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A backplane or backplane system is a group of electrical connectors in parallel with 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 to connect several printed circuit boards together to make up a complete computer system. Backplanes commonly use a printed circuit board, but Wire wrap backplanes have also been used in and high-reliability applications.
A backplane is generally differentiated from a motherboard by the lack of on-board processing and storage elements. A backplane uses plug-in cards for storage and processing.
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 or removed from the system; 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, DIN 41612 connectors (used in the VMEbus system) have three durability grades built to withstand (respectively) 50, 400 and 500 insertions and removals, or "mating cycles". To transmit information, Serial Back-Plane technology uses a low-voltage differential signaling transmission method for sending information. (2025). 9780780393073 ISBN 9780780393073
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 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.
True 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 clear, but may become an important issue if a whole system is expected to not have a single point of failure (SPOF) . Common myth around passive backplane, even if it is single, is not usually considered a SPOF. Active back-planes are even more complicated and thus have a non-zero risk of malfunction. However one situation that can cause disruption both in the case of Active and Passive Back-planes is while performing maintenance activities i.e. while swapping boards there is always a possibility of damaging the Pins/Connectors on the Back-plane, this may cause full outage for the system as all boards mounted on the back-plane should be removed in order to fix the system. Therefore, we are seeing newer architectures where systems use high speed redundant connectivity to interconnect system boards point to point with No Single Point of Failure anywhere in the system.
However, backplane architecture is somewhat unrelated to the SBC technology plugged into it. There are some limitations to what can be constructed, in that the SBC chip set and processor have to provide the capability of supporting the slot types. In addition, virtually an unlimited number of slots can be provided with 20, including the SBC slot, as a practical though not an absolute limit. Thus, a PICMG backplane can provide any number and any mix of ISA, PCI, PCI-X, and PCI-e slots, limited only by the ability of the SBC to interface to and drive those slots. For example, an SBC with the latest i7 processor could interface with a backplane providing up to 19 ISA slots to drive legacy I/O cards.
Midplanes are often used in computers, mostly in , where server blades reside on one side and the peripheral (power, networking, and other I/O) and service modules reside on the other. Midplanes are also 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.
Orthogonal midplanes connect vertical cards on one side to horizontal boards on the other side.Kevin O’Connor. "Orthogonal Backplane Connector Technology Offers Design Flexibility" . 2010.Pete. "High-Speed Orthogonal Connectors Optimize Signal Integrity" . 2011. One common orthogonal midplane connects many vertical telephone on one side, each one connected to copper telephone wires, to a horizontal communications card on the other side. "AirMax VS Orthogonal" .
A "virtual midplane" is an imaginary plane between vertical cards on one side that directly connect to horizontal boards on the other side; the card-slot aligners of the card cage and self-aligning connectors on the cards hold the cards in position. Michael Fowler. "Virtual Midplane Realizes Ultrafast Card Interconnects". Electronic Design. 2002.
Some people use the term "midplane" to describe a board that sits between and connects a hard drive hot-swap backplane and redundant power supplies. "HP StorageWorks Modular Smart Array 70 Enclosure - Replacing the Backplane". "Intel Server System SR2612UR Service Guide".
Backplanes for SAS and SATA HDDs most commonly use the SGPIO protocol as means of communication between the host adapter and the backplane. Alternatively SCSI Enclosure Services can be used. With Parallel SCSI subsystems, SAF-TE is used.
In the Intel Single-Board Computer world, PICMG provides standards for the backplane interface: PICMG 1.0, 1.1 and 1.2 provide ISA and PCI support, with 1.2 adding PCIX support. PICMG 1.3 provides PCI-Express support.
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