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Bar code technology has been helping businesses
minimize data entry errors, speed processes, and
reduce costs for over thirty years. The fact is,
bar code systems work. Yes, some are configured
better than others, some are easier to use, but
even the most unusual application has reaped substantial
return on investment in a reasonably short time
frame.
This document is
designed to introduce this very effective technology
to potential new users. Written in non-technical
language, it covers the components of a bar code
label, scanning options, and a short glossary
of common industry terms.
What is a Bar
Code?
A bar code is a
machine readable code consisting of a series of
bars and spaces printed in defined ratios. Bar
code symbologies are essentially alphabets in
which different widths of bars and spaces are
combined to form characters and, ultimately, a
message. Because there are many ways to arrange
these bars and spaces, numerous symbologies are
possible. Common linear symbologies include UPC/EAN,
Interleaved 2 of 5 (I of 5), Codabar, Code 39,
and Code 128.
While each symbology
is in some way unique, the composition of a complete
message (bar code) is surprisingly similar regardless
of the symbology used. For example, all bar codes
are based on some "X" dimension. The
"X" dimension is the narrowest bar or
space in the bar code. Designated in "mils"
(thousandths of an inch), symbology standards
usually specify a minimum value "X"
to insure compatibility between reading and printing
equipment used in open systems.
The "X"
dimension determines a bar code's density. Density
refers to the amount of information that can be
captured in the bar code in a particular space,
usually a linear inch. While not intuitively obvious,
high density bar codes have low numbers (e.g.,
5 mil) and low density bar codes have high numbers
(e.g., 55 mil). This is because individual characters
consist of some combination of bars and spaces
that are each multiples of "X". When
"X" is small, the area required for
each character is less than when "X"
is large; thus the bar code can hold more per
linear inch and is said to be of higher density.
Similarly, increasing the width of the narrowest
element ("X") increases the space required
for each character and reduces the number of characters
per inch. Because the resulting code is often
quite large, very low density codes are often
associated with applications such as warehousing
that require reading bar codes from a significant
distance (3 to 30 feet).
All bar codes have
start/stop characters that allow the bar code
to be read from both left to right and right to
left. Unique characters placed at both the beginning
and end of each bar code, the stop/start characters
provide timing references, symbology identification,
and direction of read information to the scanner.
By convention, the unique character on the left
of the bar code is considered the "start"
and the character on the right of the bar code
is considered the "stop."
Immediately preceding
the start character and following the stop character
is an area of no markings called the quiet zone.
Because there is no printing in this area, a scanning
signal is not produced, thus the term "quiet."
The quiet zone helps the scanner find the leading
edge of the bar code so reading can begin. As
a rule, the quiet zone should be ten times the
"X" dimension or 1/4", whichever
is greater.
Putting all these
components together, we get a complete bar code
such as the one found below. Notice the leading
quiet zone followed by a start character, data,
a stop character, and a final quiet zone.
Bar Code Scanners
The function of
the bar code scanner is to "read" the
image presented by the bar code. In its most basic
form, the scanner sees and measures the absence
or presence of light in the code's bars and spaces,
and converts that information into an electrical
signal that can be translated into recognizable
or computer-compatible data.
Common hand held
scanning technologies include wands, lasers, and
CCDs. While they are all dedicated to the same
task, reading a bar code , each scanner type offers
both advantages and disadvantages, and none is
clearly superior in all cases. The following discussion
outlines how each technology works and the relative
advantages/disadvantages of each. For information
on selecting an appropriate scanner for a specific
application see "Which Bar Code Reader is
Right for You?"
Wands
Hand-held contact
scanners, or wands, are the oldest and most cost
effective method of bar code scanning.
How Wands Work
An operator manually
places the scanner in contact with the bar code
label. A tiny spot of light is projected through
the scanner lens. As the scan spot is drawn across
the bar code, reflections from each white space
and light absorption from each black bar produce
voltage variations that are amplified and shaped
for decoding (see "Decoding and Interfacing"
below).
Wand technology
is an excellent choice for many bar code applications.
Specific advantages include: contact with the
code makes it easy to determine which bar code
is being read, and allows the operator to read
bar codes of virtually any length; relative cost
is low when compared to other scanning technologies;
and with no moving parts, wands are the most rugged,
compact and lightweight of the technologies available.
Wands are not completely
without their limitations, however. Some applications
are simply not appropriate for contact scanning.
