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What
is a bar code?
There is a
mystique surrounding bar codes which
intimidates many people. Let's eliminate it
quickly. First the bar code usually doesn't
contain descriptive data, (just like your
social security number or car's license plate
number doesn't have anything about your name
or where you live). The data in a bar code is
just a reference number which the computer
uses to look up associated computer disk
record(s) which contain descriptive data and
other pertinent information.
For example, the bar codes found on food items
at grocery stores don't contain the price or
description of the food item; instead the bar
code has a "product number" (12
digits) in it. When read by a bar code reader
and transmitted to the computer, the computer
finds the disk file item record(s) associated
with that item number. In the disk file is the
price, vendor name, quantity on-hand,
description, etc. The computer does a
"price lookup" by reading the bar
code, and then it creates a register of the
items and adds the price to the subtotal of
the groceries purchased. (It also subtracts
the quantity from the "on-hand"
total.)
Another example of bar code data might be in a
quality reporting application, the bar code
may have only a single digit in it, but it may
be titled "Failed Vibration Test".
The computer associates the single digit with
the test result.
So, bar codes typically have only ID data in
them; the ID data is used by the computer to
look up all the pertinent detailed data
associated with the ID data.
Bar
Code Structure
A bar code is a
series of varying width vertical lines (called
bars) and spaces. Bars and spaces together are
named "elements". There are
different combinations of the bars and spaces
which represent different characters.
When a bar code scanner is passed over the bar
code, the light source from the scanner is
absorbed by the dark bars and not reflected,
but it is reflected by the light spaces. A
photocell detector in the scanner receives the
reflected light and converts the light into an
electrical signal.
So, as the wand is passed over the bar code,
the scanner creates a low electrical signal
for the spaces (reflected light) and a high
electrical signal for the bars (nothing is
reflected); the duration of the electrical
signal determines wide vs. narrow elements.
This signal can be "decoded" by the
bar code reader's decoder into the characters
that the bar code represents. The decoded data
is then passed to the computer in a
traditional data format.
Types
of Bar Codes
There are lots of different bar codes. Some
bar codes are numeric only, (i.e.
UPC,EAN,Interleaved 2 of 5). Some bar codes
are fixed length, (i.e. UPC-A is 12 digits,
UPC-E is 6 digits, EAN-13 is 13 digits, and
EAN-8 is 8 digits). Some bar codes can have
numbers and alphabetic characters, (i.e. Code
93, Code 128, and Code 39). One bar codes
allow you to encode all 128 characters, (Code
128).
Many were invented some time ago and have been
superseded by newer bar codes. Some industries
standardized on older bar codes before the
better ones had been invented, and therefore
there is a continuing requirement for their
use in particular industries.
Many of this booklet's readers have to comply
with their customer's or industry's bar coding
specifications; no choice is possible, just
compliance. The classic bar code type is Code
39, (also called Code 3 of 9) which has 9 bars
and spaces; three are wide, and the other 6
are narrow. In Code 39, 3 of 9 total bars and
spaces are wide; hence the name, Code 3 of 9.
There are two widths of bars and two widths of
spaces. If you wished to print a bar code of
ABCD, you would need to start and end it with
a special Start/Stop code character - the *
(asterisk) is used for Code 39. So to print a
bar code of ABCD, it would need to be printed
as *ABCD*. There should be at least 1/4"
of white space to the left and right of the
code; this helps the reader pick out where a
bar code begins and ends. Other bar code types
are similarly constructed. UPC and EAN bar
codes have four widths of bars and spaces; so
does Code 128.
Bar
Code Selection Recommendations
For new bar coding projects that don't have
industry or customer standards, Code 39 is the
typical non-food standard, because almost all
bar code equipment reads/prints Code 39.
However, Code 39 produces relatively long bar
codes; it is not particularly efficient in bar
code density, (the maximum density is 9.4
characters per inch including 2 start/stop
characters). Where the label width is an issue
and there is numeric data or lower case data,
Code 128 is the best alternative; Code 128
also has an extra efficient numeric only
packing scheme to produce very dense bar
codes, and Code 128 has all 128 ASCII
characters. No all readers read Code 128, so
before you settle on it as a standard be sure
that your reader is 128 capable. Code 93 has
been promoted by only one vendor; it requires
two characters to make Full ASCII; and it
doesn't have a numeric packing option. For
these reasons, Code 128 is preferable over
Code 93. The larger the width of the elements,
the more space it takes to print the bar code;
therefore, the lower the bar code density. The
thinner the bar and spaces, the less space is
required and the higher the bar code density.
For examples of different bar code densities,
see the complete Bar Code Primer available
from Worthington Data Solutions.Lower density
bar codes are more reliably printed and more
consistently read than higher density bar
codes, because minor variations (due to
printing or damage) are much more serious with
high density bar codes - the percentage of
distortion is larger.
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