History
Even vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board. Processor, memory and I/O cards became feasible with the development of integrated circuits. Expansion cards allowed a processor system to be adapted to the needs of the user, allowing variations in the type of devices connected, additions to memory, or optional features to the central processor (such as a floating point unit). 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 to establish a de facto standard was the Altair 8800, developed 1974-1975, which later became a mulit-manufacturer standard, the S-100 bus.Many of these computers were also passive backplane designs, where all elements of the computer, (processor, memory, and I/O) plugged into a card carge which passsively distributed signals and power between the cards.
Proprietary bus implementations for systems such as the Apple II co-existed with multi-manufacturer standards.
Read more about this topic: Expansion Slots
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