Hardware Reference
In-Depth Information
Many kinds of I/O devices are available today. A few of the common ones are
discussed below. Computer terminals consist of two parts: a keyboard and a moni-
tor. In the mainframe world, these parts are often integrated into a single device
and attached to the main computer by a serial line or over a telephone line. In the
airline reservation, banking, and other mainframe-oriented industries, these devices
are still in use. In the personal computer world, the keyboard and monitor are in-
dependent devices. Either way, the technology of the two parts is the same.
Keyboards
Keyboards come in several varieties. The original IBM PC came with a
keyboard that had a snap-action switch under each key that gave tactile feedback
and made a click when the key was depressed far enough. Nowadays, the cheaper
keyboards have keys that just make mechanical contact when depressed. Better
ones have a sheet of elastometric material (a kind of rubber) between the keys and
the underlying printed circuit board. Under each key is a small dome that buckles
when depressed far enough. A small spot of conductive material inside the dome
closes the circuit. Some keyboards have a magnet under each key that passes
through a coil when struck, thus inducing a current that can be detected. Various
other methods, both mechanical and electromagnetic, are also in use.
On personal computers, when a key is depressed, an interrupt is generated and
the keyboard interrupt handler (a piece of software that is part of the operating sys-
tem) is started. The interrupt handler reads a hardware register inside the keyboard
controller to get the number of the key (1 through 102) that was just depressed.
When a key is released, a second interrupt is caused. Thus if a user depresses the
SHIFT key, then depresses and releases the M key, then releases the SHIFT key,
the operating system can see that the user wants an uppercase ''M'' rather than a
lowercase ''m.'' Handling of multikey sequences involving SHIFT, CTRL, and
ALT is done entirely in software (including the infamous CTRL-ALT-DEL key se-
quence that is used to reboot PCs).
Touch Screens
While keyboards are in no danger of going the way of the manual typewriter,
there is a new kid on the block when it comes to computer input: the touch screen.
While these devices only became mass-market items with the introduction of
Apple's iPhone in 2007, they go back much further. The first touch screen was de-
veloped at the Royal Radar Establishment in Malvern, U.K. in 1965. Even the
much-heralded pinching capability of the iPhone dates back to work at the Univer-
sity of Toronto in 1982. Since then, many different technologies have been devel-
oped and marketed.
