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In
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
, a bus (a contraction of the Latin ''
omnibus Omnibus may refer to: Film and television * Omnibus (film), ''Omnibus'' (film) * Omnibus (broadcast), a compilation of TV episodes * Omnibus (UK TV series), ''Omnibus'' (UK TV series), an arts-based documentary programme * Omnibus (U.S. TV series ...
'', and historically also called ''data highway'') is a communication system that transfers
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
between components inside a
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

computer
, or between computers. This expression covers all related
hardware Hardware may refer to: Technology Computing and electronics * Computer hardware, physical parts of a computer * Digital electronics, electronics that operate on digital signals * Electronic component, device in an electronic system used to affect e ...

hardware
components (wire,
optical fiber An optical fiber (or fibre in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codificati ...

optical fiber
, etc.) and
software Software is a collection of instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Computer program, a collection of instructions Music * I ...

software
, including
communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networ ...
s. Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel . Modern computer buses can use both
parallel Parallel may refer to: Computing * Parallel algorithm In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their a ...

parallel
and
bit serial 300px, Parallel versus serial communication. In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and th ...
connections, and can be wired in either a
multidropA multidrop bus (MDB) is a computer bus In computer architecture, a bus (a contraction of the Latin '' omnibus'', and historically also called ''data highway'') is a communication system that transfers data Data are units of information ...
(electrical parallel) or
daisy chain Daisy chain may refer to: * Daisy chain, a garland created from daisy flowers * Daisy chain (climbing), a type of strap * Daisy chain (electrical engineering), a wiring scheme * Daisy chain (fishing), a type of fishing lure * Daisy chain (knot), or ...
topology, or connected by switched hubs, as in the case of
USB Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Brot ...
.


Background and nomenclature

Computer systems generally consist of three main parts: *The
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electroni ...

central processing unit
(CPU) that processes data, *The
memory Memory is the faculty of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exis ...
that holds the programs and data to be processed, and *I/O (
input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Information, Inputs are ...
) devices as
peripherals A peripheral or peripheral device is an auxiliary device used to put information into and get information out of the computer. The term peripheral device refers to all hardware components that are attached to a computer and are controlled by the co ...
that communicate with the outside world. An early computer might contain a hand-wired CPU of
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
s, a
magnetic drum Magnetism is a class of physical phenomena that are mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Magnetism is one ...
for main memory, and a
punch tape Five- and eight-hole punched paper tape Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop Punched tape or perforated paper tape is a form of data storage ...
and
printer Printers may be: Technology * Printer (publishing) In publishing, printers are both companies A company, abbreviated as co., is a Legal personality, legal entity representing an association of people, whether Natural person, natural, Legal ...
for reading and writing data respectively. A modern system might have a
multi-core CPU A multi-core processor is a computer processor on a single integrated circuit with two or more separate Central processing unit, processing units, called cores, each of which reads and executes Instruction set, program instructions. The instruc ...
,
DDR4 SDRAM Double Data Rate 4 Synchronous Dynamic Random-Access Memory (DDR4 SDRAM) is a type of with a high ("") interface. Released to the market in 2014, it is a variant of (DRAM), of which some have been in use since the early 1970s, and a higher-sp ...
for memory, a
solid-state drive A solid-state drive (SSD) is a solid-state storage Solid-state storage (SSS) is a type of non-volatile computer storage that stores and retrieves digital information Digital usually refers to something using digits, particularly binary di ...
for
secondary storage A spindle of DVD-RW's. Computer data storage is a technology consisting of computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perfor ...
, a
graphics card A graphics card (also called a video card, display card, graphics adapter, video adapter, or display adapter) is an expansion card Modern EEPROM chip suitable for storing expansion card configuration electronically In computing Compu ...
and
LCD A liquid-crystal display (LCD) is a flat-panel display A flat-panel display (FPD) is an electronic display device s, LED display and Vacuum fluorescent display, VF display, top to bottom. A display device is an output device for presentation ...
as a display system, a
mouse A mouse, plural mice, is a small mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which ...

mouse
and for interaction, and a
Wi-Fi Wi-Fi () is a family of wireless network A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installa ...

