Backplane
Encyclopedia
A backplane is a group of connectors
connected 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 wrap
ped backplanes have also been used in minicomputer
s and high reliability applications.
Early personal computer
s like the Apple II and the IBM PC
integrated an internal backplane for expansion card
s. While a motherboard
may include a backplane for the addition of feature cards, a backplane can stand alone as a separate entity. 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 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. To transmit information Serial Back-Plane technology uses a low voltage differential signaling
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.
(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.
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 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.
(SHB), the combination provides the same functionallity 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.
On the other hand, backplane architecture is somewhat unrelated to the SBC technology that is 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.
or multiple connectors that can be connected to one or more controllers in arbitrary way. Backplanes are commonly found in disk enclosure
s, disk array
s, and server
s.
Backplanes for SAS and SATA HDDs most commonly use the SGPIO
protocol as means of communication between the HBA
and the backplane. Alternatively SCSI Enclosure Services
can be used. With Parallel SCSI
subsystems, SAF-TE
is used in computers, mostly in blade server
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.
.
In the Intel Single Board Computer world, PICMG
provides standards for the backplane interface:
PICMG 1.0, 1.1 and 1.2 provide for ISA and PCI support with 1.2 adding PCIX support.
PICMG 1.3 provides for PCI-Express support.
Electrical connector
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. 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...
connected in parallel with each other, so that each pin
Pin
A pin is a device used for fastening objects or material together.Pin may also refer to:* Award pin, a small piece of metal or plastic with a pin attached given as an award for some achievement...
of each connector is linked to the same relative pin of all the other connectors forming a computer bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
. 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
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
but wire wrap
Wire wrap
Wire wrap is a technology used to assemble electronics. It is a method to construct circuit boards without having to make a printed circuit board. Wires can be wrapped by hand or by machine, and can be hand-modified afterwards. It was popular for large-scale manufacturing in the 60s and early 70s,...
ped backplanes have also been used in minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s and high reliability applications.
Early personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
s like the Apple II and the IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
integrated an internal backplane for expansion card
Expansion card
The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...
s. While a motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
may include a backplane for the addition of feature cards, a backplane can stand alone as a separate entity. 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
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
. 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
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 mostly widely known use of DIN 41612...
connectors used in the VMEbus
VMEbus
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,...
system can withstand 50 to 500 insertions and removals (called mating cycles), depending on their quality. To transmit information Serial Back-Plane technology uses a low voltage differential signaling
Low voltage differential signaling
Low-voltage differential signaling, or LVDS, is an electrical digital signaling system that can run at very high speeds over inexpensive twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and...
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 ISAIndustry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
(used in the original IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
) or S-100
S-100 bus
The S-100 bus or Altair bus, IEEE696-1983 , was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer...
style where all the connectors were connected to a common bus.
Because of limitations inherent in the PCI
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
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 (computer science)
In computer science, a buffer is a region of a physical memory storage used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device or just before it is sent to an output device...
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
Single point of failure
A single point of failure is a part of a system that, if it fails, will stop the entire system from working. They are undesirable in any system with a goal of high availability or reliability, be it a business practice, software application, or other industrial system.-Overview:Systems can be made...
. A passive backplane, even if it is single, is not usually considered a SPOF
Single point of failure
A single point of failure is a part of a system that, if it fails, will stop the entire system from working. They are undesirable in any system with a goal of high availability or reliability, be it a business practice, software application, or other industrial system.-Overview:Systems can be made...
. 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 boardSystem Host Board
thumb|right|300px|PICMG 1.3 SHB and BackplaneSystem Host Board is a term applied to a Single Board Computer meeting the PICMG 1.3 specification. PICMG 1.3 extended the previous PICMG specifications to continue support for PCI/PCI-X plug in cards as well as new support for PCI-Express.- External...
(SHB), the combination provides the same functionallity as a motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
, 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.
On the other hand, backplane architecture is somewhat unrelated to the SBC technology that is 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.
Butterfly Backplanes
Some backplanes are constructed with slots on both sides. These are not the same as midplane backplanes. A butterfly backplane is typically constructed to maximize the number of slots for the least vertical height. The backplane would be mounted vertically in a chassis oriented front to back and the plug-in SBC and cards would lay flat, protruding out both sides of the backplane. This, for example, allows the use of up to four full height boards in a 2U chassis.Backplanes in storage
Servers commonly have a backplane to attach hot swappable hard drives, backplane pins passing directly into hard drive sockets without cables. They may have single connector to connect one disk array controllerDisk array controller
A disk array controller is a device which 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 additional disk cache.A disk array controller name is...
or multiple connectors that can be connected to one or more controllers in arbitrary way. Backplanes are commonly found in disk enclosure
Disk enclosure
A disk enclosure is essentially a specialized chassis 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 data sent across their native data bus into a...
s, disk array
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 and virtualization.Components of a typical disk array include:...
s, and server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
s.
Backplanes for SAS and SATA HDDs most commonly use the SGPIO
SGPIO
SGPIO is an acronym for Serial General Purpose Input/Output which is a 4-signal bus used between a host bus adapter and a backplane; of the 4 signals, 3 are driven by the HBA and 1 by the backplane...
protocol as means of communication between the HBA
Host adapter
In computer hardware, a host controller, host adapter, or host bus adapter connects a host system to other network and storage devices...
and the backplane. Alternatively SCSI Enclosure Services
SCSI Enclosure Services
Most recent SCSI enclosure products support a protocol called SCSI Enclosure Services . The initiator can communicate with the enclosure using a specialised set of SCSI commands to access power, cooling, and other non-data characteristics.-SES devices:...
can be used. With Parallel SCSI
Parallel SCSI
Parallel SCSI is one of the interface implementations in the SCSI family. In addition to being a data bus, SPI is a parallel electrical bus: There is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it...
subsystems, SAF-TE
SAF-TE
In computer storage, a SCSI Accessed Fault-Tolerant Enclosure is an industry standard to interface an enclosure to a SCSI subystem to gain access to information or control concerning* temperature* fan status* slot status...
is used in computers, mostly in blade server
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. Whereas a standard rack-mount server can function with a power cord and network cable, blade servers have many components removed to save space, minimize power...
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.
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 BoardSystem Host Board
thumb|right|300px|PICMG 1.3 SHB and BackplaneSystem Host Board is a term applied to a Single Board Computer meeting the PICMG 1.3 specification. PICMG 1.3 extended the previous PICMG specifications to continue support for PCI/PCI-X plug in cards as well as new support for PCI-Express.- External...
.
In the Intel Single Board Computer world, PICMG
PICMG
The PCI Industrial Computer Manufacturers Group is a consortium of over 227 companies. The group, founded in 1994, was originally formed to adapt PCI technology for use in high-performance telecommunications, military and industrial computing applications but its work has now grown to include...
provides standards for the backplane interface:
PICMG 1.0, 1.1 and 1.2 provide for ISA and PCI support with 1.2 adding PCIX support.
PICMG 1.3 provides for PCI-Express support.