Bar codes need to be of good quality, of a particular
density (wand dependent), and reside on a flat
hard surface if acceptable scanning performance
is to be achieved. Some training is required for
operators to develop an appropriate scanning technique
since factors such as scan speed, wand angle,
and pressure can adversely affect scanning performance
if not performed properly. Finally, because this
is a contact device, damage to the bar code label
can occur if the proper paper stock or protective
coating has not been used.
Lasers
Hand held laser
scanners are the most expensive of the scanning
devices, but offer the largest depth of field
making them an appropriate choice for a wide variety
of non-contact applications.
How Lasers Work
Hand held laser
scanners use a laser diode to create a scan line
by projecting a beam of energy off a rotating
prism or oscillating mirror. The beam is reflected
out the scanner window onto the bar code, where
light energy from the bars and spaces is reflected
back to the scanner, collected on a mirror, focused,
and read by a photodetector. The resulting signal
may then be read using decoding software within
the scanner or at the terminal or host.
Laser technology
is an excellent choice for non-contact applications,
and the only choice for applications that require
reading distances of a foot (.3048m) or more.
Available in both hand held and fixed mount form,
lasers are easy to use, read a wide variety of
code densities, and allow for easy reading of
bar codes from irregular surfaces or through glass.
Because they are non-contact devices, lasers will
not wear out repeatedly scanned labels.
The two disadvantages
inherent in laser scanning are durability and
cost. Because lasers use both moving parts and
mirrors, they are not as rugged as CCDs or wands.
The reality is that hand held scanners will be
dropped no matter how diligent the operator, and
even if the internal parts do not break, misalignment
of the laser can easily reduce performance or
render the scanner unusable. Finally, laser technology
is the most expensive, both in terms of initial
purchase price and product life costs.
CCDs
Charge coupled
devices (CCDs) are extremely durable scanners
for near contact and contact applications. Less
expensive than their laser counterparts, CCDs
having no moving parts to wear out or break.
How CCDs Work
CCD scanners use
one or more LEDs to flood the bar code area with
light, and an image of the code is transferred
to an array of photodetectors. The characteristics
of the bar code are determined by electronically
sampling each individual photodetector which interprets
each bar and space by the number of adjacent detectors
sensing black or white. In other words, instead
of reading each bar and space in succession, the
CCD "takes a picture" of a very thin
portion of the complete bar code which it then
converts into a signal that may be decoded.
CCDs offer numerous
advantages over competing technologies. Though
less expensive than lasers CCDs also read various
code densities, are easy to use, and require very
little training. They are lighter and more rugged
than lasers and, unlike wands, may be used for
non-contact applications. New models offer depth
of field that is well suited for most retail,
banking and manufacturing applications. Welch
Allyn's 3400LR, for example, reads low density
codes to 12 inches (30.48 cm) and 100% UPC to
6 inches (15.24 cm).
Limitations to
CCD technology include depth of field and scan
width. While CCDs are an excellent choice for
the applications listed above, they are not appropriate
for long range scanning applications such as warehousing.
CCDs are also not the best technology choice for
applications in which a wide variety of label
lengths and formats are used. Long messages or
very low density codes can easily result in bar
codes that exceed the width of the scan head,
rendering them unreadable.
The Common
Denominator - Decoding and Interfacing
While each technology
uses a different method for reading bar codes,
all result in a digital signal that must be translated
into recognizable, or computer-compatible, data.
This is accomplished using decoding software that
resides in the scanner itself, or in a separate
device placed between the scanner and the terminal
or host. Using an algorithm, the decoder identifies
and interprets each bar coded message, and transmits
that data to the host computer.
Transmitting the
data requires a link, or interface, to the host
computer. Every interface has two different "layers":
a physical connection (hardware), and a logical
communications protocol. Common interfaces for
bar code scanners include keyboard wedge, serial
wedge, and direct connect.
The term "wedge"
refers to any device inserted between the keyboard
and the terminal that translates digital signals
into keyboard codes. In a keyboard wedge application,
the data resulting from the scanning of a bar
code symbol is treated by the PC or terminal as
if it originated from the keyboard, while the
keyboard itself remains fully functional. Because
the terminal or PC cannot differentiate between
bar coded data and actual keyboard data, a keyboard
wedge interface allows bar code reading capability
to be rapidly added to an existing computer without
changing the application software.