Wi-Fi
connection for networking. In both examples, computer buses of one form or another move data between all of these devices. In most traditional
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
s, the CPU and main memory tend to be tightly coupled. A
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

microprocessor
conventionally is a single chip which has a number of
electrical connection An electrical connector is an electromechanical device used to join electrical conductor In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , ...
s on its
pin A pin is a device used for fastening objects or material together, and can have three sorts of body: a shaft of a rigid inflexible material meant to be inserted in a slot, groove, or hole (as with pivots, hinges, and jigs); a shaft connected to ...
s that can be used to select an "address" in the main memory and another set of pins to read and write the data stored at that location. In most cases, the CPU and memory share signalling characteristics and operate in synchrony. The bus connecting the CPU and memory is one of the defining characteristics of the system, and often referred to simply as the
system bus A system bus is a single computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a ...
. It is possible to allow peripherals to communicate with memory in the same fashion, attaching adaptors in the form of
expansion card Modern EEPROM chip suitable for storing expansion card configuration electronically In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentati ...
s directly to the system bus. This is commonly accomplished through some sort of standardized electrical connector, several of these forming the
expansion bus File:AT24C02 EEPROM 1480355 6 7 HDR Enhancer.jpg, Modern EEPROM chip suitable for storing expansion card configuration electronically In computing, an expansion card (also called expansion board, adapter card or accessory card) is a printed ...
or
local bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine that can be programmed to carry ou ...

local bus
. However, as the
performance A performance is an act of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Management science In the work place ...
differences between the CPU and peripherals varies widely, some solution is generally needed to ensure that peripherals do not slow overall system performance. Many CPUs feature a second set of pins similar to those for communicating with memory, but able to operate at very different speeds and using different protocols. Others use smart controllers to place the data directly in memory, a concept known as
direct memory access Direct memory access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the retention of inform ...
. Most modern systems combine both solutions, where appropriate. As the number of potential peripherals grew, using an expansion card for every peripheral became increasingly untenable. This has led to the introduction of bus systems designed specifically to support multiple peripherals. Common examples are the
SATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a interface that connects to s such as s, s, and s. Serial ATA succeeded the earlier (PATA) standard to become the predominant interface for storage devices. Serial ATA industry co ...

SATA
ports in modern computers, which allow a number of hard drives to be connected without the need for a card. However, these high-performance systems are generally too expensive to implement in low-end devices, like a mouse. This has led to the parallel development of a number of low-performance bus systems for these solutions, the most common example being the standardized
Universal Serial Bus Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Bro ...
(USB). All such examples may be referred to as
peripheral busIn computing, a peripheral bus is a computer bus In computer architecture, a bus (a contraction of the Latin '' omnibus'', and historically also called ''data highway'') is a communication system that transfers data Data are units of info ...
es, although this terminology is not universal. In modern systems the performance difference between the CPU and main memory has grown so great that increasing amounts of high-speed memory is built directly into the CPU, known as a
cache Cache, caching, or caché may refer to: Places * Cache (Aosta) Cache is a frazione of the city of Aosta, in the Aosta Valley region of Italy. Frazioni of Aosta Valley Aosta {{Aosta-geo-stub ..., a frazione in Italy * Cache Creek (disambig ...
. In such systems, CPUs communicate using high-performance buses that operate at speeds much greater than memory, and communicate with memory using protocols similar to those used solely for peripherals in the past. These system buses are also used to communicate with most (or all) other peripherals, through adaptors, which in turn talk to other peripherals and controllers. Such systems are architecturally more similar to multicomputers, communicating over a bus rather than a network. In these cases, expansion buses are entirely separate and no longer share any architecture with their host CPU (and may in fact support many different CPUs, as is the case with PCI). What would have formerly been a system bus is now often known as a
front-side bus A front-side bus (FSB) is a computer communication interface (bus trolleybus in Toronto Toronto is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a recorded population of 2,731,571 in 2 ...
. Given these changes, the classical terms "system", "expansion" and "peripheral" no longer have the same connotations. Other common categorization systems are based on the bus's primary role, connecting devices internally or externally, PCI vs.
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
for instance. However, many common modern bus systems can be used for both;
SATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a interface that connects to s such as s, s, and s. Serial ATA succeeded the earlier (PATA) standard to become the predominant interface for storage devices. Serial ATA industry co ...

SATA
and the associated
eSATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implemen ...

eSATA
are one example of a system that would formerly be described as internal, while certain automotive applications use the primarily external
IEEE 1394 IEEE 1394 is an interface standard for a serial communication, serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple Inc., Apple in cooperation with a number ...
in a fashion more similar to a system bus. Other examples, like
InfiniBand InfiniBand (IB) is a computer networking communications standard used in high-performance computing A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer ...
and
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orch ...
were designed from the start to be used both internally and externally.


