Backplanes

A backplane (or "backplane system") is a group of electrical connectors in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a complete computer system. Backplanes commonly use a printed circuit board, but wire-wrapped backplanes have also been used in minicomputers and high-reliability applications.

Early microcomputer systems like the Altair 8800 used a backplane for the processor and expansion cards. A backplane is generally differentiated from a motherboard by the lack of on-board processing and storage elements. A backplane uses plug-in cards for storage and processing.

Backplanes are normally used in preference to cables because of their greater reliability. In a cabled system, the cables need to be flexed every time that a card is added or removed from the system; this flexing eventually causes mechanical failures. A backplane does not suffer from this problem, so its service life is limited only by the longevity of its connectors. For example, the DIN 41612 connectors used in the VMEbus system can withstand 50 to 500 insertions and removals (called mating cycles), depending on their quality. To transmit information, Serial Back-Plane technology uses a low voltage differential signaling transmission method for sending information.

In addition, there are bus expansion cables which will extend a computer bus to an external backplane, usually located in an enclosure, to provide more or different slots than the host computer provides. These cable sets have a transmitter board located in the computer, an expansion board in the remote backplane, and a cable between the two.

Backplanes have grown in complexity from the simple Industry Standard Architecture (ISA) (used in the original IBM PC) or S-100 style where all the connectors were connected to a common bus. Due to limitations inherent in the Peripheral Component Interconnect (PCI) specification for driving slots, backplanes are now offered as passive and active.

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