An ASCII or serial
wedge is an RS-232 scanner that is connected between
the ASCII terminal and a host controller. This
connection is used when keyboard wedge transmission
is too slow, or when the interface is not supported
by the product.
The term direct
connect actually has two meanings. To some, direct
connect refers to decoded output, or the ability
of the scanner to read a bar code and output data
directly to the host without an external decoder.
Direct connect has also been used to describe
a decoded output scanner connecting to a PC or
host without a keyboard.
Some Other Common
Terms
Dual Interface:
The ability of the scanner to connect directly
to either of two different host devices and to
automatically configure itself to communicate
with each host. For example, a hand-held CCD may
be attached to an IBM POS (Point of Sale) terminal
during the day, and a portable data terminal for
maintaining inventory at night. A built-in dual
interface makes it easy to move a scanner between
applications.
Flash Memory: A
memory chip that holds its content without power.
The term was coined by Toshiba for the chip's
ability to be erased "in a flash". Flash
memory is used by Welch Allyn in most products
as an alternative to PROMs (Programmable Read
Only Memory) because flash memory can be easily
updated. Flash capability allows cloning, PC Menuing
and full firmware updates.
HHLC (Hand Held
Laser Compatible): "Dumb" or undecoded
lasers have a unique way of communicating with
an external decoder. This protocol, also known
as laser emulation, is used by devices such as
CCD's or decoded output lasers to communicate
with external decoders.
RS-232 (Recommended
Standard 232): TIA/EIA standard for serial transmission
between computers and peripheral devices such
as barcode scanners, modems, and mice. RS-232
uses a 25-pin DB-25 or 9-pin DB-9 connector. RS-232
is generally used for distances of 50 feet (15.24
m) or less from the host, though this distance
may be extended if high quality cable is used.
Snappiness: A term
used to reference the speed of the scanner. Depending
on the testing method employed, snappiness may
be measured by reads per minute, trigger to beep
time, or trigger to output time. Various factors
can affect snappiness, including ease of use (aiming),
decoding software, bar code quality, and interface
speed.
Wand Emulation:
When a wand scans a bar code, it sends a digital
picture of the bar code to an external decoder.
When a decoded output scanner connects to an external
decoder (such as a portable data terminal), wand
emulation mode is used. The decoded output scanner
decodes the bar code and outputs the information
as a digital picture just as if a wand had scanned
the bar code.
SELECTING A
HAND HELD SCANNER
Which Bar Code
Reader is Right For You?
How to pick a hand
held scanner that is compatible with your symbology
and ideal for your application.
Hand held bar code
readers have been a key part of automatic identification
applications since the inception of the industry,
and they remain a crucial part of bar code systems
in a range of industries and applications. Today's
manufacturers offer customers a variety of choices
and price/performance options for tailoring hand
held scanner solutions to individual requirements.
There are three
primary types of hand held readers: contact wands,
CCD readers, and laser scanners. In considering
which of these readers offers the best solution
for your scanning activities, it is useful to
understand the key functional components of a
hand held reader: 1) illumination and image or
code capture; 2) decoding, and 3) connectivity.
The three types
of hand held readers are differentiated by the
type of "reader engine" used to illuminate
and read the bar code. A contact wand uses a light
emitting diode (LED), the CCD's engine reader
is a charge-coupled device (CCD) and the laser
scanner uses a visible laser diode (VLD). The
type of reader engine is a primary factor in the
hand held reader's price / performance and also
determines its appropriateness for individual
applications. By understanding the distinctions,
users can select a hand held reader offering the
best fit and value.
To successfully
match reader choices to project requirements,
users need to consider three key application criteria:
working distance, label size, and label density.
Working distance refers to the distance between
the label and the reader while scanning. There
may be none (contact) or several feet, but each
requires a different reader. Label size refers
to the width of the bar codes being read, while
label density addresses the minimum resolution
of the bar/space patterns. Each of these criteria
is interrelated. For example, the larger the label
and the bigger the bar/space pattern, the greater
the attainable working distance. Contact wands,
CCD readers and laser scanners offer different
levels of scanning performance, and, given different
price points and life cycle costs, there are a
variety of trade offs to consider.
Working Distance
The required working
distance should be clearly defined. Will the operator
bring the reader in contact with the label, or
will he/she scan the bar code from a distance?
Retail point of sale, office, and factory applications
often support contact scanning, while warehousing,
distribution, and transportation applications
typically require greater working distances.
