A Backplane is a system of parallel electrical conductors in a PCB which are used to connect the connectors. Backplanes are advantageous over traditional cable-type connections as they are mechanically stronger. The cards are mounted on backplanes on slots. This facilitates easy replacement of the cards.
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Backplanes can be classified into Active and Passive Backplanes
Active backplanes are backplanes which have some computing circuitry in built into them. Passive backplanes are backplanes with no computing circuits.
A mid-plane is a backplane which has got slots on both sides.