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Major components on a PICMG 1.3 active backplane PICMG-Backplane-Details.jpg
Major components on a PICMG 1.3 active backplane
Wire-wrapped backplane from a 1960s PDP-8 minicomputer PDP-8I-backplane.jpg
Wire-wrapped backplane from a 1960s PDP-8 minicomputer

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 as a backbone to connect several printed circuit boards together to make up a complete computer system. Backplanes commonly use a printed circuit board, but wire-wrapped backplanes have also been used in minicomputers and high-reliability applications.

Electrical connector electro-mechanical device used to connect electrical parts

An electrical connector is an electro-mechanical device used to join electrical terminations and create an electrical circuit. Electrical connectors consist of plugs (male-ended) and jacks (female-ended). The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or devices. An adapter can be used to effectively bring together dissimilar connectors.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Wire wrap

Wire wrap was invented to wire telephone crossbar switches, and later adapted to construct electronic circuit boards. Electronic components mounted on an insulating board are interconnected by lengths of insulated wire run between their terminals, with the connections made by wrapping several turns of uninsulated sections of the wire around a component lead or a socket pin.


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.

Motherboard printed circuit board (PCB) found in all modern computers

A motherboard is the main printed circuit board (PCB) found in general purpose computers and other expandable systems. It holds and allows communication between many of the crucial electronic components of a system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a backplane, a motherboard usually contains significant sub-systems such as the central processor, the chipset's input/output and memory controllers, interface connectors, and other components integrated for general purpose use and applications.


Early microcomputer systems like the Altair 8800 used a backplane for the processor and expansion cards.

Altair 8800 microcomputer designed in 1975

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 advertisements there, in Radio-Electronics, and in other hobbyist magazines. The designers hoped to sell a few hundred build-it-yourself kits to hobbyists, and were surprised when they sold thousands in the first month. The Altair also appealed to individuals and businesses that just wanted a computer and purchased the assembled version. The Altair is widely recognized as the spark that ignited the microcomputer revolution as the first commercially successful personal computer. The computer bus designed for the Altair was to become a de facto standard in the form of the S-100 bus, and the first programming language for the machine was Microsoft's founding product, Altair BASIC.

Expansion card a printed circuit board that can be inserted into an electrical connector, or expansion slot on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus

In computing, the expansion card, expansion board, adapter card or accessory card is a printed circuit board that can be inserted into an electrical connector, or expansion slot, on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus.

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. [1]

Reliability engineering is a sub-discipline of systems engineering that emphasizes dependability in the lifecycle management of a product. Dependability, or reliability, describes the ability of a system or component to function under stated conditions for a specified period of time. Reliability is closely related to availability, which is typically described as the ability of a component or system to function at a specified moment or interval of time.

DIN 41612

DIN 41612 is 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 operate together. The most widely known use of DIN 41612 connectors is in the VMEbus system. They were also used by NuBus. The standard has subsequently been upgraded to international standards IEC 60603-2 and EN 60603-2.


VMEbus 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. It is physically based on Eurocard sizes, mechanicals and connectors, but uses its own signalling system, which Eurocard does not define. It was first developed in 1981 and continues to see widespread use today.

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

ISA Passive Backplane showing connectors and parallel signal traces on back side. Only components are connectors, capacitors, resistors and voltage indicator LEDs. ISA-Backplane.jpg
ISA Passive Backplane showing connectors and parallel signal traces on back side. Only components are connectors, capacitors, resistors and voltage indicator LEDs.

Backplanes have grown in complexity from the simple Industry Standard Architecture (ISA) (used in the original IBM PC) or S-100 style where all the connectors were connected to a common bus. Due to limitations inherent in the Peripheral Component Interconnect (PCI) specification for driving slots, backplanes are now offered as passive and active.

Industry Standard Architecture 16-bit internal bus of IBM PC/AT

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 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.

S-100 bus

The S-100 bus or Altair bus, IEEE696-1983 (withdrawn), is an early computer bus designed in 1974 as a part of the Altair 8800. The S-100 bus was the first industry standard expansion bus for the microcomputer industry. S-100 computers, consisting of processor and peripheral cards, were produced by a number of manufacturers. The S-100 bus formed the basis for homebrew computers whose builders implemented drivers for CP/M and MP/M. These S-100 microcomputers ran the gamut from hobbyist toy to small business workstation and were common in early home computers until the advent of the IBM PC.