Working distance
is the primary differentiator of the three reader
types. As the name implies, contact wands require
that the label be brought into contact with the
reader. Until recently, CCD readers were limited
to working distances of one to two inches, but
the newest generation of scanners has extended
that range significantly (7 in/17.78 cm). Laser
scanners, offer the greatest working distances
averaging 8 to 30 inches (20.32 to 76.2 cm)standard.
There are also specialty laser guns that can be
matched up with special large, reflective labels
to achieve scan distances of many feet.
Differences in
working distance are reflected in the cost of
the readers. While there are exceptions, contact
wands are generally the most economical of the
readers, and laser guns the most expensive, both
in terms of initial cost and overall life cycle.
CCD readers are priced somewhere between wands
and lasers.
Because these readers
feature solid-state designs, they offer excellent
life cycle costs as well.
Label Size/Label
Density
Label density refers
to the minimum width of a bar/space element, and
is measured in thousandths of an inch , or "mils".
For linear or one-dimensional bar codes, the key
size consideration is label width. High-density
codes (under 7.5 mil) tend to be read at closer
working distances; low-density codes (above 15
mil) can be read from greater distances.
Total code width
is particularly important to know when selecting
a CCD reader. In most cases, the widest label
a CCD reader will support is limited to the width
of the reader's scanner opening, though some custom
readers have been designed to read substantially
larger codes. Since wand scanners are moved across
the target code, and laser guns project light
to move across the code, these two readers can
handle broader code widths.
If you decide a
contact wand is ideal for your application, you
must consider the wand's aperture width when you
make your hardware selection. The aperture should
be approximately the same size as the narrow bar
(X dimension) of the bar code it is to read. If
it is much wider, adjacent bars may appear in
the scanning window at the same time, making it
harder to read the symbol. If the aperture is
too small, the scanner may see a printing defect
as a bar or space where none exists.
Decoding the
Image
Once the reader's
illumination and acquisition system has captured
the code's bar/space pattern, the information
must be converted into a signal with a format
that can be understood by the host computer system.
This is called decoding. The decoding function
identifies the type of symbology being scanned
(autodiscrimination), loads the appropriate decoding
algorithms, and decodes the data encoded on the
label.
Decoded information
is typically formatted as serial RS-232 data,
or is converted into keyboard commands for transmission
to the host system. The decoded signal is transmitted
via an interface cable to an RS-232 communication
port (serial data) or to the terminal's keyboard
port via a "Y-cable" (keyboard data).
The term "wedge"
is used to refer to one technique for interfacing
the bar code reader to the host system or terminal.
A "serial" wedge inserts the scanned
data in an RS-232 signal between a host computer
and terminal, while a "keyboard" wedge
presents the data as a series of key strokes.
Software in the readers are programmed via a bar
code menu to select terminal and interface parameters.
Until a few years
ago, the decoding and wedge functions were typically
handled by separate devices. The bar code reader
would output a hand held laser-compatible (HHLC)
or wand emulation (Wand Em) signal to an external
decoder bow, which performed the decoding and
wedging functions. Today, reader manufacturers
have integrated the decoding in all three reader
types, resulting in a one-piece, decoded output
scanner (DOS). Decoded output readers offer the
same decoding performance of box decoders in a
single package at a lower overall price.
External wedge
decoders are necessary when two or more different
types of scanning devices are required at each
decoding station (e.g., a badge swipe reader and
contact wand), or when an AUX port is required
(for integration of scales, printers, or other
serial I/O devices). But for ease of integration
and a lower overall system cost, integrated decoded
readers offer excellent value.
Connectivity
Once a reader has
decoded the label data, the information must be
transmitted to the host system. The serial and
keyboard wedge functions for electronically formatting
decoded output data are described above. Of course,
the reader must also be physically connected to
the host system, so customers will need to specify
the physical interface type of the terminal, PC,
or system. Since manufacturers of hand held readers
support hundreds of terminal types, they also
carry hundreds of interface cables. Customers
should work with the equipment supplier to identify
and order the correct cable.
Since decoded output
readers typically support many terminal interfaces
in a single device, some manufacturers of CCD
and laser hand held readers have standardized
common cabling schemes so that customers can stock
fewer cable and use common cable on both CCD and
laser scanners to save both time and money.
All three types
of hand held readers -contact wands, CCD readers
and laser scanners - offer excellent price/performance
for particular applications.
Understanding a
few key functional and price point differences,
however, can help you select the best hand held
scanner for your requirements.
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