Expansion card
Expansion cards allow the capabilities and interfaces of a computer system to be extended or supplemented in a way appropriate to the tasks it will perform.
Expansion cards can often be installed or removed in the field, allowing a degree of user customization for particular purposes.
In personal computing, notable expansion buses and expansion card standards include the S-100 bus from 1974 associated with the CP/M operating system, the 50-pin expansion slots of the original Apple II computer from 1977 (unique to Apple), IBM's Industry Standard Architecture (ISA) introduced with the IBM PC in 1981, Acorn's tube expansion bus on the BBC Micro also from 1981, IBM's patented and proprietary Micro Channel architecture (MCA) from 1987 that never won favour in the clone market, the vastly improved Peripheral Component Interconnect (PCI) that displaced ISA in 1992, and PCI Express from 2003 which abstracts the interconnect into high-speed communication "lanes" and relegates all other functions into software protocol.
Vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board.
This used a second connector for extending the address and data bus over the XT, but was backward compatible; 8-bit cards were still usable in the AT 16-bit slots.
For their 1000 EX and 1000 HX models, Tandy Computer designed the PLUS expansion interface, an adaptation of the XT-bus supporting cards of a smaller form factor.
8-bit ISA or XT-ISA), a passive adapter can be made to connect XT cards to a PLUS expansion connector.
This may have been electrically comparable to the XT bus; it most certainly had some similarities since both essentially exposed the 8088 CPU's address and data buses, with some buffering and latching, the addition of interrupts and DMA provided by Intel add-on chips, and a few system fault detection lines (Power Good, Memory Check, I/O Channel Check).
One notable exception to the above is the inclusion of a single internal slot for a special reduced size version of the desktop standard.
Such slots were usually intended for a specific purpose such as offering "built-in" wireless networking or upgrading the system at production with a discrete GPU.
Most other computer lines, including those from Apple Inc., Tandy, Commodore, Amiga, and Atari, Inc., offered their own expansion buses.
DEC Alpha, IBM PowerPC, and NEC MIPS workstations used PCI bus connectors.
One specific example is HP-IB (or Hewlett Packard Interface Bus) which was ultimately standardized as IEEE-488 (aka GPIB).
Some well-known historical standards include VMEbus, STD Bus, SBus (specific to Sun's SPARCStations), and numerous others.
In fact, the cartridge slots of many cartridge-based consoles (not counting the Atari 2600) would qualify as expansion buses, as they exposed both read and write capabilities of the system's internal bus.
Some single-board computers made no provision for expansion cards, and may only have provided IC sockets on the board for limited changes or customization.
In this case, the motherboard provides basic functionality but the expansion card offers additional or enhanced ports.
Depending on the form factor of the motherboard and case, around one to seven expansion cards can be added to a computer system.
When many expansion cards are added to a system, total power consumption and heat dissipation become limiting factors.
[8] Daughterboards are sometimes used in computers in order to allow for expansion cards to fit parallel to the motherboard, usually to maintain a small form factor.
