The expansion card (also expansion board, adapter card or accessory card) in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard to add functionality to a computer system.
One edge of the expansion card holds the contacts (the edge connector) that fit exactly into the slot. They establish the electrical contact between the electronics (mostly integrated circuits) on the card and on the motherboard.
Connectors mounted on the bracket allow the connection of external devices to the card. Depending on the form factor of the motherboard and case, around one to seven expansion cards can be added to a computer system. In the case of a backplane system, up to 19 expansion cards can be installed. There are also other factors involved in expansion card capacity. For example, most graphics cards on the market as of 2010 are dual slot graphics cards, using the second slot as a place to put an active heat sink with a fan.
Some cards are "low-profile" cards, meaning that they are shorter than standard cards and will fit in a lower height computer chassis. (There is a "low profile PCI card" standard that specifies a much smaller bracket and board area). The group of expansion cards that are used for external connectivity, such as a network, SAN or modem card, are commonly referred to as input/output cards (or I/O cards).
The primary purpose of an expansion card is to provide or expand on features not offered by the motherboard. For example, the original IBM PC did not provide graphics or hard drive capability as the technology for providing that on the motherboard did not exist. In that case, a graphics expansion card and an ST-506 hard disk controller card provided graphics capability and hard drive interface respectively.
In the case of expansion of on-board capability, a motherboard may provide a single serial RS232 port or Ethernet port. An expansion card can be installed to offer multiple RS232 ports or multiple and higher bandwidth Ethernet ports. In this case, the motherboard provides basic functionality but the expansion card offers additional or enhanced ports.
The first microcomputer to feature a slot-type expansion card bus was the Altair 8800, developed 1974-1975. Initially, implementations of this bus were proprietary (such as the Apple II and Macintosh), but by 1982 manufacturers of Intel 8080/Zilog Z80-based computers running CP/M had settled around the S-100 standard. IBM introduced the XT bus, with the first IBM PC in 1981; it was then called the PC bus, as the IBM XT, using the same bus (with slight exception,) was not to be introduced until 1983. XT (a.k.a. 8-bit ISA) was replaced with ISA (a.k.a. 16-bit ISA), originally known as AT bus, in 1984. IBM's MCA bus, developed for the PS/2 in 1987, was a competitor to ISA, also their design, but fell out of favor due to the ISA's industry-wide acceptance and IBM's closed licensing of MCA. EISA, the 32-bit extended version of ISA championed by Compaq, was used on some PC motherboards until 1997, when Microsoft declared it a "legacy" subsystem in the PC 97 industry white-paper. Proprietary local buses (q.v. Compaq) and then the VESA Local Bus Standard, were late 1980s expansion buses that were tied but not exclusive to the 80386 and 80486 CPU bus. The PC104 bus is an embedded bus that copies the ISA bus.
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