Chapter 18: Keyboards, Mice, and Pointing Devices^467
A mouse with a ball that is 21 millimeters (mm) in diameter has rollers that are 7mm in
diameter. As stated above, the optical encoding disk has 36 holes. So if the user moves the
mouse one inch (25.4mm), the ball rotates slightly more than once; the rollers rotate a little
more than three times and cause the disk to spin a little more than one complete revolution,
matching the movement of the ball, which results in the sensor detecting about 40 light
pulses. The PC is sent 40 bits of data to indicate the mouse’s movement.
Theprocessusedbythemousetoconvertmovementintolightpulses(andeventually
binary data) is called optomechanical. The ball, rollers, and disk move mechanically, and
the LED and sensor convert light pulses optically. A mouse may have two sets of LEDs
and sensors on each optical encoding disk, one on the left side of the disk and one on the
right. Having two LED and sensor sets allows the processor to detect the direction of the
disk’s rotation.
Although not shown in Figure 18-20, there is also a small plastic window placed
between each LED and disk to aim the LED’s infrared light beam so that the sensor can be
focused on it. The plastic windows on each side of a disk are set at slightly different
heights so that the sensors see light pulses at different times. Figure 18-21 shows the effect
this has on the infrared light beams. As the disk rotates, the beams show through the
disks’ holes at slightly different times. The processor can detect the direction of the rota-
tion (and the mouse) by which beam is detected first.Mouse Connectors
Nearly all mice sold today have a six-pin mini-DIN (PS/2) connector, shown earlier in
Figure 18-17, on the PC end of their cable. This connector, which was first introduced
with the IBM PS/2 system, has essentially replaced the DB-9 serial connector that was
used before that. Serial mice are still available, but since newer PC systems rarely offer
more than a single serial port and include PS/2 connectors for both the keyboard and
mouse, the serial mouse has all but disappeared.Figure 18-21. One of the mouse’s LED beams is set slightly higher than the other to help detect
the mouse’s travel direction