Internal buses

The internal bus, also known as internal data bus,
memory bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structur ...
,
system bus A system bus is a single computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a ...
or
front-side bus A front-side bus (FSB) is a computer communication interface (bus trolleybus in Toronto Toronto is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a recorded population of 2,731,571 in 2 ...
, connects all the internal components of a computer, such as CPU and memory, to the motherboard. Internal data buses are also referred to as local buses, because they are intended to connect to local devices. This bus is typically rather quick and is independent of the rest of the computer operations.


External buses

The external bus, or
expansion bus File:AT24C02 EEPROM 1480355 6 7 HDR Enhancer.jpg, Modern EEPROM chip suitable for storing expansion card configuration electronically In computing, an expansion card (also called expansion board, adapter card or accessory card) is a printed ...
, is made up of the electronic pathways that connect the different external devices, such as printer etc., to the computer.


Address bus

An address bus is a bus that is used to specify a
physical address 300px, Diagram of relationship between the virtual and physical address spaces In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorith ...
. When a or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus (the value to be read or written is sent on the
data bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine that can be programmed to carry ...
). The width of the address bus determines the amount of memory a system can address. For example, a system with a 32-bit address bus can address 232 (4,294,967,296) memory locations. If each memory location holds one byte, the addressable memory space is 4 GiB.


Address multiplexing

Early processors used a wire for each bit of the address width. For example, a 16-bit address bus had 16 physical wires making up the bus. As the buses became wider and lengthier, this approach became expensive in terms of the number of chip pins and board traces. Beginning with the
Mostek Mostek was an integrated circuit manufacturer, founded in 1969 by L. J. Sevin, Louay E. Sharif, Richard L. Petritz and other ex-employees of Texas Instruments. Initially their products were manufactured in Worcester, Massachusetts, however by 19 ...
4096
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum fr ...

DRAM
, address multiplexing implemented with
multiplexer In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplif ...

multiplexer
s became common. In a multiplexed address scheme, the address is sent in two equal parts on alternate bus cycles. This halves the number of address bus signals required to connect to the memory. For example, a 32-bit address bus can be implemented by using 16 lines and sending the first half of the memory address, immediately followed by the second half memory address. Typically 2 additional pins in the control bus -- a row-address strobe (RAS) and the column-address strobe (CAS) -- are used to tell the DRAM whether the address bus is currently sending the first half of the memory address or the second half.


Implementation

Accessing an individual byte frequently requires reading or writing the full bus width (a
word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most lang ...
) at once. In these instances the least significant bits of the address bus may not even be implemented - it is instead the responsibility of the controlling device to isolate the individual byte required from the complete word transmitted. This is the case, for instance, with the
VESA Local Bus The VESA Local Bus (usually abbreviated to VL-Bus or VLB) is a short-lived expansion bus File:AT24C02 EEPROM 1480355 6 7 HDR Enhancer.jpg, Modern EEPROM chip suitable for storing expansion card configuration electronically In computing, an ...
which lacks the two least significant bits, limiting this bus to aligned 32-bit transfers. Historically, there were also some examples of computers which were only able to address words -- word machines.


Implementation details

Buses can be
parallel bus In , parallel communication is a method of conveying multiple binary digits (s) simultaneously. It contrasts with , which conveys only a single bit at a time; this distinction is one way of characterizing a . The basic difference between a para ...
es, which carry data words in parallel on multiple wires, or
serial bus In telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or r ...
es, which carry data in bit-serial form. The addition of extra power and control connections, differential drivers, and data connections in each direction usually means that most serial buses have more conductors than the minimum of one used in
1-Wire 1-Wire is a device communications bus system designed by Dallas Semiconductor Corp. that provides low-speed (16.3 kbit/s) data, signaling, and power over a single conductor Conductor or conduction may refer to: Music * Conductor (music), ...
and
UNI/O and chip scale package, wafer level chip scale packages sitting on the face of a U.S. penny The UNI/O bus is an asynchronous serial communication, asynchronous Serial communications, serial Bus (computing), bus created by Microchip Technology ...
. As data rates increase, the problems of timing skew, power consumption, electromagnetic interference and
crosstalk In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particl ...
across parallel buses become more and more difficult to circumvent. One partial solution to this problem has been to double pump the bus. Often, a serial bus can be operated at higher overall data rates than a parallel bus, despite having fewer electrical connections, because a serial bus inherently has no timing skew or crosstalk.
USB Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Brot ...
,
FireWire IEEE 1394 is an interface standard In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defi ...
, and
Serial ATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implemen ...

