A backplane (or "backplane system") is a group of

electrical connector Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection between ...

s 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 In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...

. It is used as a backbone to connect several printed circuit boards together to make up a complete

computer system A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These progr ...

. Backplanes commonly use a

printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...

, but wire-wrapped backplanes have also been used in

minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...

s and high-reliability applications. A backplane is generally differentiated from a

motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...

by the lack of on-board processing and storage elements. A backplane uses plug-in cards for storage and processing.

Usage

Early microcomputer systems like the

Altair 8800 The Altair 8800 is a microcomputer designed in 1974 by MITS and based on the Intel 8080 CPU. Interest grew quickly after it was featured on the cover of the January 1975 issue of Popular Electronics and was sold by mail order through advertiseme ...

used a backplane for the processor and

expansion card In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus slo ...

s. Backplanes are normally used in preference to cables because of their greater

reliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), a ...

. 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 DIN 41612 was a DIN standard for electrical connectors that are widely used in rack based electrical systems. Standardisation of the connectors is a pre-requisite for open systems, where users expect components from different suppliers to operat ...

connectors (used in the

VMEbus VMEbus (Versa Module Europa or Versa Module Eurocard bus) is a computer bus standard, originally developed for the Motorola 68000 line of CPUs, but later widely used for many applications and standardized by the IEC as ANSI/IEEE 1014-1987. I ...

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 Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...

transmission method for sending information. 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.

Active versus passive backplanes

Backplanes have grown in complexity from the simple

Industry Standard Architecture Industry Standard Architecture (ISA) is the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8 ...

(ISA) (used in the original

IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...

) or S-100 style where all the connectors were connected to a common bus. Due to limitations inherent in the

Peripheral Component Interconnect Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format th ...

(PCI) specification for driving slots, backplanes are now offered as passive and active. 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 Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion 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 A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working. SPOFs are undesirable in any system with a goal of high availability or reliability, be it a business practice, software appl ...

(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.

Backplanes versus motherboards

When a backplane is used with a plug-in

single-board computer A single-board computer (SBC) is a complete computer built on a single circuit board, with microprocessor(s), memory, input/output (I/O) and other features required of a functional computer. Single-board computers are commonly made as demonstrati ...

(SBC) or system host board (SHB), the combination provides the same functionality as a

, providing processing power, memory, I/O and slots for plug-in cards. While there are a few motherboards that offer more than 8 slots, that is the traditional limit. In addition, as technology progresses, the availability and number of a particular slot type may be limited in terms of what is currently offered by motherboard manufacturers. 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.

Midplane

Some backplanes are constructed with slots for connecting to devices on both sides, and are referred to as midplanes. 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. Midplanes are often used in computers, mostly in

blade server A blade server is a stripped-down server computer with a modular design optimized to minimize the use of physical space and energy. Blade servers have many components removed to save space, minimize power consumption and other considerations, whil ...

s, 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. One common orthogonal midplane connects many vertical telephone

line card A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network. A line card typically interfaces the twisted pair cable of a plain ...

s on one side, each one connected to copper telephone wires, to a horizontal communications card on the other side. 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. 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.

Backplanes in storage

Servers commonly have a backplane to attach hot swappable hard drives; backplane pins pass directly into hard drive sockets without cables. They may have single connector to connect one

disk array controller A disk array controller is a device that manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides a ...

or multiple connectors that can be connected to one or more controllers in arbitrary way. Backplanes are commonly found in

disk enclosure A disk enclosure is a specialized casing designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the dat ...

disk array A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID, deduplication, encryption and virtualization. Compone ...

s, and

server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...

s. Backplanes for SAS and SATA HDDs most commonly use the

SGPIO Serial general purpose input/output (SGPIO) is a four-signal (or four-wire) bus used between a host bus adapter (HBA) and a backplane. Of the four signals, three are driven by the HBA and one by the backplane. Typically, the HBA is a storage con ...

protocol as means of communication between the

host adapter In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other network and storage devices. The terms are primarily used to refer to devices for conne ...

and the backplane. Alternatively

SCSI Enclosure Services SCSI Enclosure Services (SES) is a protocol for more modern SCSI enclosure products. An initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics. SES device ...

can be used. With

Parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...

subsystems,

SAF-TE In computer storage, SAF-TE (abbreviated from SCSI Accessed Fault-Tolerant Enclosure) is an industry standard to interface an enclosure in-band to a (parallel) SCSI subsystem in order to gain access to information or control for various elements ...

is used.

Platforms

PICMG

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. In the Intel Single-Board Computer world,

PICMG PICMG, or PCI Industrial Computer Manufacturers Group, is a consortium of over 140 companies. Founded in 1994, the group was originally formed to adapt PCI technology for use in high-performance telecommunications, military, and industrial comp ...

provides standards for the backplane interface:

PICMG 1.0 PICMG 1.0 is a PICMG specification that defines a CPU form factor and corresponding backplane connectors for PCI- ISA passive backplanes. This standard moves components typically located on the motherboard (i.e. memory, CPUs and chipset component ...

, 1.1 and 1.2 provide ISA and PCI support, with 1.2 adding PCIX support. PICMG 1.3 provides PCI-Express support.