Hardware Reference
In-Depth Information
Figure 15.6. A capacitive buckling spring keyswitch.
A capacitive switch does not work by making contact between conductors. Instead, two plates usually
made of plastic are connected in a switch matrix designed to detect changes in the capacitance of the
circuit. The membrane version of the switch uses upper and lower electrical contact sheets separated
by an insulating sheet with small holes.
When a key is pressed, the plunger moves the top plate in relation to the fixed bottom plate, or it
presses a hammer over the membranes, allowing the top and bottom contacts to touch. The buckling
spring mechanism provides for distinct over-center tactile feedback with a resounding “click.” As the
capacitance between the two plates changes or the two membranes make contact, the comparator
circuitry in the keyboard detects this as a keypress.
The tactile feedback is unsurpassed because the buckling spring design provides a relatively loud
click and a strong over-center feel. The only drawback to the design is the cost. Buckling spring
keyboards are among the most expensive designs, whether they use capacitive or membrane switches.
The quality of the feel and their durability make them worth the price, however.
Although some of IBM's older keyboards featured capacitive keyswitches, most current IBM/Lenovo
keyboards use either membrane, rubber dome, or other lower-cost keyswitches. In 1991, IBM spun
off its keyboard/printer division as Lexmark, which then spun off the keyboard division as Unicomp
in 1996. Today, Unicomp still manufactures and sells “IBM” keyboards with buckling spring
membrane switch (“clickety” as some would say) technology. You can purchase new Unicomp (IBM)
keyboards direct by visiting its online store (
www.pckeyboard.com
).
Because of the buckling spring keyswitches (and the resulting clickety feel), I am a huge fan of the
IBM, Lexmark, and Unicomp keyboards.
Touch keyboards
The need for super thin and light portable keyboards to go with tablet systems has prompted the
introduction of touch sensitive keyboards. These are keyboards that lack traditional keys. The keypad
area is completely flat, with keys literally just printed on the surface. The “keys” themselves are
merely areas that use pressure sensors (similar to a touchpad) to detect a tap. Some may be made of
glass and use the same touch sensitive technologies found in new touch-sensitive displays.
Although these thin and light touch keyboards may be useful for temporary use with a tablet system,
they are less than ideal as a substitute for a traditional keyboard on desktop or laptop systems.
The Keyboard Interface
A keyboard consists of a set of switches mounted in a grid or an array called the key matrix. When a
switch is pressed, a processor in the keyboard identifies which key is pressed by determining which
grid location in the matrix shows continuity. The keyboard processor, which also interprets how long
the key is pressed, can even handle multiple keypresses at the same time. A 16-byte hardware buffer
in the keyboard can handle rapid or multiple keypresses, passing each one to the system in
succession.
When you press a key, the contact bounces slightly in most cases, meaning that several rapid on/off
cycles occur just as the switch makes contact. This is called
bounce
. The processor in the keyboard is
designed to filter this, or “debounce” the keystroke. The keyboard processor must distinguish bounce
from a double keystroke the keyboard operator intends to make. This is fairly easy, though, because
the bouncing is much more rapid than a person could simulate by striking a key quickly several times.
