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In a conventional desktop scanner, the document is placed face-down on a glass bed, and a scanning array consisting of a lamp, a mirror, a lens, and an image pickup moves back and forth underneath the glass. The image sensor may be a CCD or a Compact Image Sensor (CIS), a single row of sensor elements mounted very close to the document. Light from the lamp bounces off the original and is, with the CCD, reflected by the mirror into the lens, which focuses the image into the CCD. In the case of the CIS, the light and dark areas are picked up directly by the sensor. The CCD/CIS digitizes the results via an analog-to-digital converter, or ADC, and sends the resulting information to the scanner's own hardware and then to the host PC.
Scanner Interfaces
Scanners
have used just about every interface available on the PC, short of the IDE
bus. The first PC-based scanners used either the printer port or a custom
port using an ISA card. With printer-port scanners, the printer was often
daisy-chained through the scanner. While this method was slow and clumsy
(since printer ports were never designed to support this kind of chaining
of devices), it was almost universally effective: just about every
computer out there had a printer port, even laptops or "luggables."
Scanners that user their own interface didn't do as well, partly because
of the cost and because not everyone always had a free ISA slot to spare.
The SCSI interface, popularized on the Macintosh platform,
became the next de facto standard for scanners. Since the Mac was
(and to a great degree still is) predominantly used by graphic designers
and artists, low-cost desktop scanners for the Mac market were inevitable,
and so the first Mac scanners were also SCSI. There was one big drawback
to SCSI scanners for the PC: most PCs didn't have a SCSI interface, so the
scanner had to be packaged with a SCSI controller card of some kind. If
you had no spare slots for the card, or no free IRQs for the card to use,
this posed a big problem. (Even today, the vast majority of PCs don't come
with a SCSI connector.)
Then along came two innovations that made it far more practical to add a scanner to a PC without undue difficulty. These two things were USB and FireWire (a/k/a IEEE 1394). Because of USB's widespread adoption (every PC manufactured today has at least one USB port) and its versatility (up to 63 devices can be chained together), USB is now the most widely-used interface for scanners. It's also the most convenient: installing a USB scanner involves little more than plugging the scanner in and providing the right driver.
USB has another advantage that neither SCSI nor parallel-port scanners had: the USB bus itself can be used to explicitly supply power to the device. Canon's CanoScan series of scanners, for instance, derive their power from the USB link. There is no external power "brick." The downside of doing this is the scanner motors can't draw a lot of power, and therefore don't move very fast. If scanning speed is a priority, you may want to think twice before buying a USB-powered scanner. On the other hand, such scanners only need one cord and are usually a lot more compact.
FireWire, or IEEE 1394, is also used as a scanner interface, although to a lesser extent. Like USB, all you need to do is plug the scanner in and add the drivers. Note that FireWire interfaces generally don't appear on lower-end scanners; the expense in adding a IEEE 1394 interface is greater than USB. Scanners such as the high-end Epson Expression series use FireWire, but even they come with a choice of USB or FireWire. FireWire devices can also be powered by the bus, but there do not appear to be any scanners that use this option yet.
With really high-end scanners, the higher-end interface is less a luxury and more of a requirement. The greater the scanner's resolution, the more data needs to be shunted across the wire. Epson's Expression 1640XL can manage up to 12,800 x 12,800 DPI (interpolated). Sending that kind of data stream across a USB bus would be problematic, especially since USB can only manage 12 megabits per second at absolute best--and most USB buses are shared by other devices, too. FireWire's speed limit? 400 megabits per second--more than enough bandwidth for the most massive scans. (Granted, USB 2.0 does 480 megabits, but FireWire scanners out there far outnumber any using USB 2.0.)
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All Rights Reserved. No portion of this website may be reproduced in any
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Copyright
© 2003 The Cheap Computer Parts Handbook
All Rights Reserved. No portion of this website may be reproduced in any
way without prior written consent of the authors.