A backplane (or "backplane system") 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 [ [,,sid44_gci211632,00.html Definition] ] , 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. Early personal computers like the Apple II and the IBM PC integrated an internal backplane for expansion cards.

While a motherboard may include a backplane, the backplane is actually a separate entity. A backplane is generally differentiated from a motherboard by the lack of on-board processing power where the CPU is on a plug-in card.

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.

A backplane provides minimal functionality without a controlling Single Board Computer installed providing the CPU and other computer functions. A Single Board Computer meeting the PICMG 1.3 specification and compatible with a PICMG 1.3 backplane is referred to as a System Host Board.

A backplane can be used without an associated Single Board Computer to simply provide power to the plug-in cards. This is a common usage for companies manufacturing plug-in cards to power them for burn-in.

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 what 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. Bus expansion cables do not need a Single Board Computer in the remote bus to control the I/O cards as that is provided by the expansion electronics.

Active backplanes

Backplanes have grown in complexity from the simple ISA (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. Any desired arbitration logic is placed on the daughter cards. Active backplanes include chips which buffer the various signals to the slots.

The distinction between the two isn't always very clear, but may become an important issue if a whole system is expected to have no single point of failure. A passive backplane, even if it "is" single, is not usually considered a SPOF. Active backplanes are more complicated and thus have a non-zero risk of malfunction.

In the Intel Single Board Computer world, PICMG provides standards for the backplane interface:PICMG 1.0, 1.1 and 1.2 [ [ PICMG 1.0, 1.1 and 1.2] ] provide for ISA and PCI support with 1.2 adding PCIX support.PICMG 1.3 [ [ PICMG 1.3] ] [ [ PICMG 1.3 SHB Express Resources] ] provides for PCI-Express support.

Backplanes in storage

Backplanes have also become commonplace for connecting multiple hard drives to a single disk array controller. Backplanes are commonly found in disk enclosures, disk arrays, and servers.

Backplanes for SAS and SATA HDDs most commonly use the SGPIO protocol as means of communication between the HBA and the backplane.


Whereas cards and devices connect to only one side of a backplane, a midplane has cards and devices connected to both sides. This ability to plug cards into either side of a midplane is often useful in larger systems made up primarily of modules attached to the 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. Most blade server systems also use this type of design, with server blades on one side, and peripheral (power, networking, and other I/O) and service modules on the other.

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