Computer peripheral devices

Any device connected internally or externally to a computer and used in the transfer of data. A personal computer or workstation processes information and, strictly speaking, that is all the computer does. Data (unprocessed information) must get into the computer, and the processed information must get out. Entering and displaying information is carried out on a wide variety of accessory devices called peripherals, also known as input/output (I/O) devices. Some peripherals, such as keyboards, are only input devices; other peripherals, such as printers, are only output devices; and some are both. See also Digital computer; Microcomputer.

The monitor is the device on which images produced by the computer operator or generated by the program are displayed on a cathode-ray tube (CRT). Electron guns—one in a monochrome monitor, three in a color monitor—irradiate phosphors on the inside of the vacuum tube, causing them to glow. The flat-panel displays on most portable computers, known as liquid-crystal displays (LCDs), use two polarizing filters with liquid crystals between them to produce the image. See also Electronic display; Liquid crystals.

The computer keyboard, based on the typewriter keyboard, contains keys for entering letters, numbers, and punctuation marks, as well as keys to change the meaning of other keys. The function keys perform tasks that vary from program to program. See also Typewriter.

The mouse is a device that is rolled on the desktop to move the cursor on the screen. A ball on the bottom of the mouse translates the device's movements to sensors within the mouse and then through the connecting port to the computer. There are also mice that substitute optical devices for mechanical balls, and mice that use infrared rather than physical connections.

The trackball is essentially an upside-down mouse, with the ball that is used to move the cursor located on the top rather than on the bottom.

The joystick is a pointing device used principally for games.

The light pen performs the same functions as a mouse or trackball, but it is held up to the screen, where its sensors detect the presence of pixels and send a signal through a cable to the computer.

The graphics, or digitizing, tablet is a pad with electronics beneath the surface which is drawn upon with a pointed device, called a stylus. The shapes drawn appear on the monitor's screen.

The most common input, or storage, device in personal computers or workstations is a hard disk drive, a stack of magnetized platters on which information is stored by heads generating an electrical current to represent either 1 or 0 in the binary number system. The device is called hard because the platters are inflexible, and is called a drive because it spins at 3600 revolutions or more a minute, within a sealed case. Diskettes, made of flexible film like that used in recording tape, are usually stored within a hard shell and are spun by their drives at about 360 revolutions per minute. See also Computer storage technology.

Data can also be stored and retrieved with light, the light of a laser beam reading a pattern of pits on an optical disk. The most familiar type of optical disk is the CD-ROM (compact disk-read only memory). Another kind of optical disk is the WORM (write once read many times). See also Compact disk; Multimedia technology; Optical recording.

As a consequence of their greater efficiency and speed, disk drives have quickly replaced tape drives as the primary means of data and program storage. Tape drives are still in use for backup storage, copying the contents of a hard disk as insurance against mechanical failure or human error. See also Magnetic recording.

The scanner converts an image of something outside the computer, such as text, a drawing, or a photograph, into a digital image that it sends into the computer for display or further processing. The image is viewed as a graphics image, not a text image, so it can be altered with a graphics program but cannot be edited with a word-processing program, unless the scanner is part of a character-recognition system. To digitize photographs, a scanner may dither the image (put the dots a varying amount of space apart), or use the tagged image file format (TIFF), storing the image in 16 gray values. Some scanners can use standard video cameras to capture images for the computer. See also Character recognition.

The printer puts text or other images produced with a computer onto paper or other surfaces. Printers are either impact or nonimpact devices.

Daisy-wheel or thimble printers are so called from the shape of the elements bearing raised images of the characters. Their speed, perhaps 30 characters per second, is now considered unacceptably slow. Dot-matrix printers produce their images by striking a series of wire pins, typically, 9, 18, or 24, through the ribbon in the pattern necessary to form the letter, number, line, or other character.

Ink-jet printers carry their ink in a well, where it is turned into a mist by heat or vibration and sprayed through tiny holes to form the pattern of the character on paper. Laser printers are similar to photocopying machines. The quality of laser-printer output is the highest generally available. See also Photocopying processes; Printing.

The modem connects one computer to another, ordinarily through the telephone lines, to exchange information. See also Modem.