An expansion card is a printed circuit board that can be inserted into an expansion slot of a computer motherboard to enhance the function of a computer system. One edge of the expansion card, called the edge connector, holds the contacts so that they fit perfectly into the slot. They also establish the electrical contact between the integrated circuits and other electronics on the card and on the motherboard. Connectors are mounted on the bracket to allow the connection of external devices to the card. The number of expansion cards that can be added to the computer system is dependent on the format of the motherboard and case. While the normal number of cards is between one and seven, up to nineteen expansion cards are known to have been installed in a single system. There are also other factors involved in expansion card capacity, such as dual slots. Some cards are considered low-profile cards because they are shorter than standard cards and will fit in a smaller computer chassis. Input and output cards are used for external connectivity, such as a network, SAN or modem card.
The Altair 8800 was the first computer to feature a slot-type expansion card bus. Initially, performances of this bus were exclusive to the systems they were working with. However, by 1982 manufacturers created the S-100 standard. IBM introduced the XT bus and the first IBM PC in 1981. XT, or 8-bit ISA, was replaced with ISA, or 16-bit ISA. IBM’s MCA bus was a competitor to ISA, but ended up falling short 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 motherboards until Microsoft declared it a “legacy” subsystem in the PC 97 industry white-paper in 1997. Proprietary local buses and 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. In 1991, the ISA was replaced with the PCI bus. In 1993, Intel launched their PCI bus chipsets along with the P5-based Pentium CPUs. The version 3.0 standard is found on PC motherboards to this day. The PCI standard supports bridging, and daisy chained PCI buses have been tested. Cardbus is a PCI format that uses the PCMCIA connector to attach peripherals to the Host PCI Bus via PCI to PCI Bridge. Cardbus is being replaced by ExpressCard format. In 1997, Intel introduced the AGP bus as a dedicated video acceleration solution. PCI-Express has been replacing both PCI and AGP since 1995. This standard implements the logical PCI protocol over a serial communication interface. PC104-Plus, Mini PCI, or PCI-104 are commonly added for expansion on small form factor boards such as Micro ITX.
The USB format has become an actual expansion bus standard especially for laptop computers. USBs can now duplicate all the functions of add-in card slots, including audio, video, and storage. IEEE 1394 is a serial expansion bus originally promoted for Apple Inc. Also adopted for PCs, often used for storage and video cameras; it has applications for networking, video, and audio.
Expansion card slots standards are PCI Express, AGP, PCI, ISA, MCA, VLB, ExpressCard, and CompactFlash. Expansion card types include video cards, sound cards, network cards, TV tuner cards, modems, host adapters, and compatibility cards.