-generation bus for the telecommunications industry