Serial ATA
are examples of this.
MultidropA multidrop bus (MDB) is a computer bus In computer architecture, a bus (a contraction of the Latin '' omnibus'', and historically also called ''data highway'') is a communication system that transfers data Data are units of information ...
connections do not work well for fast serial buses, so most modern serial buses use or hub designs.
Network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
connections such as
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
are not generally regarded as buses, although the difference is largely conceptual rather than practical. An attribute generally used to characterize a bus is that power is provided by the bus for the connected hardware. This emphasizes the
busbar In , a busbar (also bus bar) is a metallic strip or bar, typically housed inside , , and for local high current power distribution. They are also used to connect high voltage equipment at electrical switchyards, and low voltage equipment i ...

busbar
origins of bus architecture as supplying switched or distributed power. This excludes, as buses, schemes such as serial
RS-232 In telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It has its origin in the desire of ...
, parallel
Centronics Centronics Data Computer Corporation was an American manufacturer of computer printers, now remembered primarily for the parallel interface In computing Computing is any goal-oriented activity requiring, benefiting from, or creating ...

Centronics
,
IEEE 1284 IEEE 1284 is a standard that defines bi-directional parallel communications 300px, Parallel versus serial communication In data transmission Data transmission and data reception (or, more broadly, data communication or digital communications) is ...
interfaces and Ethernet, since these devices also needed separate power supplies.
Universal Serial Bus Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Bro ...
devices may use the bus supplied power, but often use a separate power source. This distinction is exemplified by a
telephone A telephone is a telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anyt ...
system with a connected
modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telep ...

modem
, where the
RJ11 A registered jack (RJ) is a standardized telecommunication network interface device, network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces w ...

RJ11
connection and associated modulated signalling scheme is not considered a bus, and is analogous to an
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
connection. A phone line connection scheme is not considered to be a bus with respect to signals, but the uses buses with cross-bar switches for connections between phones. However, this distinctionthat power is provided by the busis not the case in many , where data connections such as
ARINC 429 ARINC 429, "Mark33 Digital Information Transfer System (DITS)," is also known as the Aeronautical Radio INC. (ARINC) technical standard for the predominant used on most higher-end commercial and transport aircraft. It defines the physical and elec ...
,
ARINC 629 The ARINC 629 computer bus was introduced in May 1995 and is used on the Boeing 777. The ARINC 629 bus operates as a multiple-source, multiple-sink system; each terminal can transmit data to, and receive data from, every other terminal on the data b ...
, MIL-STD-1553B (STANAG 3838), and EFABus (
STANAG 3910STANAG 3910 ''High Speed Data Transmission Under STANAG 3838 or Fibre Optic Equivalent Control'' is a protocol defined in a NATO The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also cal ...
) are commonly referred to as “data buses” or, sometimes, "databuses". Such avionic data buses are usually characterized by having several equipments or Line Replaceable Items/Units (LRI/LRUs) connected to a common, shared
media Media may refer to: Physical means Communication * Media (communication), tools used to deliver information or data ** Advertising media, various media, content, buying and placement for advertising ** Broadcast media, communications deliv ...
. They may, as with ARINC 429, be
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given space. For e ...
, i.e. have a single source LRI/LRU or, as with ARINC 629, MIL-STD-1553B, and STANAG 3910, be duplex, allow all the connected LRI/LRUs to act, at different times (
half duplex A duplex communication system 400px, Communication system A communications system or communication system is a collection of individual telecommunications network A telecommunications network is a group of nodes interconnected by links ...
), as transmitters and receivers of data.Avionic Systems Standardisation Committee, ''Guide to Digital Interface Standards For Military Avionic Applications'', ASSC/110/6/2, Issue 2, September 2003


Bus multiplexing

The simplest
system bus A system bus is a single computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a ...
has completely separate input data lines, output data lines, and address lines. To reduce cost, most microcomputers have a bidirectional data bus, re-using the same wires for input and output at different times. Don Lancaster
"TV Typewriter Cookbook"
(
TV Typewriter The TV Typewriter is a video terminal that could display two pages of 16 lines of 32 upper case characters on a standard television set A Sony Wega CRT television set A television set or television receiver, more commonly called the television ...
). Section "Bus Organization". p. 82.
Some processors use a dedicated wire for each bit of the address bus, data bus, and the control bus. For example, the 64-pin
STEbus The STEbus (also called the IEEE-1000 bus) is a non-proprietary, processor-independent, computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functional ...
is composed of 8 physical wires dedicated to the 8-bit data bus, 20 physical wires dedicated to the 20-bit address bus, 21 physical wires dedicated to the control bus, and 15 physical wires dedicated to various power buses. Bus multiplexing requires fewer wires, which reduces costs in many early microprocessors and DRAM chips. One common multiplexing scheme, address multiplexing, has already been mentioned. Another multiplexing scheme re-uses the address bus pins as the data bus pins, an approach used by
conventional PCI Peripheral Component Interconnect (PCI) is a local Local may refer to: Geography and transportation * Local (train), a train serving local traffic demand * Local, Missouri, a community in the United States * Local government, a form of public a ...
and the
8086 The 8086 (also called iAPX 86) is a 16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations aut ...
. The various "serial buses" can be seen as the ultimate limit of multiplexing, sending each of the address bits and each of the data bits, one at a time, through a single pin (or a single differential pair).