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). 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.

Backplanes versus motherboards

When a backplane is used with a plug-in single board computer (SBC) or system host board (SHB), the combination provides the same functionality as a motherboard, 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.


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 servers, 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. [2] [3] One common orthogonal midplane connects many vertical telephone line cards on one side, each one connected to copper telephone wires, to a horizontal communications card on the other side. [4]

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. [5]

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. [6] [7]

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 or multiple connectors that can be connected to one or more controllers in arbitrary way. 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 host adapter and the backplane. Alternatively SCSI Enclosure Services can be used. With Parallel SCSI subsystems, SAF-TE is used.



A single-board computer installed into a passive backplane SBC-Backplane.jpg
A single-board computer installed into a passive backplane

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 provides standards for the backplane interface: PICMG 1.0, 1.1 and 1.2 [8] provide ISA and PCI support, with 1.2 adding PCIX support. PICMG 1.3 [9] [10] provides PCI-Express support.

See also

Related Research Articles

Eurocard (printed circuit board)

Eurocard is a European standard format for printed circuit board (PCB) cards that can be plugged together into a standard chassis which, in turn, can be mounted in a 19-inch rack. The chassis consists of a series of slotted card guides on the top and bottom, into which the cards are slid so they stand on end, like books on a shelf. At the spine of each card is one or more connectors which plug into mating connectors on a backplane that closes the rear of the chassis.

VESA Local Bus VESA Local Bus

The VESA Local Bus was a short-lived expansion bus introduced during the i486 generation of x86 IBM compatible personal computers. Created by VESA the VESA Local Bus worked alongside the then dominant ISA bus to provide a standardized high-speed conduit intended primarily to accelerate video (graphics) operations. VLB provided a standardized "fast path" that add-in (video) card makers could tap for greatly accelerated memory-mapped I/O and DMA, while still using the familiar ISA bus to handle basic device duties such as interrupts and port-mapped I/O.

A video card is an expansion card which generates a feed of output images to a display device. Frequently, these are advertised as discrete or dedicated graphics cards, emphasizing the distinction between these and integrated graphics. At the core of both is the graphics processing unit (GPU), which is the main part that does the actual computations, but should not be confused as the video card as a whole, although "GPU" is often used to refer to video cards.

Single-board computer complete computer built on a single circuit board

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 were made as demonstration or development systems, for educational systems, or for use as embedded computer controllers. Many types of home computers or portable computers integrate all their functions onto a single printed circuit board.

Edge connector

An edge connector is the portion of a printed circuit board (PCB) consisting of traces leading to the edge of the board that are intended to plug into a matching socket. The edge connector is a money-saving device because it only requires a single discrete female connector, and they also tend to be fairly robust and durable. They are commonly used in computers for expansion slots for peripheral cards, such as PCI, PCI Express, and AGP cards.

Autoconfig is an auto-configuration protocol of Amiga computers which is intended to automatically assign resources to expansion devices without the need for jumper settings. It is analogous to PCI configuration.


CompactPCI is a computer bus interconnect for industrial computers, combining a Eurocard-type connector and PCI signaling and protocols. Boards are standardized to 3U or 6U sizes, and are typically interconnected via a passive backplane. The connector pin assignments are standardized by the PICMG US and PICMG Europe organizations. The connectors and the electrical rules allow for eight boards in a PCI segment. Multiple bus segments are allowed with bridges.

Blade server type of server computer

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, while still having all the functional components to be considered a computer. Unlike a rack-mount server, a blade server fits inside a blade enclosure, which can hold multiple blade servers, providing services such as power, cooling, networking, various interconnects and management. Together, blades and the blade enclosure form a blade system, which may itself be rack-mounted. Different blade providers have differing principles regarding what to include in the blade itself, and in the blade system as a whole.

Advanced Telecommunications Computing Architecture is the largest specification effort in the history of the PCI Industrial Computer Manufacturers Group (PICMG), with more than 100 companies participating. Known as AdvancedTCA, the official specification designation PICMG 3.x was ratified by the PICMG organization in December 2002. AdvancedTCA is targeted primarily to requirements for "carrier grade" communications equipment, but has recently expanded its reach into more ruggedized applications geared toward the military/aerospace industries as well. This series of specifications incorporates the latest trends in high speed interconnect technologies, next-generation processors, and improved Reliability, Availability and Serviceability (RAS).

