Before Intel introduced the 32-bit Peripheral Component Interconnect bus in 1993, computer hardware manufacturers produced add-on cards -- such as sound cards, network cards and many other types -- based on the 16-bit ISA architecture used since the early 1980s. With the release of the PCI standard, manufacturers were able to create cards that were literally four times faster and used fewer system resources than the old ISA devices. Since the release of the original PCI standard, manufacturers have continued to develop faster versions of the PCI protocol that add enhanced functionality for personal computers as well as support for modern 64-bit processors.
Ever since manufacturers ceased to produce motherboards with ISA slots, component makers have been using the PCI platform for most add-on cards. Consequently, virtually all computer add-on cards now use the PCI bus type. Common PCI devices include sounds cards, network cards, port add-on USB, parallel or serial cards, internal modems and basic 2-D video cards. However, there are many other types of PCI devices and add-on cards available. If a manufacturer produces a card designed to be added to a personal computer to enhance or expand functionality, chances are good that the card uses a PCI slot.
Original PCI and PCI 2.0
The original PCI bus -- or regular PCI as many call it -- runs at a core bus speed of 33 MHz, which is four times faster than the ISA bus it replaced. While the original PCI bus is considerably slower than modern PCI variants found in most modern computers, its 1 Gbps maximum transfer rate is still more than fast enough for many common devices such as modems, port add-on cards and network Ethernet cards. Most regular PCI slots are 3.5 inches long and produce a signaling voltage of 3.3 volts. Consequently, most PCI cards operate at 3.3 volts. Some server motherboards also have 5-volt slots. However, there are only a few PCI devices that require or are compatible with 5-volt slots. Unless you purchase a specialty server motherboard, all the 3.5-inch PCI slots on it probably produce 3.3 volts. PCI 2.0 slots are identical in appearance to original PCI slots. The PCI 2.0 bus increases the core bus speed for 32-bit cards to 2 Gbps and provides support for 64-bit devices with a core bus speed of 4 Gbps.
An extension of the original PCI standard, PCI-X is a 64-bit bus type that supports core clock speeds of up to 533 MHz, or roughly eight times of those of PCI 2.0 devices. PCI-X slots are roughly five inches in length and support both 3.3- and 5-volt signaling voltages. Common devices designed for use in a PCI-X slot include SCSI drive controller cards, high-end Serial ATA RAID controllers and some Gigabit Ethernet cards. While some high-end consumer or workstation motherboards may have one PCI-X slot, the bus is more common on server motherboards used in data-center or Web-server computers. While regular PCI slots are shorter than PCI-X slots, many modern PCI 2.0 cards will fit into a PCI-X slot and function normally.
PCI-Express is the fastest PCI-variant and the slot type used by most high-performance graphics cards in modern computers. PCI-Express replaced AGP, or Advanced Graphics Port, as the fastest bus type for video cards in desktop computers. PCI-Express comes in a variety of sizes and speeds. PCI Express x16 is the most common and the one used for high-end video cards. Other variants of PCI-Express include x1, x2, x4 and x8. The "x" in the variation name refers to the number of lanes or paths the card supports for transferring data to and from the processor and system bus. The "x" factor also determines the physical length of the slot required for certain cards. A single PCI-Express path can transfer data full duplex at up 2 Gbps in each direction. That means the path can transfer 4 Gpbs per second. Each incremental increase in the PCI-Express "x" factor speeds up the half-duplex, or one way, transfer rate by 2 Gbps or the full-duplex rate by 4 Gbps. Therefore, a PCI-Express x16 video card can transfer data to and from the processor and system bus at up to 64 Gbps. Besides x16 PCI-Express video cards, there are not many other available consumer oriented cards that use x1, x2, x4 or x8 variants of the bus. However, there are add-on cards designed for server use that use the smaller PCI-Express slots. Additionally, specialty cards that ship with some high-end gamer and workstation motherboards and provide additional ports use the smaller "x" factor PCI-Express slots.
- KarbosGuide: PC Architecture - Chapter 25 From ISA to PCI Express
- PCI-SIG: PCI Family History
- PCI-SIG: PCI Express Architecture Frequently Asked Questions
- PCI System Architecture; Dan Anderson, et al.
- ACME: Build a PC -- PCI Types
- Direction.org: What is PCI Express? A Layman's Guide to High Speed PCI-E Technology
- National Instruments: What Is the Difference Between Half-Length and Full-Length PCI Slots?
- We Love Macs: How Can I Tell the Difference Between PCI, PCI-X and PCI Express?
- Comstock/Comstock/Getty Images