History

Over time, several groups of people worked on various computer bus standards, including the IEEE Bus Architecture Standards Committee (BASC), the IEEE "Superbus" study group, the open microprocessor initiative (OMI), the open microsystems initiative (OMI), the "Gang of Nine" that developed EISA, etc.


First generation

Early
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

computer
buses were bundles of wire that attached
computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
and peripherals. Anecdotally termed the "''digit trunk''", they were named after electrical power buses, or
busbar In , a busbar (also bus bar) is a metallic strip or bar, typically housed inside , , and for local high current power distribution. They are also used to connect high voltage equipment at electrical switchyards, and low voltage equipment i ...

busbar
s. Almost always, there was one bus for memory, and one or more separate buses for peripherals. These were accessed by separate instructions, with completely different timings and protocols. One of the first complications was the use of
interrupt In digital computer A computer is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of pe ...

interrupt
s. Early computer programs performed
I/O In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Information, Inputs are t ...
by waiting in a loop for the peripheral to become ready. This was a waste of time for programs that had other tasks to do. Also, if the program attempted to perform those other tasks, it might take too long for the program to check again, resulting in loss of data. Engineers thus arranged for the peripherals to interrupt the CPU. The interrupts had to be prioritized, because the CPU can only execute code for one peripheral at a time, and some devices are more time-critical than others. High-end systems introduced the idea of
channel controllerIn computing, channel I/O is a high-performance input/output (I/O) architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. In the past, channels were generally implemented with custo ...
s, which were essentially small computers dedicated to handling the input and output of a given bus.
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
introduced these on the
IBM 709 The IBM 709 was a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. ...
in 1958, and they became a common feature of their platforms. Other high-performance vendors like
Control Data Corporation Control may refer to: Basic meanings Economics and business * Control (management) Control is a function of management which helps to check errors in order to take corrective actions. This is done to minimize deviation from standards and ensur ...
implemented similar designs. Generally, the channel controllers would do their best to run all of the bus operations internally, moving data when the CPU was known to be busy elsewhere if possible, and only using interrupts when necessary. This greatly reduced CPU load, and provided better overall system performance. To provide modularity, memory and I/O buses can be combined into a unified
system bus A system bus is a single computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a ...
. In this case, a single mechanical and electrical system can be used to connect together many of the system components, or in some cases, all of them. Later computer programs began to share memory common to several CPUs. Access to this memory bus had to be prioritized, as well. The simple way to prioritize interrupts or bus access was with a
daisy chain Daisy chain may refer to: * Daisy chain, a garland created from daisy flowers * Daisy chain (climbing), a type of strap * Daisy chain (electrical engineering), a wiring scheme * Daisy chain (fishing), a type of fishing lure * Daisy chain (knot), or ...
. In this case signals will naturally flow through the bus in physical or logical order, eliminating the need for complex scheduling.


Minis and micros

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights ...
(DEC) further reduced cost for mass-produced
minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold for much less than Mainframe computer, mainframe and mid-size computers from IBM and BUNCH, its direct competitors. In ...
s, and mapped peripherals into the memory bus, so that the input and output devices appeared to be memory locations. This was implemented in the
Unibus The Unibus was the earliest of several computer bus trolleybus in Toronto Toronto is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a recorded population of 2,731,571 in 2016, it is th ...

Unibus
of the
PDP-11 The PDP-11 is a series of 16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically ...
around 1969. Early
microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted on a single printed ci ...
bus systems were essentially a passive
backplane A backplane (or "backplane system") is a group of electrical connector An electrical connector is an electromechanical device used to join electrical conductors and create an electrical circuit An electrical network is an interconnec ...

backplane
connected directly or through buffer amplifiers to the pins of the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
. Memory and other devices would be added to the bus using the same address and data pins as the CPU itself used, connected in parallel. Communication was controlled by the CPU, which read and wrote data from the devices as if they are blocks of memory, using the same instructions, all timed by a central clock controlling the speed of the CPU. Still, devices
interrupt In digital computer A computer is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of pe ...

interrupt
ed the CPU by signaling on separate CPU pins. For instance, a
disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
controller would signal the CPU that new data was ready to be read, at which point the CPU would move the data by reading the "memory location" that corresponded to the disk drive. Almost all early microcomputers were built in this fashion, starting with the
S-100 bus The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and ...
in the
Altair 8800 The Altair 8800 is a microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) cir ...

