Bar codes, auto ID and portable data capture - there are many terms
and technologies to baffle those who are new to this niche of the
What is auto ID and how can it help ?
The term auto ID covers a number of different technologies that
enable computers to automatically identify an item, usually by reading
data from it. There are of course many application areas for auto
ID and companies have developed technologies like bar codes, RF
(radio frequency) tags, magnetic stripe, smart card, optical character
recognition (OCR), optical mark readers (OMR), etc. Each of these
technologies has its own benefits and weaknesses. In the majority
of applications, product or asset identification is best achieved
using one of two technologies - bar codes or RFID (short range radio
tags), and it is these that this guide will focus on. In addition,
we will explain how to transfer data to and from the product, explaining
terms such as "keyboard wedge", "batch" and
"RF data network".
What is a bar code and how does it work ?
A bar code is a way of representing characters (numbers and/or letters)
in a printed form that can be read and decoded by suitable reading
equipment. There is a variety of different "types" of
bar code (the various types are called "symbologies")
and a range of different technologies for reading them. Because
a bar code comprises just encoded characters, the act of "reading"
or decoding it simply makes those characters available to the host
computer equipment to which the decoder is attached.
The structure of a bar code varies from symbology to symbology,
but the diagram below represents the components of a typical bar
are special codes that define
each end of the bar code. Different bar code symbologies use different
start and stop characters.
is a character appearing in the bar code that
is generated from the data in the bar code. The check character
is used as a verification check that the data in the code has been
correctly decoded. Different types of bar code use different methods
for calculating the check character. In some symbologies a check
character is optional, while in others it is always present.
are areas of blank space to the left
and right sides of the bar code.
(see above) are horizontal bars printed across
the top and bottom of some bar codes. Bearer bars can help avoid
partial reads should the reader move off the top or bottom of the
code. These are usually only required for certain types of code
(most usually Interleaved 2 of 5) as the start and stop characters
in most bar codes make bearer bars unnecessary.
When a bar code is scanned (see fig 2 below), the optical elements
of the reader convert the black and white bars of the code into
an analogue (i.e. non-digital) electrical signal that varies according
to the light/dark parts of the code. Thus the optics of a reader
scanning a code "see" the bar code as an electrical signal
with a corresponding high/low pattern.
Now the code has been represented in an electrical form, the next
step is for the decoding element of the reader to convert the analogue
signal into digital data that the attached computer can understand.
The decoder analyses and decodes the signal according to a pre-defined
set of rules. The decoding rules (correctly referred to as algorithms)
are defined by the symbology type being decoded.
So, providing the bar code has been properly printed and scanned,
the decoder can apply the decode algorithm to output the encoded
characters as data.
Why do some bar codes have numbers and letters ?
The practice of printing the encoded characters below the bar code
is really a case of common sense. If, for some reason, a bar code
cannot be scanned (it may be damaged, badly printed, or the scanner
might fail), then a sensible back-up measure is for the characters
to appear in human readable form and for any associated application
software to allow the user to manually enter the characters or digits.
Some bar code symbology rules explicitly state that the numbers
should always be printed below. While there may be applications
where having the numbers printed below is not required, not desirable
or not possible, it is generally good practice to represent the
bar code in human readable form.