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Encyclopedia > Computer bus

In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers and typically is controlled by device driver software. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. Each bus defines its set of connectors to physically plug devices, cards or cables together. In computer engineering, computer architecture is the conceptual design and fundamental operational structure of a computer system. ... A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ... Windows XP loading drivers during a Safe Mode bootup A device driver, or a software driver is a specific type of computer software, typically developed to allow interaction with hardware devices. ... A point-to-point link is an exclusive connection between addressable units. ... A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. ... A peripheral is a type of computer hardware that is added to a host computer in order to expand its abilities. ... An electrical connector is a device for joining electrical circuits together. ...


Early computer buses were literally parallel electrical buses with multiple connections, but the term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical bus. Modern computer buses can use both parallel and bit-serial connections, and can be wired in either a multidrop (electrical parallel) or daisy chain topology, or connected by switched hubs, as in the case of USB. An electrical bus (sometimes spelled buss) is a physical electrical interface where many devices share the same electric connection. ... This article needs to be cleaned up to conform to a higher standard of quality. ... The elementary meaning of daisy chain is a garland created from the daisy flower, generally as a childrens game. ... The USB trident Icon The USB (Type A and B) Connectors A USB Series “A” plug USB Logo High Speed USB Logo USB OTG Logo High Speed USB OTG Logo Wireless USB Logo Universal Serial Bus (USB) is a serial bus standard to interface devices. ...

Contents


History

First generation

Early computer buses were bundles of wire that attached memory and peripherals. They were named after electrical buses, or busbars. Almost always, there was one bus for memory, and another for peripherals, and these were accessed by separate instructions, with completely different timings and protocols. A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ... An electrical bus (sometimes spelled buss) is a physical electrical interface where many devices share the same electric connection. ...


One of the first complications was the use of interrupts. Early computers performed I/O 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 lost 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. In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... In computing, Input/output, or I/O, is the collection of interfaces that different functional units (sub-systems) of an information processing system use to communicate with each other, or the signals (information) sent through those interfaces. ...


Some time after this, some computers began to share memory between several CPUs. On these computers, access to the bus had to be prioritized, as well.


The classic, simple way to prioritize interrupts or bus access was with a daisy chain. The elementary meaning of daisy chain is a garland created from the daisy flower, generally as a childrens game. ...


DEC noted that having two buses seemed wasteful and expensive for small, mass-produced computers, and mapped peripherals into the memory bus, so that the devices appeared to be memory locations. At the time, this was a very daring design. Cynics predicted failure. Digital Equipment Corporation was a pioneering company in the American computer industry. ...


Early microcomputer bus systems were essentially a passive backplane connected to the pins of the 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. In some instances, such as the IBM PC, instructions still generated signals at the CPU that could be used to implement a true I/O bus. The Commodore 64 was one of the most popular microcomputers of its era, and is the best selling home computer of all time. ... A backplane is a circuit board (usually a printed circuit board) that connects several connectors in parallel to 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. ... CPU redirects here. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ...


In many microcontrollers and embedded systems, an I/O bus still does not exist. Communication is controlled by the CPU, which reads and writes data from the devices as if they are blocks of memory (in most cases), all timed by a central clock controlling the speed of the CPU. Devices ask for service by signalling on other CPU pins, typically using some form of interrupt. An embedded system is a special-purpose computer system, which is completely encapsulated by the device it controls. ... CPU redirects here. ... In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ...


For instance, a disk drive 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 that corresponded to the disk drive. Almost all early computers were built in this fashion, starting with the S-100 bus in the Altair, and continuing through the IBM PC in the 1980s. Disk Drive is the afternoon show on CBC Radio Two. ... The S-100 bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer. The S-100 bus was the first industry standard bus for the microcomputer industry, and S-100 computers, processor... Altair 8800 The MITS Altair 8800 was a microcomputer design from 1975, based on the Intel 8080A CPU. Sold as a kit through Popular Electronics magazine, the designers intended to sell only a few hundred to hobbyists, and were surprised when they sold over ten times that many in the... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... The examples and perspective in this article or section may not represent a worldwide view. ...


These simple bus systems had a serious drawback for general-purpose computers. All the equipment on the bus has to talk at the same speed, and thus shares a single clock.


Increasing the speed of the CPU is not a simple matter, because the speed of all the devices must increase as well. This often leads to odd situations where very fast CPUs have to "slow down" in order to talk to other devices in the computer. While acceptable in embedded systems, this problem was not tolerated for long in commercial computers. A router, an example of an embedded system. ...


Another problem is that the CPU is required for all operations, so if it becomes busy with other tasks, the real throughput of the bus could suffer dramatically. In information technology, throughput is the rate at which a computer or network sends or receives data. ...


