Chapter 18: Keyboards, Mice, and Pointing Devices^471
Other Pointing Devices
Many types of pointing devices exist, but the four that have some popularity beyond the
mouse are the touchpad, the trackball, glidepoint, and the joystick.Touchpads
A touchpad is a fixed-place pointing device that has become very common in notebook
computers. A touchpad, like the one shown in Figure 18-25, is a small, flat square or rect-
angular surface on which you slide (touch) your finger to move the cursor on the display,
select objects, and run programs. A touchpad provides the same actions as a mouse.
A touchpad works on the principle of coupling capacitance that uses a two-layer grid
of electrodes to hold an electrical charge. The upper layer of the grid has small vertical
electrodes,andthelowerlayerhassmallhorizontallyplacedelectrodes.AnICisattached
to the grid that detects any changes in the capacitance of the pad. The chip is constantly
monitoring the capacitance between each of the horizontal electrodes and a corresponding
vertical electrode. The user’s finger when placed over a pair of the electrodes serves as a
conductor and alters the capacitance of the electrode pair, since a human finger has a very
different dielectric property than air. The chip detects the change and data is sent to the
PC using the same techniques that are used by a mouse. As the finger moves over the
grid, each of the electrode pairs affected are converted intox-yplacements for the PC.
Like an optical mouse, the touchpad has no moving parts and does not require pre-
ventive maintenance. Touchpads are being integrated into desktop PC keyboards as well
as notebook computers. An external touchpad can be added to a PC via its PS/2 port.Figure 18-25. A touchpad integrated into a notebook PC