Altair 8800
computer system. In some instances, most notably in the
IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first computer released in the IBM PC model line and the basis for the IBM PC compatible IBM PC compatible computers are similar to the original IBM PC The IBM ...

IBM PC
, although similar physical architecture can be employed, instructions to access peripherals (in and out) and memory (mov and others) have not been made uniform at all, and still generate distinct CPU signals, that could be used to implement a separate I/O bus. These simple bus systems had a serious drawback when used for general-purpose computers. All the equipment on the bus had to talk at the same speed, as it shared a single clock. Increasing the speed of the CPU becomes harder, because the speed of all the devices must increase as well. When it is not practical or economical to have all devices as fast as the CPU, the CPU must either enter a
wait state A wait state is a delay experienced by a computer processor when accessing external memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the retention of information over time for the purpose of inf ...
, or work at a slower clock frequency temporarily, to talk to other devices in the computer. While acceptable in
embedded systems An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Com ...
, this problem was not tolerated for long in general-purpose, user-expandable computers. Such bus systems are also difficult to configure when constructed from common off-the-shelf equipment. Typically each added
expansion card Modern EEPROM chip suitable for storing expansion card configuration electronically In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentati ...
requires many jumpers in order to set memory addresses, I/O addresses, interrupt priorities, and interrupt numbers.


Second generation

"Second generation" bus systems like
NuBus motherboard with its six NuBus slots visible on the left. Image:Nubus graphics card.jpg, 200px, Example of a NuBus graphics card, a Radius PrecisionColor Pro 8/24xj. This is a "half-length" card, with a maximum length of 7 inches. The maximum le ...
addressed some of these problems. They typically separated the computer into two "worlds", the CPU and memory on one side, and the various devices on the other. A ''bus controller'' accepted data from the CPU side to be moved to the peripherals side, thus shifting the communications protocol burden from the CPU itself. This allowed the CPU and memory side to evolve separately from the device bus, or just "bus". Devices on the bus could talk to each other with no CPU intervention. This led to much better "real world" performance, but also required the cards to be much more complex. These buses also often addressed speed issues by being "bigger" in terms of the size of the data path, moving from 8-bit
parallel bus In , parallel communication is a method of conveying multiple binary digits (s) simultaneously. It contrasts with , which conveys only a single bit at a time; this distinction is one way of characterizing a . The basic difference between a para ...
es in the first generation, to 16 or 32-bit in the second, as well as adding software setup (now standardised as
Plug-n-play In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwar ...
) to supplant or replace the jumpers. However, these newer systems shared one quality with their earlier cousins, in that everyone on the bus had to talk at the same speed. While the CPU was now isolated and could increase speed, CPUs and memory continued to increase in speed much faster than the buses they talked to. The result was that the bus speeds were now very much slower than what a modern system needed, and the machines were left starved for data. A particularly common example of this problem was that
video card A graphics card (also called a video card, display card, graphics adapter, video adapter, or display adapter) is an expansion card which generates a feed of output images to a display device (such as a computer monitor). Frequently, these are ad ...

video card
s quickly outran even the newer bus systems like PCI, and computers began to include just to drive the video card. By 2004 AGP was outgrown again by high-end video cards and other peripherals and has been replaced by the new
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial Serial may refer to: Arts, entertainment, and media The presentation of works in sequential segments * Serial (literat ...

PCI Express
bus. An increasing number of external devices started employing their own bus systems as well. When disk drives were first introduced, they would be added to the machine with a card plugged into the bus, which is why computers have so many slots on the bus. But through the 1980s and 1990s, new systems like
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
and
IDE #REDIRECT IDE IDE, iDE, or Ide may refer to: Organisations * ''Institut für Dokumentologie und Editorik'', a German think tank for the application of digital methods on historical documents * Institute of Developing Economies, a semi-governmental ...
were introduced to serve this need, leaving most slots in modern systems empty. Today there are likely to be about five different buses in the typical machine, supporting various devices.


