Hardware Reference
In-Depth Information
Another printer is the
wax printer
. It has a wide ribbon of four-color wax that
is segmented into page-size bands. Thousands of heating elements melt the wax as
the paper moves under it. The wax is fused to the paper in the form of pixels using
the CMYK system. Wax printers used to be the main color-printing technology,
but they are being replaced by the other kinds with cheaper consumables.
Still another kind of color printer is the
dye sublimation printer
. Although it
has Freudian undertones, sublimation is the scientific name for a solid changing
into a gas without passing through the liquid state. Dry ice (frozen carbon dioxide)
is a well-known material that sublimates. In a dye sublimation printer, a carrier
containing the CMYK dyes passes over a thermal print head containing thousands
of programmable heating elements. The dyes are vaporized instantly and absorbed
by a special paper close by. Each heating element can produce 256 different tem-
peratures. The higher the temperature, the more dye that is deposited and the more
intense the color. Unlike all the other color printers, nearly continuous colors are
possible for each pixel, so no halftoning is needed. Small snapshot printers often
use the dye sublimation process to produce highly realistic photographic images on
special (and expensive) paper.
Finally, we come to the
thermal printer
, which contains a small print head
with some number of tiny heatable needles on it. When an electric current is pas-
sed through a needle, it gets very hot very fast. When a special thermally sensitive
paper is pulled past the print head, dots are made on the paper when the needles are
hot. In effect, a thermal printer is like the old matrix printers whose pins pressed
against a typewriter ribbon to make dots on the paper behind the ribbon. Thermal
printers are widely used to print receipts in stores, ATM machines, automated gas
stations, etc.
Most computers nowadays are connected to a computer network, often the In-
ternet. Achieving this access requires special equipment. In this section we will
see how this equipment works.
Modems
With the growth of computer usage in the past years, it is common for one
computer to need to communicate with another computer. For example, many peo-
ple have personal computers at home that they use for communicating with their
computer at work, with an Internet Service Provider, or with a home banking sys-
tem. In many cases, the telephone line provides the physical communication.
However, a raw telephone line (or cable) is not suitable for transmitting com-
puter signals, which generally representa0as0volts anda1as3to5volts as
shown in Fig. 2-38(a). Two-level signals suffer considerable distortion when trans-
mitted over a voice-grade telephone line, thereby leading to transmission errors. A
