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Computer Input and Output Devices

In the previous section, the history of computers and human-computer interfaces was briefly outlined. Now, a closer examination at the computer interface will be taken, beginning with input and output devices. Furthermore, some details about what happens under the surface will be detailed, providing the basis for further discussion.
Typical PCs have several devices for I/O operations, of which the most important are (Dix et al. 49):²⁸

• input devices
  • keyboard
  • mouse
• output devices
  • screen
  • speaker
  • printer
  • LEDs (Light-Emitting Diodes)

Keyboards are used for text and command input, sending a code to the computer when a key is pressed. Depending on what kind of keyboard is used, a different degree of energy is required to press the keys. Furthermore, some keyboards react faster than others, which frequently makes it difficult for users to get used to different keyboards (52-53).
The mouse became more and more important as an input device in the 1980s due to the growing popularity of graphical user interfaces. It is used to point at, position, and select items, which is essential in the principle of direct manipulation discussed in section 3.3.3. Today, a mouse only detects movements and clicks, while joysticks may also provide a force-feedback function (59-61).²⁹ A mouse may have one, two or three buttons; some also have trackballs. Mouse buttons may have different functions; typically, the left mouse button is used for selecting and positioning, the right mouse button is used in order to let a context-sensitive menu appear, and the middle button is often used for double-clicks. Often, these functions can be customized.³⁰ Frequently, novice users have difficulties with the mouse; thus, applications should never solely rely on double-clicking and dragging (Microsoft 15).
Most computers include a speaker, allowing the system and applications to send acoustic signals to the user, for example when displaying an alert (Dix et al. 72). Games often make extensive use of sound signals, augmenting visual signals, whereas the importance of sound has been ignored by most interfaces designers by now (521).
Printers are a basic computer peripheral; however, they are not as critical for this study as the other I/O devices, but they do play a special role in the principle of WYSIWYG (72-75; see also section 3.3.3).
A large variety of computer screens are currently available, from very small to very large, frequently utilising cathode ray tubes or liquid crystal displays (LCD). The resolution and colour depth do not only depend on the screen itself, but also on the graphics adapter (66-71). Computer screens can also function as an input device (touch screen), but this kind of screen is not very common as part of an average user's equipment (62).
Dix et al. does not describe the LEDs of computer hardware in detail although they may provide important information to the user (71). Particularly in IBM compatible PCs, the computer may not answer to mouse or keyboard input; however, a look at the hard disk LED of the computer may indicate that the computer is busy. Furthermore, the LEDs of a modem connected to a telephone line may indicate that the modem is still connected to a remote computer although the computer had to be restarted after a system crash.

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