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Some useful information about barcodes
A symbology is a language in bar code technology. In certain countries people speak, write and read the French language in order to communicate. In other countries the language of choice and custom might be English, or German, or Chinese. In the technology field we call bar code, a symbology is a language which we use to print and read messages accurately and conveniently comprised of bars and spaces. When a symbology is used to print a message, we call that message a bar code label. Information in a label is read through the eyes of a scanner, but the scanner and the label have to communicate through the same symbology rules or the message will not be understood. There are a number of bar code symbologies, some primitive, and some quite sophisticated. After an introduction to symbology characteristics, this section will describe three symbologies, U.P.C./EAN, Code 39 and Code 128, in some detail, and explain why you might decide to use a certain symbology to control one portion of your business, and a different symbology to control another portion.
"X" Dimension
The smallest element of interest in a bar code is called the "X" dimension. This is the narrowest bar or space in the "picket fence like" array which carries your message. In designing your bar code driven data collection system, you must first decide what this dimension should be. In the range of practical applications of bar code in use today, you may choose from an "X" of 5 mils to an "X" of 50 mils. A mil is one thousandth of an inch. Each symbology you choose for your bar code language will have a number of its own rules for the representation of each character in your message. A character is made up of bars and spaces, some of which will be a single "X" in width, and others will be two, three, or four "X"s wide.
Generally speaking, the larger the "X" dimension, the more forgiving the bar code is when read by a scanner. On the other hand, a larger "X" dimension requires a larger label area. Most applications use "X" dimensions in the range of 10 mils to 25 mils, representing a trade off among a number of considerations in bar code technology, including printer resolution.
Quiet Zone and Contrast
In order to read a bar code label dependably, a scanner must make a series of sensitive measurements of the contrast between a black (or dark) bar and the white (or light) background space between each bar. To complicate this problem, the scanning instrument projects a spot of energy that can be moving very fast, challenging the capabilities of today's most responsive electronic circuits. Symbology designers have, therefore, specifies a zone just to the left and right of the bar code that shall be free of all printing. This "quiet zone" gives the scanning device time to adjust its opto-electronic measurement circuits in order to properly determine the critical width of each bar in the message.
Bar code is most frequently expressed as black bars on white paper. This combination gives the greatest contrast, and the most reliably scanned message. However, bar code continues to work effectively when certain other combinations of color and background are used. You see this in supermarket packaging. What it important is the contrast between bars and spaces as viewed by the scanner.
Start, Stop, and Parity Characters
From the earliest applications of bar code, system designers have insisted that bar code symbologies be bi-directional. This means that the bar code message can be read from left to right, or right to left, with no difference in performance. To accomplish this bi-directionality, every symbology provides for a separate start and stop character, in addition to the message characters. By convention we refer to the start character as the unique character to the left of the bar code, and the stop character as the unique character to the right of the bar code. In a vertical label, the start would be the bottom character and the stop would be the top character.
The symbology may also specify a mandatory or optional parity, or check character, in the bar code message. This parity character is included in the bar code to minimize the risk of misreading the message. It does not add any information to the message itself. Usually this parity character is stripped by the decoding portion of the scanner, and not passed along to the information network supporting the bar code application. The start, stop, and parity character (if present) are collectively referred to as bar code message overhead.
Bar Code Message Composition
The zones to the left and right of the symbology are free from all printing so that the scanner can have time to calibrate. Most symbologies require that the quiet zone be ten times the "X" dimension of the symbol, or one quarter of an inch, whichever is greater. Since most industrial bar codes are printed with an "X" dimension of 20 mils or less, the requirement for one quarter of an inch of quiet zone, each side, is the most common rule. Of course for large "X" dimensions of 40 or 50 mils, such as warehouse bin labels or shipping container labels, the quiet zone will grow proportionately.
After the quiet zone, the start character will appear. Following the start character, the message characters are represented with the most significant characters to the left. This means that the characters are arranged in a horizontal bar code message the same way they are presented in the roman alphabet.
Symbology Standards
From the dozens of symbologies introduced in the bar code field to date, only a handful have achieved broad support. To gain support for a new bar code today, the symbology creator would have to offer the marketplace some dramatic new feature not available in earlier symbologies. As time passes, this gets more difficult.
