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[[File:PCI-Bus.jpg|thumb|PCI expansion slot]]
In [[computing]], an '''expansion card''',
An '''expansion bus''' is a [[
Note that the function of a wire is either power delivery or signalling, both of which can become exceedingly complex in the [[frequency domain]]. Below around 8 MHz wires can often be treated as DC elements, above 30 MHz wires on this length scale must be treated as [[transmission lines]]; in between a designer might get away with treating wires as DC elements if the [[ground plane]] is handled carefully. Hobby electronics rarely exceeds a 10 MHz signalling rate on [[parallel communication|parallel interconnects]].
Once the signalling frequency passes into the transmission line regime, form factor constraints become arduous and expansion cards will typically feature standardized connectors with tight manufacturing tolerances, and the expansion card itself will likely interface to the bus through a dedicated interface chip designed to respect the narrow bus tolerances for timing and [[signal integrity]]. It will no longer be possible to [[prototype]] with [[breadboard]]s or [[wire-wrapping]], and the design must go straight to PCB, which are costly to fabricate in small volumes. In this regime, the expansion bus and the expansion card become largely synonymous.
Additionally, with high-speed electronics the [[electrical enclosure]] becomes important to shield the the expansion card from electromagnetic interference and to comply with regulations on [[electromagnetic interference|radiated emissions]]. [[Electrical enclosure]]s, such as [[computer case]]s, are typically designed to accept expansion cards of predetermined dimensions, with standardized interconnects. Usually the chassis will provide an [[edge connector|edge connector socket]], and the expansion card PCB with expose a row of conductive fingers (typically double-sided) that seats securely into the connector. Most enclosures specify power consumption and cooling limits on the expansion card design. In modern computers, expansion cards are complex system elements constrained by a wide variety of complex design factors.
For [[hardware compatibility|compatibility]] reasons, expansion cards will often have a keying system which prevents cards from being inserted into inappropriate sockets (such as the wrong voltage of operation, which could damage one or both sides). For this reason, it often takes close inspection of a socket to determine what form factors are compatible. In some cases, a socket can accept a card with less than the exposed connection points, usually to operate at lower performance than the socket otherwise supports; its not uncommon in this scenario that the socket is divided into a pair of two (rarely more than two) colinear sockets, where a short card will connect with the first socket only, but a long card will connect with both.
==History==
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