In computing, an expansion card (also known as an expansion card, adapter card, peripheral card, or accessory card) is a printed circuit board that plugs into an electrical connector or expansion slot (also known as a bus slot) on the motherboard of a computer. (see also backplane), to add functionality to a computer system. Sometimes the design of the computer case and motherboard requires that most (or all) of these slots be placed on a separate removable card. These cards are typically called risers in part because they protrude upward from the motherboard, allowing expansion cards to be placed on top of and parallel to the motherboard.
Expansion cards allow you to expand or complement the capabilities and interfaces of a computer system in a task-oriented way. For example, a high-speed multi-channel data acquisition system on a personal computer used for accounting would be useless but could be a key part of a system used for industrial process control. Expansion cards can often be installed or removed in the field, allowing a degree of customization for specific purposes. Some expansion cards take the form of "daughter boards" that plug into connectors on a carrier system board.
Notable expansion buses and expansion card standards in personal computing include the 1974 S-100 bus associated with the CP/M operating system, the 50-pin expansion slots of the original 1977 Apple II computer (exclusive from Apple), IBM's Industry Standard Architecture (ISA) introduced with the IBM PC in 1981, Acorn's tube expansion bus in BBC Micro also from 1981, IBM's proprietary and proprietary Microchannel Architecture (MCA) 1987 IBM, which never succeeded in the clone market, vastly improved Peripheral Component Interconnect (PCI), which replaced ISA in 1992, and PCI Express in 2003, which abstracted connecting into communication "lanes" from high speed and relegated all other functions to the software protocol.