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Unsourced material may be challenged and removed. In computing, the expansion card, expansion board, adapter card or accessory card is a printed circuit board that can be inserted into an electrical connector, or expansion slot, on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus. It is a collection of wires and protocols that allows for the expansion of a computer.
Even vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board. O cards became feasible with the development of integrated circuits. Minicomputers, starting with the PDP-8, were made of multiple cards, all powered by and communicating through a passive backplane.
The first commercial microcomputer to feature expansion slots was the Micral N, in 1973. The first company to establish a de facto standard was the Altair 8800, developed 1974-1975, which later became a multi-manufacturer standard, the S-100 bus. Proprietary bus implementations for systems such as the Apple II co-existed with multi-manufacturer standards.
At that time, the technology was called the PC bus. The 8-bit PC and XT bus was extended with the introduction of the IBM AT in 1984. 8-bit cards were still usable in the AT 16-bit slots. IBM AT bus after other types were developed.