TouchPads

In their developmental years, notebook computers used a variety of pointing devices. Several tried integrated trackballs. The designer's whims determined placement—sometimes at the corner of the screen, sometimes below the keypad. In any case, the balls were small (they had to be to fit in a portable system), and the perfectly located trackball was rare, indeed.

Hewlett-Packard developed a pop-out mouse tethered to the right side of the system. The system detected mouse movement through the thin, flat tether, so you could hold the mouse in the air and its movements would nevertheless register. The downside was the need for space on the right, either air or desktop, a requirement not easily met in crowded aircraft.

The notebook industry needed a better alternative for a pointing device and found it in the TouchPad (the name is a trade mark of Synaptics Corporation). Almost by default the TouchPad became the most popular pointing device for notebook systems. It wasn't that the TouchPad necessarily was so good. All the alternatives were worse. Figure 21.1 shows a TouchPad.

The TouchPad detects the location of your finger on the pad by detecting the electrical capacitance of your finger. Your finger attracts a minute static electrical charge, which causes a small current flow in the circuitry of the TouchPad. The electronics associated with the TouchPad detects this minute current from two adjacent edges of the pad, so it can precisely locate your finger in two dimensions. By following changes in the current, it can determine the motion of your finger across the pad.

Mechanically, the foundation of the TouchPad is a printed circuit board. The top of the board holds a pattern of conductive sensor lines etched into place. The bottom of the board holds the electronics that make the pad work. A layer of Mylar covers the top of the board to protect it and gives your finger a smooth surface to trace across. Current designs are capable of resolving 1000 points per inch (40 points/mm).

A TouchPad is inherently an absolute position detector. The electronics of the pad converts the absolute location information it determines into mouse-like relative motion that can be sent to your computer in standard mouse protocol.

The TouchPad can be completely sealed from any kind of contamination, so it is an excellent choice as a pointing device for a machine that has to operate in a hostile environment. On the other hand, useful TouchPads must be large enough to conveniently register your finger movements. That means they steal space from the top of your notebook computer. Most notebook computer–makers take advantage of this space requirement to give you a hand rest below the keyboard—and to give the manufacturer more room for the components inside the computer.

Computer-makers can choose from several sizes of TouchPads for their products. Typically the active pad surface measures 62 by 46.5 millimeters, although larger (90.6 by 72.1 millimeters) and smaller (55.9 by 32.9 millimeters or less) versions are also readily available.

Most manufacturers locate the TouchPad just below the space bar on their keyboards. This location allows you to use your thumbs to move the mouse so you don't have to take your hands from the home row when typing, an important consideration if you touch-type on your computer. Notebook computers differ in the height and sensitivity of their TouchPads. Although the differences are subtle, they affect usability. If the TouchPad height is wrong for your typing system, you're likely to inadvertently move the mouse, or even activate it. You'll want to try out the keyboard and the TouchPad of any portable computer that has one before you commit to buying it.