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Incomputing, anexpansion card (also called anexpansion board,adapter card,peripheral card oraccessory card) is aprinted circuit board that can be inserted into anelectrical connector, orexpansion slot (also referred to as a bus slot) on a computer'smotherboard (see alsobackplane) to add functionality to a computer system. Sometimes the design of the computer's case and motherboard involves placing most (or all) of these slots onto a separate, removable card. Typically such cards are referred to as ariser card in part because they project upward from the board and allow expansion cards to be placed above and parallel to the motherboard.
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. For example, a high-speed multi-channel data acquisition system would be of no use in a personal computer used for bookkeeping, but might be a key part of a system used for industrial process control. Expansion cards can often be installed or removed in the field, allowing a degree of user customization for particular purposes. Some expansion cards take the form of "daughterboards" that plug into connectors on a supporting system board.
Inpersonal computing, notable expansion buses and expansion card standards include theS-100 bus from 1974 associated with theCP/Moperating system, the 50-pin expansion slots of the originalApple II computer from 1977 (unique to Apple), IBM'sIndustry Standard Architecture (ISA) introduced with theIBM PC in 1981,Acorn'stube expansion bus on theBBC Micro also from 1981, IBM's patented and proprietaryMicro Channel architecture (MCA) from 1987 that never won favour in theclone market, the vastly improvedPeripheral Component Interconnect (PCI) that displaced ISA in 1992, andPCI 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. Processor, memory and I/O cards became feasible with the development ofintegrated circuits.[1] Expansion cards make processor systems adaptable to the needs of the user by making it possible to connect various types of devices, including I/O, additional memory, and optional features (such as afloating point unit) to the central processor. Minicomputers, starting with thePDP-8, were made of multiple cards communicating through, and powered by, a passivebackplane.
The first commercialmicrocomputer to feature expansion slots was theMicral N, in 1973. The first company to establish ade facto standard was Altair with theAltair 8800, developed 1974–1975, which later became a multi-manufacturer standard, theS-100 bus. Many of these computers were also passive backplane designs, where all elements of the computer, (processor, memory, and I/O) plugged into a card cage which passively distributed signals and power between the cards.
Proprietarybus implementations for systems such as theApple II co-existed with multi-manufacturer standards.
IBM introduced what would retroactively be called theIndustry Standard Architecture (ISA) bus with the IBM PC in 1981. At that time, the technology was called thePC bus. TheIBM XT, introduced in 1983, used the same bus (with slight exception). The 8-bit PC and XT bus was extended with the introduction of the IBM AT in 1984. 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. Industry Standard Architecture (ISA) became the designation for the IBM AT bus after other types were developed. Users of the ISA bus had to have in-depth knowledge of the hardware they were adding to properly connect the devices, since memory addresses, I/O port addresses, and DMA channels had to be configured by switches or jumpers on the card to match the settings in driver software.
IBM'sMCA 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 licensing of MCA. EISA, the 32-bit extended version of ISA championed byCompaq, was used on some PC motherboards until 1997, when Microsoft declared it a "legacy" subsystem in thePC 97 industry white-paper. Proprietary local buses (q.v. Compaq) and then theVESA Local Bus Standard, were late 1980s expansion buses that were tied but not exclusive to the 80386 and 80486CPU bus.[2][3][4] ThePC/104 bus is anembedded bus that copies the ISA bus.
Intel launched theirPCI bus chipsets along with theP5-basedPentium CPUs in 1993. ThePCI bus was introduced in 1991 as a replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. The PCI standard supports bus bridging: as many as ten daisy-chained PCI buses have been tested.CardBus, using thePCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge. Cardbus is being supplanted byExpressCard format.
Intel introduced theAGP bus in 1997 as a dedicated video acceleration solution. AGP devices are logically attached to the PCI bus over a PCI-to-PCI bridge. Though termed a bus, AGP usually supports only a single card at a time (LegacyBIOS support issues). From 2005PCI Express has been replacing both PCI and AGP. This standard, approved[like whom?] in 2004, implements the logical PCI protocol over a serial communication interface. PC/104(-Plus) orMini PCI are often added for expansion on small form factor boards such asMini-ITX.
For their1000 EX and1000 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 IBMPCjr. 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 ofinterrupts andDMA provided by Intel add-on chips, and a few systemfault 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).
Laptops are generally unable to accept most expansion cards intended for desktop computers. Consequently, several compact expansion standards were developed.
The originalPC Card expansion card standard is essentially a compact version of the ISA bus. TheCardBus expansion card standard is an evolution of the PC card standard to make it into a compact version of the PCI bus. The originalExpressCard standard acts like it is either a USB 2.0 peripheral or a PCI Express 1.x x1 device. ExpressCard 2.0 adds SuperSpeed USB as another type of interface the card can use. Unfortunately, CardBus and ExpressCard are vulnerable toDMA attack unless the laptop has an IOMMU that is configured to thwart these attacks.
