Hardware Reference
In-Depth Information
calling the company's toll-free number (800-777-4886) or by visiting its online store
(
www.pckeyboard.com
)
.
See
“
TrackPoint II/III/IV
,”
p.
755
(this chapter).
The EnduraPro/104 is notable for including a built-in TrackPoint pointing device;
however, a design change has rendered the TrackPoint device used in these models as un-
acceptable. As any TrackPoint user knows, the pointing stick itself is a pressure sensor,
notajoystick,andassuchitshouldberigidandunmovable,Unfortunately,theTrackPoint
designnowusedbyUnicompexhibitsunwantedphysicalmovement,greatlyreducingthe
functionality. If you want a keyboard with a TrackPoint, I only recommend IBM or Len-
ovo models.
Because of the buckling spring keyswitches (and the resulting clickety feel), I am a huge
fan of the IBM, Lexmark, and Unicomp keyboards.
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 pro-
cessor in the keyboard is designed to filter this, or “debounce” the keystroke. The key-
board processor must distinguish bounce from a double keystroke the keyboard operator
intendstomake.Thisisfairlyeasy,though,becausethebouncingismuchmorerapidthan
a person could simulate by striking a key quickly several times.
The keyboard in a PC is actually a computer. It communicates with the main system in
one of two ways:
• Through a special serial data link if a standard PS/2 keyboard connector is used
• Through the USB port
The serial data link used by conventional keyboards transmits and receives data in 11-bit
packets of information, consisting of 8 data bits, plus framing and control bits. Although
it is indeed a serial link (in that the data flows on one wire), the keyboard interface is in-
compatible with the standard RS-232 serial port commonly used to connect modems.