The need for a standards authority first arose in 1972 when the supermarket industry decided to mark each of the grocery point-of-sale packages with a unique identifier to speed check out transactions. The grocery industry created an organization that today is called the Uniform Code Council, Incorporated (UCC). The UCC are the keepers and publishers of the UPC symbology, the bar code on the grocery package. Now, any company processing food, or manufacturing non-food items for retail sale in the grocery industry, can label their product with a bar code that will be guaranteed to read at the checkout stand.
The Universal Product Code, better known as U.P.C., is the symbology used throughout the American grocery industry. Related to this symbology is the International Article Numbering Code, referred to as EAN. In time, these two similar symbologies will be accepted throughout North American points of sale.
In the U.P.C.-A code, a field of six digits was set aside to the left side of the bar code for producer identification, and a field of five digits was set aside to the right of the bar code for the product identification. In an EAN-13 code, the number of digits encoding the country of registration can vary from 2 to 3, and the manufacturer prefix can vary between 4 and 6 digits, resulting in a "non-significant" number, or one that cannot easily be taken apart.
The UPC symbology is intended to be used only in the structured environment of retail point of sale. Originally planned for groceries, UPC has now expanded to convenience stores and general merchandise retailing as well. Since some retail items are very small, and label location is difficult to reserve, a short version of the UPC symbology is permitted with six characters of numeric information in this message. Essentially, the producer and product information is still available, but zeros within the producer and product numbering sequence have been squeezed out of the bar code. The missing zeros are reconstructed again after scanning within the decoding device at the cash register, for price look up. This shortened UPC symbol is called UPC-E.
A frequently used symbology in industrial bar code systems today is Code 39. This symbology has the ability to encode messages using the full alpha numeric character set. The widespread popularity of Code 39 began with its selection by the Department of Defense (DOD) as the bar code tracing label designating the contents of every package in every shipment delivered to all US military agencies. This DOD directive was made in 1981, and led to the requirement that 40,000 vendors use Code 39 bar code labels in order to fulfill their contract shipping requirements.
By today's standards, Code 39 uses a lot of label space. Over the history of bar code applications, there has been continuous growth in the amount of data within a bar code message. Ten years ago a six character bar code message was the norm. Today, sixteen character messages are typical, and thirty to forty character bar code labels are not uncommon.
Code 128 was introduced in 1981 by Computer Identics Corporation. This symbology has a number of features which set it apart from the older symbologies, making Code 128 the preferred code for most new bar code applications. These special features are:
* Encodes all 128 ASCII characters without cumbersome procedures.
* Uses the least amount of label space for any "X" dimension.
* Tested to be the most easily read code with the highest message integrity, due
to several separate message check routines.
* Contains function codes that can increase the power of complex data collection
processes.
Code 128 fits the general bar code structure including a mandatory check character. However, instead of having a single start character, Code 128 gives users the choice of three start characters. Each separate start character puts the following portion of the message in a different character set as explained below. Also, the three start characters act as character set shift codes when used within the message. To help understand this principle, the start codes are:
* Start Code A - Encodes all following characters into upper case alpha numeric
and ASCII control characters.
* Start Code B - Encodes all following characters into upper and lower case
alpha numeric characters.
* Start Code C - Encodes all following characters into pairs of numbers, 00 to
99, producing a "double density" expression of numeric information
within the symbology.
With properly designed Code 128 printing software, alpha numeric messages will be expressed in each label by the shortest possible bar code expression. The choice of start codes (or shift codes) will be made by the print software and inserted in the symbology automatically, minimizing label length. Anyone designing a bar code data collection application today should consider Code 128 first.
The appetite for including more and more detail in bar code messages seem to have no limit. These codes are sometimes referred to a stacked codes or matrix codes. Stacked bar code applications are not yet in the mainstream of bar code technology. This bar code format does not easily adapt to manual scanners on the market today. However, a new generation of CCD scanner is emerging to relieve this limitation, and stacked bar codes are an excellent match for fixed beam laser scanning of well controlled, conveyorized items.
Summary
The most efficient symbology in practical use today is Code 128, and should be the first symbology choice to consider. Using an even number of all numeric characters, within Code 128, will produce the shortest bar code label length and give your application the easiest label to scan
