Different devices have different methods of communication with the computer. A particular device's method of communication is called its interface. Many of the "extra" chips on a motherboard are actually translators for a certain kind of interface. It's as if the CPU only speaks English, but the hard drive and CD-ROM speak Russian, the keyboard speaks German, and the mouse speaks a dialect of Spanish.
Having different interfaces may seem an inefficient way to make a computer. Why not have everyone agree on the same language? There are two reasons why this hasn't happened. First, the interfaces are optimized to the needs of the devices that use them. A hard drive has to transmit a tremendous amount of data in a short time, but the keyboard sends only a handful of bytes to the computer every second, even for the fastest typists. Any interface fast enough for the hard drive would be serious overkill for a keyboard. Second, just like with CPUs, designers have to be aware of backwards compatibility.
When someone introduces a new keyboard interface that requires users to purchase new keyboards, even though the old interface and keyboard worked fine, users will complain. Some common interfaces used in computers today include USB, Firewire, IDE, and SCSI.
USB, which stands for Universal Serial Bus, is an interface for external components, like mice, keyboards, and printers. The word "serial" means that USB uses serial transmission, in which bits are sent one at a time along the same wire. In contrast, parallel transmission sends bits in groups, like one byte at a time, by having a wire for each bit that is sent. Parallel transmission would seem to be faster, but because it is more complicated, the overhead often outweighs the benefits.
The USB interface can transmit at a rate of 12 Mbps. M means "Mega," bps means "bits per second," and thus 12 Mbps means about 12 million bits per second, or 1.5 million bytes per second. USB allows chaining, which means multiple devices can share the same interface. In the case of USB, up to 127 devices can chain onto one interface. That is, if a computer has one USB socket on the back of the case, up to 127 devices can communicate with the computer through that one socket. This chaining means the devices form a bus, just like the ones on the motherboard, only one that can change over time.
To allow for the physical connection of so many devices, some larger devices like printers may have both an outgoing socket (to connect to the computer) and an incoming socket. Because some devices, like mice, won't have any incoming sockets, users can purchase USB "hubs," which are nothing more than a set of extra incoming sockets. One special feature of USB is that it allows hot-swapping, which means devices can be safely plugged or unplugged while the computer is running. The USB interface can alert the computer's operating system when a device is connected or disconnected from the system.
Firewire is another type of high-speed serial bus. Developed by Apple, it is also known as IEEE 1394, after its official standards designation. It's much faster than USB, transmitting data up to 400 Mbps. This speed is necessary because Firewire's intended use is connecting computers with video devices like camcorders and digital cameras, where video data is large. The speed of the interface is also good for devices like portable hard drives. Like USB, Firewire allows hot-swapping and can chain up to sixty-three devices.
IDE, Integrated Drive Electronics, is a standard interface for connecting hard drives and optical drives. The name was chosen originally because the hard drive contains its own interface hardware, whereas previous hard drives had separate interface "cards" that had to be installed in the computer first. IDE has been superseded by EIDE, or Enhanced IDE, which allows for faster data transmission and larger hard drives.
A single IDE interface on a motherboard, known as a port, can support two drives. If two drives are on the same port, one drive must be designated a "master" and the other a "slave." Usually this is done by setting switches on the drives themselves. Because many computers have more than two drives, motherboards using IDE have at least two ports.
SCSI (pronounced "scuzzy") stands for Small Computer System Interface and is another standard interface for hard drives and optical drives. Note that drives are built for particular interfaces. A hard drive made for IDE will not work on a SCSI interface. In general, SCSI interfaces and drives are more expensive than their IDE counterparts but perform better. SCSI interfaces can support up to seven devices.
Both SCSI and IDE use parallel transmission. If you look inside a computer case, you will probably find that the hard drives are connected to the motherboard using a ribbon cable, which, like it sounds, is flat like a ribbon. If you look closely at a ribbon cable, you will see that it is ribbed like corduroy. Each of those ribs contains a wire to allow the parallel transmission of data.
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Note: This article was sent to us by: Ryan Welsh at 02122011
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