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 or backplane to add functionality to a computer system via the expansion bus.
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.
Depending on the form factor of the motherboard and case, around one to seven expansion cards can be added to a computer system.
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.
The group of expansion cards that are used for external connectivity, such as network, SAN or modem cards, 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.
EXPANSION BUS HISTORY
Intel 8080/Zilog Z80-based computers running CP/M had settled around the S-100 standard.
IBM introduced the Industry Standard Architecture (ISA) bus, with the IBM PC in 1981; it was then called the PC bus.
The IBM XT, introduced in 1983, used the same bus (with slight exception). XT (a.k.a. 8-bit ISA) was replaced with ISA (a.k.a. 16-bit ISA)
EISA, the 32-bit extended version of ISA
All buses up to this point are called “legacy” subsystem in the PC 97 white paper
VESA Local Bus Standard, were late 1980s expansion buses that were tied to the 80386 and 80486 CPU
Cardbus, using the PCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge.
Cardbus is being supplanted by ExpressCard format.
Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution.
Though termed a bus, AGP usually supports only a single card at a time
Intel launched their PCI bus chipsets along with the P5-based Pentium CPUs in 1993.
From 2005 PCI-Express has been replacing both PCI and AGP.
2004, implements the logical PCI protocol over a serial communication interface. PC/104(-Plus) or Mini PCI
The USB format has become a de facto expansion bus standard especially for laptop computers. All the functions of add-in card slots can currently be duplicated by USB, including Video, networking, storage and audio. USB 2.0 is currently part of the ExpressCard interface and USB 3.0 is part of the ExpressCard 2.0 standard.
FireWire or IEEE 1394 is a serial expansion bus originally promoted for Apple Inc. Computer expansion replacing the SCSI bus. Also adopted for PCs, often used for storage and video cameras, it has application for networking, video, and audio.
Expansion slot standards
CardBus/PC card/PCMCIA (for notebook computers)
CompactFlash (for handheld computers)
Expansion card types
AMR Advanced Multi Rate Codec
TV tuner cards
Video processing expansion cards
Host adapters such as SCSI and RAID controllers.
BIOS Expansion ROM cards
Compatibility card (legacy)
Physics cards. (becoming obsolete as they are integrated into video cards)
Disk controller cards (for fixed- or removable-media drives)
Interface adapter cards, including parallel port cards, serial port cards, multi-I/O cards, USB port cards, and proprietary interface cards.
RAM disks, e.g. i-RAM
Memory expansion cards (legacy)
Hard disk cards (legacy)
Clock/calendar cards (legacy)
Security device cards
Radio tuner cards