A circuit board containing sockets into which other circuit boards can be plugged. In the context of PCs, it is the large circuit board with sockets for expansion cards. Active backplanes contain, in addition to the sockets, logic circuitry that performs computing functions. Passive backplanes, in contrast, contain almost no computing circuitry. Traditionally, most PCs have used active backplanes; in fact, the motherboard and the backplane have become synonymous. Recently, though, there has been a move toward passive backplanes, with the active components, such as the CPU, inserted on an additional card. Passive backplanes make it easier to repair or upgrade the components.