Such bus systems are difficult to configure when constructed from common off-the-shelf equipment. Typically each added PC board requires many jumpers in order to set memory addresses, I/O addresses, interrupt priorities, and interrupt numbers. The word jumper has many meanings: A jumper is a garment. ...


Second generation

"Second generation" bus systems like NuBus 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, with a bus controller in between. This allowed the CPU to increase in speed without affecting the bus. This also moved much of the burden for moving the data out of the CPU and into the cards and controller, so 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 buses in the first generation, to 16 or 32-bit in the second, as well as adding software setup (now standardised as Plug-n-play) to supplant or replace the jumpers. NuBus is a 32-bit parallel computer bus, originally developed at MIT as a part of the NuMachine workstation project, and eventually used by Apple Computer and NeXT Computer. ... In computing, a parallel port is an interface from a computer system where data is transferred in or out in parallel, that is, on more than one wire. ... Plug and Play is a term used in the computer field to describe a computers ability to have new devices, normally peripherals, added to it without having to restart the computer. ...


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 without fear, 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 cards quickly outran even the newer bus systems like PCI, and computers began to include AGP just to drive the video card. By 2004 AGP was outgrown again by high-end video cards and is being replaced with the new PCI Express bus. A GeForce 4 4200-based graphics card A graphics card or video card is a component of a computer which is designed to convert a logical representation of an image stored in memory to a signal that can be used as input for a display medium, most often a monitor... 32-bit PCI expansion slots on a motherboard 64-bit PCI expansion slots inside a Power Macintosh G4 The Peripheral Component Interconnect standard (in practice almost always shortened to PCI) specifies a computer bus for attaching peripheral devices to a computer motherboard. ... An AGP slot (maroon, although the color is usually brown) and two PCI slots. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... PCI Express (formerly known as 3GIO for 3rd Generation I/O, not to be mistaken with PCI-X) is an implementation of the PCI computer bus that uses existing PCI programming concepts and communications standards, but bases it on a much faster serial communications system. ...


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 and IDE 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. The examples and perspective in this article or section may not represent a worldwide view. ... See also 1990s, the band The 1990s decade refers to the years from 1990 to 1999, inclusive, sometimes informally including popular culture from the very late 1980s and from 2000 and beyond. ... SCSI stands for Small Computer System Interface, and is a standard interface and command set for transferring data between devices on both internal and external computer buses. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ...


A useful differentiation then became popular, the concept of the local bus as opposed to external bus. The former referred to bus systems that were designed to be used with internal devices, such as graphics cards, and the latter to buses designed to add external devices such as scanners. Note, though, that "local" also referred to the greater proximity to the processor of VL-Bus and PCI than ISA. IDE is an external bus in terms of how it is used, but is almost always found inside the machine. CanoScan9950F Desktop scanner, with the lid raised. ...


Third generation

"Third generation" buses are now in the process of coming to market, including HyperTransport and InfiniBand. They typically include features that allow them to run at the very high speeds needed to support memory and video cards, while also supporting lower speeds when talking to slower devices such as disk drives. 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 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. HyperTransport logo HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency computer bus. ... InfiniBand is a high-speed serial computer bus, intended for both internal and external connections. ... A computer network is a system for communication between computers. ...


On another track, integrated circuits are increasingly being designed from predesigned logic, "intellectual property." Buses such as Wishbone have been developed to permit devices on integrated circuits to talk to one another. The Wishbone Bus is an open source computer bus intended to let parts of an integrated circuits communicate with each other. ...


Description

At one time, "bus" meant an electrically parallel system, with electrical conductors similar or identical to the pins on the CPU. This is no longer the case, and modern systems are blurring the lines between buses and networks.


Buses can be parallel buses, which carry data words striped across multiple wires, or serial buses, 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 two used in the I²C serial bus. As data rates increase, the problems of timing skew and crosstalk 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 actually 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, FireWire, and Serial ATA are examples of this. Multidrop connections do not work well for fast serial buses, so most modern serial buses use daisy-chain or hub designs. In computing, a parallel port is an interface from a computer system where data is transferred in or out in parallel, that is, on more than one wire. ... A serial bus is a computer bus that sends data bit by bit down one or a few wires. ... I²C is a serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. ... Timing skew is a problem that can occur on many kinds of computer buses. ... In telecommunication, the term crosstalk (XT) has the following meanings: 1. ... In computing, a double pumped computer bus transfers data on both the rising and falling edges of the clock signal, effectively doubling the data transmission rate without having to deal with the additional problems of timing skew that increasing the number of data lines would introduce. ... The USB trident Icon The USB (Type A and B) Connectors A USB Series “A” plug USB Logo High Speed USB Logo USB OTG Logo High Speed USB OTG Logo Wireless USB Logo Universal Serial Bus (USB) is a serial bus standard to interface devices. ... FireWire (also known as i. ... First generation (1. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Daisy chain Daisy chain The elementary meaning of daisy chain is a garland created from the daisy flower, generally as a childrens game. ...