Third generation

"Third generation" buses have been emerging into the market since about 2001, including
HyperTransport HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a technology for interconnection of computer Processor (computing), processors. It is a bidirectional Serial communication, serial/Parallel communication, parallel high-B ...
and
InfiniBand InfiniBand (IB) is a computer networking communications standard used in high-performance computing A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer ...
. They also tend to be very flexible in terms of their physical connections, allowing them to be used both as internal buses, as well as connecting different machines together. This can lead to complex problems when trying to service different requests, so much of the work on these systems concerns software design, as opposed to the hardware itself. In general, these third generation buses tend to look more like a
network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
than the original concept of a bus, with a higher protocol overhead needed than early systems, while also allowing multiple devices to use the bus at once. Buses such as
Wishbone Wishbone commonly refers to: * Furcula, a fork-shaped bone in birds and some dinosaurs Wishbone may also refer to: * Wish-Bone, an American salad dressing and condiment company * Wishbone formation, a type of offense in American football * Wishbo ...
have been developed by the
open source hardware Open-source hardware (OSH) consists of physical artifacts of technology designed and offered by the open-design movement. Both free and open-source software (FOSS) and open-source hardware are created by this open-source culture movement and ...
movement in an attempt to further remove legal and patent constraints from computer design.


Fourth generation

The Compute Express Link (CXL) is an
open standard An open standard is a standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * Standard (metrology), an object that bears a de ...
interconnect In telecommunications, interconnection is the physical linking of a common carrier, carrier's telecommunications network, network with equipment or facilities not belonging to that network. The term may refer to a connection between a carrier's ...
for high-speed
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
-to-device and CPU-to-memory, designed to accelerate next-generation
data center A data center (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Eng ...
performance.


Examples of internal computer buses


Parallel

* ASUS Media Bus proprietary, used on some ASUS
Socket 7 Socket 7 is a physical and electrical specification for an x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a comp ...

Socket 7
motherboards *
Computer Automated Measurement and Control Computer-Aided Measurement And Control (CAMAC) is a standard bus and modular-crate electronics standard for data acquisitionData acquisition is the process of sampling signals that measure real world physical conditions and converting the resultin ...
(CAMAC) for instrumentation systems * Extended ISA or EISA *
Industry Standard Architecture Industry Standard Architecture (ISA) is the 16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical op ...
or ISA *
Low Pin Count The Low Pin Count (LPC) bus is a computer bus used on IBM PC compatible, IBM-compatible personal computers to connect low-bandwidth devices to the central processing unit, CPU, such as the BIOS ROM (BIOS ROM was moved to the Serial Peripheral I ...
or LPC * MBus *
MicroChannel Micro Channel architecture, or the Micro Channel bus, was a proprietary 16- or 32-bit parallel computer bus introduced by IBM International Business Machines Corporation (IBM) is an American multinational technology company headquarter ...
or MCA *
Multibus Multibus is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine t ...
for industrial systems *
NuBus motherboard with its six NuBus slots visible on the left. Image:Nubus graphics card.jpg, 200px, Example of a NuBus graphics card, a Radius PrecisionColor Pro 8/24xj. This is a "half-length" card, with a maximum length of 7 inches. The maximum le ...
or IEEE 1196 * OPTi local bus used on early
Intel 80486 The Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is the ...
motherboards. *
Conventional PCI Peripheral Component Interconnect (PCI) is a local Local may refer to: Geography and transportation * Local (train), a train serving local traffic demand * Local, Missouri, a community in the United States * Local government, a form of public a ...
*
Parallel ATA Parallel ATA (PATA), originally , also known as ATA or IDE IDE, iDE, or Ide may refer to: Organisations * ''Institut für Dokumentologie und Editorik'', a German think tank for the application of digital methods on historical documents * Instit ...
(also known as Advanced Technology Attachment, ATA, PATA, IDE, EIDE, ATAPI, etc.),
Hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

Hard disk drive
,
optical disk drive In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware an ...
,
tape drive A tape drive is a data storage device Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, Phonograph record, phonographic recording, magnetic tape, and optical discs are all examples of storage m ...
peripheral attachment bus *
S-100 bus The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and ...
or IEEE 696, used in the
Altair 8800 The Altair 8800 is a microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) cir ...

