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 additional 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, some expansion cards need two slots like some nVidia GeForce FX and newer GeForce graphics cards and there is often a space left to aid cooling on some high-end cards.
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 common on PC motherboards until 1997, when Microsoft declared it a "legacy" subsystem in the PC 97 industry white-paper. VESA Local Bus, an early 1990s expansion bus that was inherently tied to the 80486 CPU, became obsolete (along with the processor) when Intel launched the Pentium CPU in 1993.
The PCI bus was introduced in 1991 as replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. Though termed a bus, AGP supports only a single card at a time. From 2005 PCI-Express has been replacing both PCI and AGP. This standard, approved [by who?] in 2004, implements the logical PCI protocol over a serial communication interface.
After the S-100 bus, this article above mentions only buses used on IBM-compatible/Windows-Intel PCs. Most other computer lines that were not IBM compatible, including those from Tandy, Commodore, Amiga, and Atari, offered their own expansion buses. Even many video game consoles, such as the Sega Genesis, included expansion buses; at least in the case of the Genesis, the expansion bus was proprietary, and in fact the cartridge slots of many cartridge based consoles (not including the Atari 2600) would qualify as expansion buses, as they exposed both read and write capabilities of the system's internal bus. However, the expansion modules attached to these interfaces, though functionally the same as expansion cards, are not technically expansion cards, due to their physical form.
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. Because it is electrically compatible with the XT bus (a.k.a. 8-bit ISA or XT-ISA), a passive adapter can be made to connect XT cards to a PLUS expansion connector. Another feature of PLUS cards is that they are stackable. Another bus that offered stackable expansion modules was the "sidecar" bus used by the IBM PCjr. This may have been electrically the same as or similar 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). Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box (with holes exposing the connectors).