Most computers have both internal and external buses. An internal bus connects all the internal components of a computer to the motherboard (and thus, the CPU and internal memory). These types of buses are also referred to as a local bus, because they are intended to connect to local devices, not to those in other machines or external to the computer. An external bus connects external peripherals to the motherboard. CPU redirects here. ... Internal memory: In a computer, all of the storage spaces that are accessible by a processor without the use of the computer input-output channels. ... In computer science, a local bus is a computer bus that connects directly, or almost directly, from the CPU to one or more slots on the expansion bus. ...


Network connections such as Ethernet are not generally regarded as buses, although the difference is largely conceptual rather than practical. The arrival of technologies such as InfiniBand and HyperTransport is further blurring the boundaries between networks and buses. Even the lines between internal and external are sometimes fuzzy, I²C can be used as both an internal bus, or an external bus (where it is known as ACCESS.bus), and InfiniBand is intended to replace both internal buses like PCI as well as external ones like Fibre Channel. A computer network is a system for communication between computers. ... Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ... InfiniBand is a high-speed serial computer bus, intended for both internal and external connections. ... HyperTransport logo HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency computer bus. ... I²C is a serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. ... ACCESS.bus (or A.b) is a peripheral-interconnect computer bus developed by Philips in the early 1990s. ... 32-bit PCI expansion slots on a motherboard 64-bit PCI expansion slots inside a Power Macintosh G4 The Peripheral Component Interconnect standard (in practice almost always shortened to PCI) specifies a computer bus for attaching peripheral devices to a computer motherboard. ... Fibre Channel is a gigabit speed network technology primarily used for Storage Networking. ...


Modern trends in personal computers, especially laptops, have been moving towards eliminating all external connections except for modem jack, Cat5, USB, headphone jack, and optional VGA or FireWire. Category 5 cable, commonly known as Cat 5, is an unshielded twisted pair cable type designed for high signal integrity. ... A jack plug is an extremely common audio connector. ... Video Graphics Array (VGA) is a computer display standard first marketed in 1987 by IBM. VGA belongs to a family of earlier IBM video standards and largely remains backward compatible with them. ...


Bus topology

In a network, the master scheduler controls the data traffic. If data is to be transferred the requesting computer sends a message to the scheduler, which puts the request into a queue. The message contains an identification code which is broadcast to all nodes of the network. The scheduler works out priorities and notifies the receiver as soon as the bus is available.


The identified node takes the message and performs the data transfer between the two computers. Having completed the data transfer the bus becomes free for the next request in the scheduler's queue.


Bus benefit: any computer can be accessed directly and message can be sent in a relatively simple and fast way. Disadvantage: needs a scheduler to assign frequencies and priorities to organize the traffic.


See also: Bus network Image showing bus network layout A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. ...


Examples of internal computer buses

Parallel

Camac Harps (Les Harpes Camac) is a French company that manufactures pedal (concert) harps, lever (folk) harps, and electric pedal and lever harps. ... The Extended Industry Standard Architecture (in practice almost always shortened to EISA and pronounced eee-suh) is a bus standard for IBM compatible computers. ... Industry Standard Architecture (in practice almost always shortened to ISA) is a computer bus standard for IBM compatible computers. ... The Low Pin Count bus, or LPC bus, is used on PC-style personal computers to connect low-bandwidth devices to the CPU, such as the boot ROM, the legacy I/O devices (behind a Super I/O chip), and audio controllers. ... Micro Channel architecture (in practice almost always shortened to MCA) was a proprietary 16 or 32-bit parallel computer bus created by IBM in the 1980s for use on their new PS/2 computers. ... An MBus module containing a single SuperSPARC microprocessor (Sun part number 501-2218) MBus is a computer bus designed and implemented by Sun Microsystems for communication between high-speed computer system components, such as the central processing unit, motherboard and main memory. ... Multibus is a computer bus standard used in industrial systems. ... NuBus is a 32-bit parallel computer bus, originally developed at MIT as a part of the NuMachine workstation project, and eventually used by Apple Computer and NeXT Computer. ... // Overview The exposed die of an Intel 80486DX2 microprocessor. ... 32-bit PCI expansion slots on a motherboard 64-bit PCI expansion slots inside a Power Macintosh G4 The Peripheral Component Interconnect standard (in practice almost always shortened to PCI) specifies a computer bus for attaching peripheral devices to a computer motherboard. ... The S-100 bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer. The S-100 bus was the first industry standard bus for the microcomputer industry, and S-100 computers, processor... Altair (α Aql / α Aquilae / Alpha Aquilae / Atair ) is the brightest star in the constellation Aquila and the twelfth brightest star in the nighttime sky, at visual magnitude 0. ... Apple IIc Generally, a microcomputer is a computer with a microprocessor (µP) as its CPU. Another general characteristic of these computers is that they occupy physically small amounts of space. ... SBus is a computer bus system that was used in most SPARC-based computers from Sun Microsystems during the 1990s. ... The VESA Local Bus (usually shortened to VLB) is a local bus defined by the Video Electronics Standards Association, mostly used in personal computers based on the Intel 80486 CPU. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O... 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. ...

