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PublisherChapter 19. Keyboards
A
keyboard is a matrix of individual switches, one
per key. Pressing a key closes its switch, generating a signal that
the dedicated keyboard controller built into the
keyboard recognizes as the make code for that
key. Releasing the key opens the switch, which the keyboard
controller recognizes as the break code for that
key. Using a firmware lookup table, the keyboard controller
translates received make code signals to standard scan
codes, which it sends via the keyboard
buffer to a second keyboard controller located in the PC,
which recognizes those scan codes as specific characters and control
codes.
Because releasing a key generates a break code, the local keyboard
controller can recognize when two keys are pressed together (e.g.,
Shift-A or Ctrl-C) and generate a unique scan code for each such
defined key combination. For undefined key combinations (e.g.,
pressing "a" and then pressing
"s" before releasing
"a"), the keyboard controller
recognizes that, even though a break code for
"a" has not been received, the
user's intent is to type
"as", and so generates the scan
code for "a" followed immediately
by the scan code for "s".Most people do not need to work
with scan codes directly, but if you program you may need to use scan
codes to control the behavior of a program when the user presses
specific keys or key combinations. The Microsoft Keyboard
Scan Code Specification, which includes the standard
keyboard scan codes as well as alternative make and break PS/2 scan
codes and USB code responses for the Windows Key, Application Keys,
and ACPI power controls, is available from http://download.microsoft.com/download/whistler/hwdev3/1.0/WXP/EN-US/scancode.exe.
Microsoft also publishes the USB HID to PS/2 Translation
Table, which provides mappings from the USB HID keyboard
multimedia usages to PS/2. That file is available in PDF format from
http://www.microsoft.com/hwdev/download/tech/input/translate.pdf
or in Excel format from http://www.microsoft.com/hwdev/download/tech/input/translate.xls.Communication between the two keyboard controllers is bidirectional
and asynchronous, which allows the PC to illuminate and extinguish
keyboard indicator lights and set Typematic parameters such as delay
and repeat rate. The keyboard interface was standardized with the
1984 introduction of the IBM PC/AT, which means that any AT-class
keyboard works with any 286 or higher PC, assuming that the physical
connectors are compatible.In addition to basic functions, the local keyboard controller may
support various programmable functions, using either or both of these
methods:
The local keyboard controller may support creating custom definitions
for specific keys and key combinations and storing those definitions
in the keyboard itself. When a key with a custom definition is
pressed, the local keyboard controller generates the necessary scan
codes directly and sends them to the PC. Because this method uses
only standard scan codes, it has the advantage of not requiring a
driver on the PCwhich means the programmable functions can be
used with any operating systembut the disadvantage of being
limited to functions that can be performed using standard scan codes.
This method was commonly used with early programmable keyboards, but
is less common nowadays.
Programmable functions may require a driver running on the PC. That
driver recognizes nonstandard scan codes generated by the local
keyboard controller and takes whatever action the driver defines for
"hard-wired" special keys. For keys
with custom definitions, the driver uses stored user configuration
data to determine what action to take. This method has the advantage
of supporting programmable functions that would be clumsy or
impossible to implement using only standard scan codes (such as
opening your web browser or email client), and is the method used by
most current programmable keyboards. If the driver is not installed,
such keyboards function as ordinary keyboards, with special-purpose
keys and programmable features disabled.
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