The backplane is a printed circuit board used in computers for implementing multiple in-line circuit boards called daughter boards. On the backplane, rows of slots for the daughter boards are linked by a bus. Each pin in a slot is matched via the bus with the corresponding pin in the next slot over, and so on across the board. In this way, the daughter boards can be inserted and removed, adding whatever functionality they contain to the whole computer's backplane bus.
The board gets its name from its early implementations in computer systems. It often resided at the back of the computer's enclosure, where sometimes rails were used to guide the daughter boards into the expansion slots. The first microcomputer, the Altair 8800, used such a system. Early IBM® personal computers (PC) then took on the industry standard architecture (ISA) bus, which sat on a board in similar fashion.
Due to varying computer construction techniques or specific requirements, backplanes have been created to fit different circumstances. One such method is known as the butterfly plane. In this type of construction, the ports are situated on both sides of the plane to accommodate additional daughter cards. Either single-sided or butterfly-styled, a backplane provides a more stable series of expansion slots than an array of cables, which can bend and wear much more quickly.
There are two major types of backplane; the most common has its slots set out in what's considered a passive configuration. With a passive board, the bus has no additional control mechanisms in place that direct signals along the bus. Signals are thereby controlled by the circuitry and microchips on the daughter boards that are inserted into the plane. In an active backplane, though, another microchip, or special circuitry, is included on the board to monitor and buffer signals entering and leaving the daughter boards.
When used in conjunction with a system host board (SHB), as defined by the Peripheral Component Interconnect (PCI) Industrial Computer Manufacturers Group (PICMG) specifications, the components create the essential workings of a common PC motherboard. The central processing unit (CPU), memory, and input/output (I/O) are situated on separate daughter boards. When combined via a backplane, they create a working computer and allow for hardware configuration changes to happen by replacing a daughter board.