INPUT AND OUTPUT DEVICES

Basic Input Devices

Most computer systems include a keyboard and pointing device, such as a mouse, for basic data input. Additional input devices, such as scanners, digital cameras, and graphics tablets, are handy for working with graphical input. Microphones and electronic instruments provide input capabilities for sound and music. A keyboardallows the user to key in programs and data and to control the computer system. Letters, numbers, symbols and blank spaces are known as characters. The design of most computer keyboards is based on the typewriter’s QWERTY layout (because these are the first six letters on the top left of the keyboard), which was engineered to keep the typewriter’s mechanical keys from jamming. In addition to the basic typing keypad, computer keyboards include a collection of function keys designed for computer-specific tasks, a calculator-style numeric keypad, and an editing keypad with keys such as End, Home, and Page Up.

A pointing device allows you to manipulate an on-screen pointer and other screen-based graphical controls. The most popular pointing devices for personal computers include mice, trackballs, pointing sticks, trackpads, and joysticks. A standard desktop computer includes a mouseas its primary pointing device. A mouse includes one or more buttons that can be “clicked” to input command selections. To track its position, a computer mouse uses one of two technologies: mechanical or optical. A mechanical mouse reads its position based on the movement of a ball that rolls over a mouse pad placed on a desk. An optical mouse uses an onboard chip to track a light beam as it bounces off a surface, such as a desk, clipboard, or mouse pad. An optical mouse provides more precise tracking, greater durability, less maintenance, and more flexibility to use the mouse on a wide variety of surfaces without a mouse pad. A pointing stick, looks like the top of an eraser embedded in the keyboard of a notebook computer. A trackpad is a touch-sensitive surface on which you can slide your fingers to move the on-screen pointer. A trackball looks like a mechanical mouse turned upside down. A joystick looks like a small version of a car’s stick shift and is used mostly for playing games.

Display Devices

A computer display screen is usually classified as an output device because it typically shows the results of a processing task. Some screens, however, can be classified as both input and output device because they include touch-sensitive technology that accepts input. Display devices used for output offer three technology options: CRT, LCD, and plasma. Gun-like mechanisms in theCRT(cathode ray tube) spray beams of electrons toward the screen and activate individual dots of color that form an image. CRT display devices often simply called “monitors”, are bulky, however, and consume a fair amount of power. An LCD(liquid crystal display) produces an image by manipulating light within a layer of liquid crystal cells. Modern LCD technology is compact in size and lightweight, and provides an easy-to-read display. The advantages of LCD monitors (or “flat panel displays”) include display clarity, low radiation emission, portability, and compactness. Plasma screen technology creates an on-screen image by illuminating miniature colored fluorescent light arrayed in a panel-like screen. The name “plasma” comes from the type of gas that fills fluorescent lights and gives them their luminescence. Like LCD screens, plasma screens are compact, lightweight, and more expensive than CRT monitors. They are rather energy consuming, too.

Image quality is a factor of screen size, dot pitch, width of viewing angle, refresh rate, resolution, and color depth. Screen size is the measurement in inches from one corner of the screen diagonally across to the opposite corner.The quality of a screen is often measured by the number of horizontal and vertical pixels used to create it. A pixel is a dot of color on a photo image or a point of light on a display screen. It can be in one of two modes: “on” or “off”. A larger number of pixels per square inch means a higher res­olution, or clarity and sharpness of the image. The distance between one pixel on the screen and the next nearest pixel is known as dot pitch. Dot pitch (dp) is a measure of image clarity. A smaller dot pitch means a clearer image. Greater pixel den­sities and smaller dot pitches yield sharper images of higher resolution. A monitor’s viewing angle width indicates how far to the side you can still clearly see the screen image. A wide viewing angle indicates that you can view the screen from various positions without compromising image quality. CRT and plasma screens offer the widest viewing angles. A CRT’s refresh rate (also referred to as“vertical scan rate”) is the speed at which the screen is repainted. The faster the refresh rate, the less the screen flickers. Refresh rate is measured in cycles per second, or Hertz (Hz). The number of colors a monitor can display is referred to as color depth or “bit depth”. Most PC display devices have the capability to display millions of colors. The number of horizontal and vertical pixels that a device displays on a screen is referred to as its resolution. The resolution for many early PC displays was referred to as VGA (Video Graphics Array). Higher resolutions were later provided by SVGA (SuperVGA), XGA (eXtended Graphics Array), SXGA(Super XGA), and UXGA (Ultra XGA).

Printers

Printers are one of the most popular output devices, they usually use ink jet or laser technology. An ink jet printer has a nozzle-like print head that sprays ink onto paper to form characters and graphics. A laser printer uses the same technology as a photocopier to paint dots of light on a light-sensitive drum. Laser technology is more complex than ink jet technology, which accounts for the higher price of laser printers. A recurring cost of using a printer is the ink-jet or laser cartridge that must be replaced every few thousand pages of output. Printers differ in resolution, speed, duty cycle, operating costs, duplex capability, and memory. Printer resolution – the density of the gridwork of dots that create an image – is measured by the number of dots printed per linear inch, abbreviated as dpi, normally 900 dpi is enough. Printer speeds are measured either by pages per minute (ppm) or character per second (cps). A printer’s duty cycle determines how many pages a printer is able to process, usually measured per month (ppm). A printer with duplex capability can print on both sides of the paper, though it will slow down the print process. A computer sends data for a printout to the printer along with a set of instructions on how to print that data. Printer Control Language (PCL) is the most widely used language for communication between computers and printers, but PostScript is an alternative printer language that many publishing professionals prefer. The data that arrives at a printer along with its printer language instructions requires memory.

Installing Peripheral Devices

Today, many peripheral devices connect to an external USB (universal serial bus) port and Windows automatically loads their service drivers, making installation as simple as plugging in a table lamp. USB is currently the most popular technology for connecting peripherals.

USB ports are conveniently located on the front of the system unit for easy access. When you install a peripheral device, you are creating a connection for data to flow between the device and the computer. Within a computer, data travels from one component to another over circuits called a data bus. One part of the data bus runs between RAM and the microprocessor. Other parts of the data bus connect RAM to various storage and peripheral devices. The segment of the data bus that extends between RAM and peripheral devices is called the expansion bus. As data moves along the expansion bus, it can travel through expansion slots, cards, ports,and cables. An expansion slotis along, narrow socket on the system board into which you can plug an expansion card. An expansion card is a small circuit board that gives a computer the capability to control a storage device, an input device, or an output device. Expansion cards are also called “expansion boards”, “controller cards”, or “adapters”. Expansion cards are built for only one type of slot. An expansion portis any connector that passes data in and out of a computer or peripheral device. Built in ports supplied with a computer usually include a mouse port, keyboard port, serial port, and USB ports. Most notebook computers are equipped with several USB ports.

Some devices require software, called a device driver, to set up communication between your computer and the device. The directions supplied with your peripheral device include instructions on how to install the device driver. Typically, you use the device driver disk or CD one time to get everything set up, and then you can put the disk away in a safe place. Installing a peripheral device you should remember that the cable you use must match the peripheral device and a port on the computer. If the right type of port is not available, you might have to add an expansion card. Once the connection is made, PnP should recognize the new device. If not, you will probably have to install driver software.

Comprehension check. Match the beginnings of the sentences in the first column with the endings in the second.

Date: 2016-01-03; view: 1420