One notable exception to the above is the inclusion of a single internal slot for a special reduced size version of the desktop standard. The most well known examples areMini-PCI orMini PCIe. 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 fromApple Inc.,Tandy,Commodore,Amiga, andAtari, Inc., offered their own expansion buses. TheAmiga usedZorro II. Apple used a proprietary system with seven 50-pin-slots forApple II peripheral cards, then later used both variations onProcessor Direct Slot andNuBus for its Macintosh series until 1995, when they switched to a PCI Bus.
Generally speaking, most PCI expansion cards will function on anyCPU platform which incorporates PCI bus hardware provided there is a software driver for that type. PCI video cards and any other cards that contain their ownBIOS or other ROM are problematic, although video cards conforming to VESA Standards may be used for secondary monitors. DEC Alpha, IBM PowerPC, and NEC MIPS workstations used PCI bus connectors.[5] Both Zorro II and NuBus wereplug and play, requiring no hardware configuration by the user.
Other computer buses were used for industrial control, instruments, and scientific systems. One specific example is HP-IB (or Hewlett Packard Interface Bus) which was ultimately standardized asIEEE-488 (aka GPIB). Some well-known historical standards includeVMEbus,STD Bus,SBus (specific to Sun's SPARCStations), and numerous others.
Many othervideo game consoles such as theNintendo Entertainment System and theSega Genesis included expansion buses in some form; In the case of at least the Genesis, the expansion bus was proprietary. In fact, the cartridge slots of many cartridge-based consoles (not counting theAtari 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.
The primary purpose of an expansion card is to provide or expand on features not offered by the motherboard. For example, the originalIBM PC did not have on-board graphics or hard drive capability. In that case, a graphics card and anST-506 hard disk controller card provided graphics capability and hard drive interface respectively. Somesingle-board computers made no provision for expansion cards, and may only have provided IC sockets on the board for limited changes or customization. Since reliable multi-pin connectors are relatively costly, some mass-market systems such ashome computers had no expansion slots and instead used a card-edge connector at the edge of the main board, putting the costly matching socket into the cost of the peripheral device.
In the case of expansion of on-board capability, a motherboard may provide asingle serial RS232 port orEthernet port. An expansion card can be installed to offer multiple RS232 ports or multiple and higher bandwidthEthernet ports. In this case, the motherboard provides basic functionality but the expansion card offers additional or enhanced ports.
One edge of the expansion card holds the contacts (the edge connector orpin header) that fit into the slot. They establish the electrical contact between the electronics on the card and on the motherboard. Peripheral expansion cards generally have connectors for external cables. In the PC-compatible personal computer, these connectors were located in the support bracket at the back of the cabinet. Industrial backplane systems had connectors mounted on the top edge of the card, opposite to the backplane pins.
Depending on theform factor of the motherboard andcase, around one to seven expansion cards can be added to a computer system. 19 or more expansion cards can be installed in backplane systems. When many expansion cards are added to a system, total power consumption and heat dissipation become limiting factors. Some expansion cards take up more than one slot space. For example, manygraphics cards on the market as of 2010 are dual slot graphics cards, using the second slot as a place to put an activeheat 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 such asHTPC andSFF. (There is a "low profile PCI card" standard[6] that specifies a much smaller bracket and board area). The group of expansion cards that are used for external connectivity, such asnetwork,SAN ormodem cards, are commonly referred to asinput/output cards (or I/O cards).
Adaughterboard,daughtercard,mezzanine board orpiggyback board is an expansion card that attaches to a system directly.[7] Daughterboards often have plugs, sockets, pins or other attachments for other boards. Daughterboards often have only internal connections within a computer or other electronic devices, and usually access the motherboard directly rather than through acomputer bus. Such boards are used to either improve various memory capacities of a computer, enable the computer to connect to certain kinds of networks that it previously could not connect to, or to allow for users to customize their computers for various purposes such as gaming.[8]
Daughterboards are sometimes used in computers in order to allow for expansion cards to fit parallel to the motherboard, usually to maintain a smallform factor. This form are also calledriser cards, or risers. Daughterboards are also sometimes used to expand the basic functionality of an electronic device, such as when a certain model has features added to it and is released as a new or separate model. Rather than redesigning the first model completely, a daughterboard may be added to a special connector on the main board. These usually fit on top of and parallel to the board, separated byspacers or standoffs, and are sometimes calledmezzanine cards due to being stacked like themezzanine of atheatre.Wavetable cards (sample-based synthesis cards) are oftenmounted onsound cards in this manner.
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Some mezzanine card interface standards includethe 400 pinFPGA Mezzanine Card (FMC);the 172 pin High-Speed Mezzanine Card (HSMC);[9][10]thePCI Mezzanine Card (PMC);XMC mezzanines;theAdvanced Mezzanine Card;IndustryPacks (VITA 4), theGreenSpring Computers Mezzanine modules;etc.
Examples of daughterboard-style expansion cards include:
