Hardware Reference
In-Depth Information
photocopy machines. In fact, many companies make devices that combine copying
and printing (and sometimes fax as well).
The basic technology is illustrated in Fig. 2-36. The heart of the printer is a
rotating precision drum (or in some high-end systems, a belt). At the start of each
page cycle, it is charged up to about 1000 volts and coated with a photosensitive
material. Then light from a laser is scanned along the length of the drum by
reflecting it of a rotating octagonal mirror. The light beam is modulated to produce
a pattern of light and dark spots. The spots where the beam hits lose their charge.
Rotating octagonal
mirror
Laser
Drum sprayed and charged
Light beam
strikes drum
Drum
Toner
Scraper
Discharger
Heated
rollers
Blank
paper
Stacked
output
Figure 2-36.
Operation of a laser printer.
After a line of dots has been painted, the drum rotates a fraction of a degree to
allow the next line to be painted. Eventually, the first line of dots reaches the toner,
a reservoir of an electrostatically sensitive black powder. The toner is attracted to
those dots that are still charged, thus forming a visual image of that line. A little
later in the transport path, the toner-coated drum is pressed against the paper, trans-
ferring the black powder to the paper. The paper is then passed through heated
rollers to fuse the toner to the paper permanently, fixing the image. Later in its
rotation, the drum is discharged and scraped clean of any residual toner, preparing
it for being charged and coated again for the next page.
That this process is an exceedingly complex combination of physics, chem-
istry, mechanical engineering, and optical engineering hardly needs to be said.
Nevertheless, complete assemblies, called
print engines
, are available from several
vendors. Laser printer manufacturers combine the print engines with their own
electronics and software to make a complete printer. The electronics consist of a
fast embedded CPU along with megabytes of memory to hold a full-page bit map
and numerous fonts, some of them built in and some of them downloadable. Most
