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A backplane is a circuit board (usually a printed circuit board) that connects several connectors in parallel to 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 board cards together to make up a complete computer system. One popular early computer system that used this approach was called the S-100 bus because the connectors used had one hundred pins. Some computers like the Apple II and the IBM PC integrated an internal backplane for expansion cards.
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 to or removed from the system; and 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.
Active vs. passive
Backplanes have grown in complexity from the simple ISA (Industry Standard Architecture used in the original IBM PC) or S-100 style where all the connectors were connected to a common bus. Because of limitations inherent in the PCI specification for driving slots, backplanes are now offered as passive and active. Passive backplanes offer no active bus driving circuitry. Active backplanes include chips which buffer the various signals to the slots.
Backplanes in Storage
Whereas a backplane resides at the back side of chassis, a midplane is located in the middle of a system chassis. This affords the system operator the ability to plug cards into either side of a midplane. Midplane system design is popular in networking and telecommunications equipment where one side of the chassis accepts system processing cards and the other side of the chassis accepts network interface cards.