Barcodes are like a printed, machine-readable version of Morse code, where information is represented in the widths and spaces between lines.  More recently, two-dimensional barcodes have been developed which contain information in other geometric shapes or matrices.  A group of specificed patterns is called a "symbology", and linear barcodes are very common today.  Almost every retail package contains a Universal Produce Code or UPC-encoded information.  Although there are many types of barcode symbologies, each having their own special characteristics, features and limitations, we will only explain Code 39 in detail here, specifically because it is a readily available as a simple TrueType font, is therefore easy to implement, generally requires no additional software, and is therefore  very commonly used for identification card purposes.  Some of the many other barcode symbologies include: UPC, EAN, RSS-14, CodaBar, Interleaved 2 of 5, Discrete 2 of 5, Code 93, Code 128, PostNet, PDF417, Data Matrix, MaxiCode, and Aztec Code.

CODE 39              

The "Normal" CODE 39 (also known as "USS Code 39", "Code 3/9", "Code 3 of 9", "USD-3", "Alpha39", "Type 39") can encode the following 44 characters: Numeric 0-9, Uppercase A - Z and the special characters -. *$/+%. Code 39 is one of the most popular symbologies in the non-retail world and is commonly used in manufacturing, military, and health applications. It is a variable length symbology, does not require a check digit and is readable by virtually all barcode scanners.  Each Code 39 bar code must contain a start/stop character at the beginning and end, represented by an asterisk (*). The Asterisk may only be used for this purpose and therefore the data to be encoded may not contain an asterisk.  Additionally, data to be encoded with "Normal" CODE 39 must only contains Uppercase alphabetic characters.

Even though Code 39 does not require a check digit, it is considered by some to be self-checking because a single incorrectly read bar cannot generate another valid character.  Additionally, it applications where data security is important, a modulo 43 check character can be included, as has been adopted in the healthcare industry.  The check character is calculated by the following logic: Sum the appropriate value of each character in the barcode, excluding start and stop (0 - 42) and then divide by 43.  The remainder is the check character to be appended.

Certain applications and barcode readers will allow Code 39 to be used to concatenate two or more barcodes, with the use of a leading space in the barcode data. This is particularly beneficial when it is necessary to break one long barcode message into multiple shorter ones.

The "Full" ASCII version of Code 39 is an enhancement of the standard "Normal" Code 39 symbology, which has been modified to encode the complete 128 ASCII character set (including asterisks and lowercase). The "Full" ASCII version uses four characters $/+% as shift characters to modify the meaning of the "Normal" Code 39 character set.  Because of the use of these shift characters, nonstandard characters require twice the width to display in the barcode as standard characters.  However, even barcode readers that do not support the expanded "Full" Code 39 symbology will normally still read the data accurately but will display or output the data as normal uppercase Code 39 characters.

One of the major limitations of the Code 39 symbology is its relatively low data density, and because of this, it requires more space to encode data than other symbologies (i.e.Code 128).  This means that a Code 39 based barcode is impractical for labelling very small items.  It is also important to remember this limitation when designing an identification, membership or loyalty program card, since there are usually limitations in the card design to the size of the area that can be allocated to the barcode.