Using an external USB hard disk that supports USB 2.0 provides a fast exchange of data between the computer and the HDD. The current standard for USB is USB 2.0, which is backward compatible to earlier 1.0 and 1.1 standards, and supports bandwidths of 1.5Mbps (megabits per second), 12.5Mbps, and 480Mbps. Once plugged into the port, the computer then detects the device and installs it, allowing you to access any data on the external hard disk. Hard disks can be mounted in cases that provide a USB connection that plugs into a USB port on the computer. In addition to peripherals, USB provides an interface for external hard disks. Because USB uses a bus topology, the devices can be daisy-chained together or connected to a USB hub, allowing up to 127 devices to be connected to the computer at one time. As we'll discuss later in this chapter, USB is used for a variety of different peripherals, including keyboards, mice, and other devices that previously required serial and parallel ports, as well as newer technologies such as digital cameras and digital audio devices. USB is an acronym for Universal Serial Bus. Depending on the SCSI version, either eight or 16 SCSI IDs can be attached to one controller (with the controller using one ID, thus allowing seven or 15 SCSI peripherals). Devices can be “chained” on a SCSI bus, each with a different SCSI ID number. There are a number of different versions of SCSI later forms provide faster transfer rates and other improvements. Some motherboards have SCSI connectors and controllers built in for those that don't, you can add SCSI disks by installing a SCSI controller card in one of the expansion slots. SCSI is another ANSI standard that provides faster data transfer than IDE/EIDE. SCSI (pronounced “skuzzy”) is an acronym for Small Computer System Interface. The ability to hot-swap devices has made SATA a possible successor to USB connections used with such things as external hard disks, which can be plugged into the computer to provide large removable storage or data. It provides high data transfer rates between the motherboard and storage device, and uses thinner cables that can be used to hot-swap devices (plug in or unplug the devices while they're still operating). SATA is an acronym for Serial Advanced Technology Attachment, and is the next generation that will probably replace ATA. If the latter is selected for both drives, the first drive in the chain will be the master drive. Most drives have three settings: master, slave, and cable-controlled. User-configurable settings on the drives determine which will act as master and which as slave. One drive functions as the “master,” which responds first to probes or signals on the interrupt (a signal from a device or program to the operating system that causes the OS to stop briefly to determine what task to do next) that is shared with the other, “slave” drive that shares the same cable. Up to two ATA devices (hard disks or CD-ROM drives) can be connected to each connector, in a master/slave configuration. Almost all modern PC motherboards include two EIDE connectors. It is also referred to as Advanced Technology Attachment (ATA), a standard of the American National Standards Institute (ANSI). Integrated Drive Electronics is so named because the disk controller is built into, or integrated with, the disk drive's logic board. IDE is an acronym for Integrated Drive Electronics, and EIDE is an acronym for Enhanced IDE. The most common hard disk interfaces include: ▪ Over the years, a number of different interfaces have been developed, allowing the hard disk to be connected to a disk controller that's usually mounted directly on the computer's motherboard. The interface used by an HDD serves as a communication channel, allowing data to flow between the computer and the HDD. The hard disk interface is one of several standard technologies used to connect the hard disk to the computer so that the machine can then access data stored on the hard disk.
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