Advanced Mezzanine Cards are printed circuit boards (PCBs) that follow a specification of the PCI Industrial Computers Manufacturers Group (PICMG), with more than 100 companies participating.

In electronic systems a diagnostic board is a specialized device with diagnostic circuitry on a printed circuit board that connects to a computer or other electronic equipment replacing an existing module, or plugging into an expansion card slot.

SGI Origin 200

The SGI Origin 200, code named Speedo, was an entry-level server computer developed and manufactured by SGI, introduced in October 1996 to accompany their mid-range and high-end Origin 2000. It is based on the same architecture as the Origin 2000 but has an unrelated hardware implementation. At the time of introduction, these systems ran the IRIX 6.4, and later, the IRIX 6.5 operating systems. The Origin 200 was discontinued on 30 June 2002.

CompactPCI Serial is an industrial standard for modular computer systems. It is based on the established PICMG 2.0 CompactPCI standard, which uses the parallel PCI bus for communication among a system's card components. In contrast to this, CompactPCI Serial uses only serial point-to-point connections. CompactPCI Serial was officially adopted by the PCI Industrial Computer Manufacturers Group PICMG as PICMG CPCI-S.0 CompactPCI Serial in March 2011. Its mechanical concept is based on the proven standards of IEEE 1101-1-1998 and IEEE 1101-10-1996. CompactPCI Serial includes different connectors that permit very high data rates. The new technology standard succeeding parallel CompactPCI comprises another specification called PICMG 2.30 CompactPCI PlusIO. This is why CompactPCI Serial and CompactPCI PlusIO as a whole were also called CompactPCI Plus. PICMG's first working title of CompactPCI Serial was CPLUS.0. CompactPCI Serial backplanes and chassis are developed by Schroff, Elmа, and Pixus Technologies companies, as for the CompactPCI Serial board level electronics – they are developed by MEN Mikro Elektronik, Fastwel, EKF, Emerson Embedded Computing, ADLINK, and Kontron.

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 to a single plug-in card. PICMG 1.0 CPU Cards look much like standard ISA cards with extra gold finger connections for the ISA bus and the root PCI bus. The "motherboard" is replaced with a simple "passive backplane" that has only PCI and ISA connectors attached to it. These backplane connections include a dedicated system slot of the PICMG 1.0 CPU and various connections for standard ISA and PCI peripheral cards. This backplane is simple and robust, with a very low likelihood of failure, given its passive nature. This allows a much lower Mean Time to Repair than classic computer motherboard approaches, as electronics associated with CPUs can be replaced without having to remove peripheral devices.

PICOe is a computer form factor in which a half sized card slot Single Board Computer (SBC) is inserted into a gold fingers card slot of a passive or active backplane. Expansion peripherals of the computer system are connected to other slots of the backplane.

Modular crate electronics are a general type of electronics and support infrastructure commonly used for trigger electronics and data acquisition in particle detectors. These types of electronics are common in such detectors because all the electronic pathways are made by discrete physical cables connecting together logic blocks on the fronts of modules. This allows circuits to be designed, built, tested, and deployed very quickly as an experiment is being put together. Then the modules can all be removed and used again when the experiment is done.


  1. Varnarvas, Kosta. "Serial Back-Plane Technologies in Advanced Avionics Architectures". NASA Technical Reports Server. Marshall Space Flight Center. Retrieved 19 October 2011.
  2. Kevin O’Connor. "Orthogonal Backplane Connector Technology Offers Design Flexibility". 2010.
  3. Pete. "High-Speed Orthogonal Connectors Optimize Signal Integrity" Archived 28 April 2015 at the Wayback Machine . 2011.
  4. "AirMax VS Orthogonal".
  5. Michael Fowler. "Virtual Midplane Realizes Ultrafast Card Interconnects". Electronic Design. 2002.
  6. "HP StorageWorks Modular Smart Array 70 Enclosure - Replacing the Backplane".
  7. "Intel Server System SR2612UR Service Guide".
  8. "PICMG 1.0, 1.1 and 1.2". Archived from the original on 26 June 2012. Retrieved 2012-09-20.
  9. "PICMG 1.3". Archived from the original on 26 June 2012. Retrieved 2012-09-20.
  10. "PICMG 1.3 SHB Express Resources". Archived from the original on 30 November 2012. Retrieved 2012-09-20.