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What is a bus?
Takin' the bus...
By: David K. Every
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2002-03-11 05:48:19
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 Bus" is short for omnibus -- Omnibus is Latin meaning "for all". Which is exactly what a bus is -- a way for all devices to talk to each other. Think of it as public transportation for your computer chips or peripherals.
Imagine that a computer is made up of many separate chips (devices) that all need to talk to each other though wires (leads). If you have a dozen devices that need to talk to each other, then you need a dozens of separate "channels", "ports" or "paths", so that each device can talk to any other device.And each path often requires many "pins" or "leads" (wires). So it isn't done this way - just too expensive and complex.
Another example might be to have all your peripherals (printer, modem, disk drive, etc.) to each have their "own" port/channel to connect to the computer - this is done more often, but with scanners, and many printers and digital cameras and all the things we're adding, it just doesn't make sense.
Instead we have all the devices share one "bus".
The "bus" is a common set of wires (leads), that all the chips share, and talk to each other through.
Basically each device listens to the bus (to make sure no one else is "talking"), and when it becomes available, it takes its turn to send information. Each device on the bus has a device "address", and they all listen to the bus constantly to see if anything is "addressed" to them; when it is, they listen, and then take their turn to talk back to the sending device (with a return address) - like really fast mail.
Now because all the information is on one shared channel, it can also be a "bottleneck". If one device "hogs" the bus or is busy talking, then the others have to wait their turn, and the only real solution is to get a faster device, a faster bus, or add another bus (and put some of the devices on each). Think of a bus like a freeway; you can make a "faster" bus, by either raising the speed limit (speed), or by adding lanes (making it wider and send more at the same time).
The "busses" have become so popular, that your computer actually has many of them, each with their own specialty. One is used for all the memory chips to talk to each other and the processor (called the memory bus, processor bus or main-bus), sometimes there are specialized ones for special types of high speed memory (cache or video memory may have it's own bus). And many devices actually have their own bus. Some are internal only; like your hard drive, CD-ROM, and storage device may use IDE or SCSI BUS. And now many are external busses with their own connectors, like USB (Universal Serial Bus) or 1394 (FireWire or iLink), which allow you to connect printers, scanners and other devices all together, using a shared "bus". Even a network or the Internet is nothing but a "super-bus" of sorts, but it isn't usually referred to as such.
That is basically all there is to how a bus works.
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