Serial

1-Wire is a computer bus system designed by Dallas Semiconductor that provides low-speed data, signaling and power over a single wire. ... HyperTransport logo HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency computer bus. ... I²C is a serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. ... PCI Express (formerly known as 3GIO for 3rd Generation I/O, not to be mistaken with PCI-X) is an implementation of the PCI computer bus that uses existing PCI programming concepts and communications standards, but bases it on a much faster serial communications system. ... The Serial Peripheral Interface Bus or SPI bus is a very loose standard for controlling almost any digital electronics that accepts a clocked serial stream of bits. ... Note: USB may also mean upper sideband in radio. ... FireWire (also known as i. ...

Examples of external computer buses

Parallel

  • Advanced Technology Attachment or ATA (aka PATA, IDE, EIDE, ATAPI, etc.) disk/tape peripheral attachment bus
    (the original ATA is parallel, but see also the recent
  • HIPPI HIgh Performance Parallel Interface
  • IEEE-488 (aka GPIB, Ge

neral-Purpose Instrumentation Bus, and HPIB, Hewlett-Packard Instrumentation Bus) ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA) is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... HIPPI (HIgh Performance Parallel Interface) is a computer bus for the attachment of high speed storage devices to supercomputers. ... The Hewlett-Packard Instrument Bus (HP-IB), is a short-range digital communications cable standard developed by Hewlett-Packard (HP) in the 1970s for connecting electronic test and measurement devices (e. ...

  • PCMCIA, now known as PC card, much used in laptop computers and other portables, but fading with the introduction of USB and built-in network and modem connections.
  • SCSI Small Computer System Interface, disk/tape peripheral attachment bus

Portable Computer Cards (PC cards) are interchangeable peripherals designed to be inserted into laptop computers in order to enable extra hardware functions. ... SCSI stands for Small Computer System Interface, and is a standard interface and command set for transferring data between devices on both internal and external computer buses. ...

Proprietary

  • Floppy drive connector

Examples of internal/external computer buses

Futurebus (IEEE 896) is a computer bus standard, intended to replace all local bus connections in a computer, including the CPU, memory, plug-in cards and even, to some extent, LAN links between machines. ... InfiniBand is a high-speed serial computer bus, intended for both internal and external connections. ... QuickRing was a gigabit-rate interconnect that combined the functions of a computer bus and a network. ... SCI, for Scalable Coherent Interconnect, is a high-speed computer bus that supports a variety of topologies, speeds and connection systems. ...

See also

Bus contention is an undesirable state of the bus of a computer, in which more than one memory mapped device or the CPU is attempting to place output values onto the bus at once. ... In computers, the front side bus (FSB) is a term for the physical bi-directional data bus that carries all electronic signal information between the central processing unit (CPU) and other devices within the system such as random access memory (RAM), the system BIOS, AGP video cards, PCI expansion cards... An address bus is (part of) a computer bus, used by CPUs or DMA-capable units for communicating the physical addresses of computer memory elements/locations that the requesting unit wants to access (read/write). ... A control bus is sabilas bum has cheese crust (part of) a computer bus, used by CPUs for communicating with other devices within the computer. ...

External links


  Results from FactBites:
 
Bus (computer) - MSN Encarta (591 words)
In computer architecture, a bus is a subsystem that transfers data between computer components inside a computer or between computers.
Bus (computer), in computer science, a set of wires used for data transfer among the components of a computer system.
Because the bus is integral to internal data transfer and yet computer users often need to add extra components to the system, most microcomputer buses allow for expansion through one or more expansion slots (connectors for add-on circuit boards).
Method and system for avoiding live lock conditions on a computer bus by insuring that the first retired bus master is ... (6177 words)
The bus controller may prevent execution of the transaction request from the other bus masters by transmitting retry commands to all bus masters that submit transaction requests after the transaction request from the first bus master is received and before the first bus master re-submits the transaction request.
The computer system of claim 14 wherein the bus controller includes a bus interface and a buffer associated with the target device, the bus interface determining whether the target device is available to execute the first transaction request by determining whether the buffer is in a state that enables execution of the first transaction request.
Bus master 1 re-submits its second read transaction request and the PCI target interface processes it normally because the write buffer is still empty and the bus master 1 no longer has an ineligible flag associated with it.
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