8.6 Input Devices
Disk units, tape units, and drum units are all input and output (I/O) devices, and they share a common use for mass storage. In this section, we look at a few devices that are used exclusively for input of data. We start with one of the most prevalent devices – the keyboard.
8.6.1 KEYBOARDS
Keyboards are used for manual input to a computer. A keyboard layout using the ECMA-23 Standard (2nd ed.) is shown in Figure 8-23. The “QWERTY” layout (for the upper left row of letters D01 – D06) conforms to the traditional layout used in typewriters. Frequently used letters are placed far apart so that the typist is slowed and jams in mechanical typewriters are reduced. Although jams are not a problem with electronic keyboards, the traditional layout prevails.
When a character is typed, a bit pattern is created that is transmitted to a host computer. For 7-bit ASCII characters, only 128 bit patterns can be used, but many keyboards that expand on the basic ECMA-23 standard use additional
modifier keys (shift, escape, and control) and so a seven-bit pattern is no longer large enough. A number of alternatives are possible, but one method that has gained acceptance is to provide one bit pattern for each modifier key and other bit patterns for the remaining keys.
Other modifications to the ECMA-23 keyboard include the addition of function keys (in row F, for example), and the addition of special keys such as tab, delete, and carriage return. A modification that places frequently used keys together was developed for the Dvorak keyboard as shown in Figure 8-24. Despite the perfor-
mance advantage of the Dvorak keyboard, it has not gained wide acceptance.
8.6.2 BIT PADS
A digitizing tablet, or bit pad, is an input device that consists of a flat surface and a stylus or puck as illustrated in Figure 8-25. The tablet has an embedded two-dimensional mesh of wires that detects an induced current created by the puck as it is moved about the tablet. The bit pad transmits X-Y (horizontal-vertical) positions and the state of the buttons on the puck (or stylus) either continuously, or for an event such as a key click or a movement, depending on the control method. Bit pads are commonly used for entering data from maps, pho-
8.6.3 MICE AND TRACKBALLS
A mouse is a hand-held input device that consists of a rubber ball on the bottom and one or more buttons on the top as illustrated in the left side of Figure 8-26.
As the mouse is moved, the ball rotates proportional to the distance moved. Potentiometers within the mouse sense the direction of motion and the distance traveled, which are reported to the host along with the state of the buttons. Two button events are usually distinguished: one for the key-down position and one for the key-up position.
A trackball can be thought of as a mouse turned upside down. The trackball unit is held stationary while the ball is manually rotated. The configuration of a trackball is shown in the right side of Figure 8-26.
An optical mouse replaces the ball with a light emitting diode (LED) and uses a special reflective mousepad that consists of alternating reflective and absorptive horizontal and vertical stripes. Motion is sensed through transitions between reflective and absorptive areas. The optical mouse does not accumulate dirt as readily as the ball mouse, and can be used in a vertical position or even in a weightless environment. The natural rotation of the wrist and elbow, however, do not match the straight horizontal and vertical stripes of the optical mousepad, and so some familiarity is required by the user in order to use the device effectively.
8.6.4 LIGHTPENS AND TOUCHSCREENS
Two devices that are typically used for selecting objects are lightpens and touch- screens. A lightpen does not actually produce light, but senses light from a video screen as illustrated in Figure 8-27. An electron beam excites a phosphor coating
on the back of the display surface. The phosphor glows and then dims as it returns to its natural state. Each individual spot is refreshed at a rate of 30 – 60 Hz, so that a user perceives a continuous image.
When a dim spot is refreshed, it becomes brighter, and this change in intensity signals the location of the beam at a particular time. If the lightpen is positioned at a location where the phosphor is refreshed, then the position of the electron beam locates the position of the pen. Since the lightpen detects intensity, it can only distinguish among illuminated areas. Dark areas of the screen all appear the same since there is no change in intensity over time.
A touchscreen comes in two forms, photonic and electrical. An illustration of the photonic version is shown in Figure 8-28. A matrix of beams covers the screen in
the horizontal and vertical dimensions. If the beams are interrupted (by a finger for example) then the position is determined by the interrupted beams.
In an alternative version of the touchscreen, the display is covered with a touch sensitive surface. The user must make contact with the screen in order to register a selection.
8.6.5 JOYSTICKS
A joystick indicates horizontal and vertical position by the distance a rod that protrudes from the base is moved (see Figure 8-29). Joysticks are commonly used
in video games, and for indicating position in graphics systems. Potentiometers within the base of the joystick translate X-Y position information into voltages, which can then be encoded in binary for input to a digital system. In a spring-loaded joystick, the rod returns to the center position when released. If the rod can be rotated, then an additional dimension can be indicated, such as height.