(^462) PC Hardware: A Beginner’s Guide
circuit. Keyboards with this type of switches tend to have a soft feel and have
had some durability problems.
Rubber dome keyswitch Also called a carbon-contact keyswitch, this design
is very much like the foam and foil contact switch. In each rubber dome switch
is a small rounded dome of rubber that has a pad of carbon material on its
underside. When the key is pressed, a plunger presses down on the rubber
and the carbon contacts the circuit board completing the circuit. This type
of keyswitch is the most common type used in current keyboards.
Capacitive Keyswitches A capacitor is an electronic device that stores an electrical charge
between two plates. The charge in the capacitor is measured as its capacitance. When the
plates of the capacitor move closer or further away, the capacitance changes. It is on this
principle that capacitive keyswitches operate.
A capacitive keyswitch is built very much like a foam and file switch except that the
plunger has a metal plate attached to its bottom. When the plunger is pressed down, the
space between the plate and another plate located below the plunger is reduced. In some
designs, the distance between the plates actually increases, but either way a change takes
place. The keyboard’s circuitry detects the change in the keyswitch’s capacitance and a
keystroke is detected. This type of keyswitch is very expensive and is usually found only
in proprietary or high-end devices.
Keycaps
Keycaps serve a couple of fairly important functions: they provide the service for your
finger to press, and they identify the keys so users can find the character they wish to
enter. Keycaps can be removed from the keyboard, but depending on the keyboard and
the type of switch in use, this is not typically recommended. The keyboard can be cleaned
without removing the keycaps and once removed, it is not always easy to replace them.
Keyboard Controller
The keyboard controller is the circuitry inside the keyboard that processes keystrokes
and exchanges information with the PC. The keyboard controller is a microprocessor and
a ROM (read-only memory) that holds the keyboard processor’s instructions. The
controller constantly scans the key grid for keystrokes and then translates the scan codes
for the keystrokes it finds and transmits the data to the PC.Keyboard Cable
The cable that connects the keyboard to the PC is a four-wire cable that provides the four
signals carried between the PC and the keyboard: data, clocking, ground, and power. A
metal grounding sheath binds the four wires of the keyboard cable, and the whole bundle
is covered with a thick plastic or rubber outer sheath. The keyboard cable is usually four
to six feet in length and is usually straight. If the cable is not long enough for a particular
application, keyboard cable extensions are available to lengthen it.