Altair 8800
and similar
microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted on a single printed ci ...
s *
SBus right , upright=1.8 , SBus male connector SBus is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of c ...
or IEEE 1496 *
SS-50 Bus The SS-50 bus was an early computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a ...
*
Runway busThe Runway bus is a front-side bus A front-side bus (FSB) is a computer communication interface ( bus) that was often used in Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara ...
, a proprietary front side CPU bus developed by Hewlett-Packard for use by its PA-RISC microprocessor family * GSC/HSC, a proprietary peripheral bus developed by Hewlett-Packard for use by its PA-RISC microprocessor family * Precision Bus, a proprietary bus developed by Hewlett-Packard for use by its HP3000 computer family *
STEbus The STEbus (also called the IEEE-1000 bus) is a non-proprietary, processor-independent, computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functional ...
* STD Bus (for STD-80 [8-bit] and STD32 [16-/32-bit])
FAQ
*
Unibus The Unibus was the earliest of several computer bus trolleybus in Toronto Toronto is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a recorded population of 2,731,571 in 2016, it is th ...

Unibus
, a proprietary bus developed by
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights ...
for their
PDP-11 The PDP-11 is a series of 16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically ...
and early VAX computers. * Q-Bus, a proprietary bus developed by
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights ...
for their Programmed Data Processor, PDP and later VAX computers. *
VESA Local Bus The VESA Local Bus (usually abbreviated to VL-Bus or VLB) is a short-lived expansion bus File:AT24C02 EEPROM 1480355 6 7 HDR Enhancer.jpg, Modern EEPROM chip suitable for storing expansion card configuration electronically In computing, an ...
or VLB or VL-bus * VMEbus, the VERSAmodule Eurocard bus * PC/104 * PC/104#PC/104-Plus, PC/104-Plus * PCI-104 * PCI/104-Express * PC/104#PCI/104, PCI/104 * Zorro II and Zorro III, used in Amiga computer systems


Serial

*
1-Wire 1-Wire is a device communications bus system designed by Dallas Semiconductor Corp. that provides low-speed (16.3 kbit/s) data, signaling, and power over a single conductor Conductor or conduction may refer to: Music * Conductor (music), ...
*
HyperTransport HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a technology for interconnection of computer Processor (computing), processors. It is a bidirectional Serial communication, serial/Parallel communication, parallel high-B ...
*
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orch ...
*
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial Serial may refer to: Arts, entertainment, and media The presentation of works in sequential segments * Serial (literat ...

PCI Express
or PCIe *
Serial ATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implemen ...

Serial ATA
(SATA),
Hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

Hard disk drive
, solid state drive, optical disc drive,
tape drive A tape drive is a data storage device Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, Phonograph record, phonographic recording, magnetic tape, and optical discs are all examples of storage m ...
peripheral attachment bus * Serial Peripheral Interface (SPI) bus *
UNI/O and chip scale package, wafer level chip scale packages sitting on the face of a U.S. penny The UNI/O bus is an asynchronous serial communication, asynchronous Serial communications, serial Bus (computing), bus created by Microchip Technology ...
* System Management Bus, SMBus


Examples of external computer buses


Parallel

* HIPPI High Performance Parallel Interface * IEEE-488 (also known as GPIB, General-Purpose Interface Bus, and HPIB, Hewlett-Packard Instrumentation Bus) * PC Card, previously known as ''PCMCIA'', much used in laptop computers and other portables, but fading with the introduction of USB and built-in network and modem connections


Serial

* Camera Link * CAN bus ("Controller Area Network") *
eSATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implemen ...

eSATA
* ExpressCard * Fieldbus * IEEE 1394 interface (FireWire) *
RS-232 In telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It has its origin in the desire of ...
* RS-485 * Thunderbolt (interface), Thunderbolt * USB


Examples of internal/external computer buses

* Futurebus *
InfiniBand InfiniBand (IB) is a computer networking communications standard used in high-performance computing A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer ...
* PCI Express External Cabling * QuickRing * Scalable Coherent Interface (SCI) * Small Computer System Interface (SCSI),
Hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

Hard disk drive
and
tape drive A tape drive is a data storage device Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, Phonograph record, phonographic recording, magnetic tape, and optical discs are all examples of storage m ...
peripheral attachment bus * Serial Attached SCSI (SAS) and other serial SCSI buses * Thunderbolt (interface), Thunderbolt * Yapbus, a proprietary bus developed for the Pixar Image Computer


See also

* Address decoder * Bus contention * Bus error * Bus mastering * Communication endpoint * Control bus * Crossbar switch * Memory address * Memory bus (Data bus) * Front-side bus (FSB) * External Bus Interface (EBI) * Harvard architecture * Master/slave (technology) * Network On Chip * List of device bandwidths * List of network buses * Software bus


References


External links

*
Computer hardware buses and slots pinouts with brief descriptions
{{DEFAULTSORT:Bus (Computing) Computer buses, Digital electronics Motherboard