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Symbology Types

Checksums

Checksums are additional characters appended to bar codes to guarantee good reads Checksums are necessary on some

bar codes that are prone to errors For example, Interleaved 2 of 5 is a very dense, numeric-only bar code, but it is prone to

substitution errors You should always use a checksum on this code and on any other code where checksums are available

Most bar code readers can be congured to use checksums in the following ways (on symbologies where there is a choice):

Do not expect a checksum

Use a checksum to guarantee a good read

Use a checksum to guarantee a good read, and pass the checksum on to the computer system

Choice 2 is generally the best Some symbologies, such as UPC, require a checksum, and the checksum processing is invisible to

the programmer Choice 2 guarantees only a good read It does not guarantee a good connection to your computer

Choice 3 guarantees both a good read and a good connection between the computer and the bar code reader It requires

special software on the computer to verily the checksum This gives the most secure system, but at the expense of extra logic in

your program

Code 128

This symbology is a very compact bar code for all alphanumeric and numeric only

applications The full (128-character) ASCII character set can be encoded in this symbology

without the double characters found in Full ASCII Code 39 If the bar code has four or more

consecutive numbers (0–9), the numbers are encoded in double-density mode (where two

characters are encoded into one character position) Code 128 also has ve special, non data

function characters These are generally used to set reader parameters or return parameters

Code 128 actually has three different character-code subsets It has two forms of error checking, making it a very stable bar code

Checksums are required If you have your choice, Code 128 is generally the best all-around symbology you can use

Code 39

Code 39 (or Code 3 of 9) is the most common bar code in use

for custom applications It is popular because it can support both

text and numbers (A–Z, 0–9, +, -, , and <space>), it can be read by

almost any bar code reader in its default conguration, and it is

one of the oldest of the modern bar codes Code 39 is a binary or

2 width bar code, and it can support any number of characters that the reader can scan Code 39 is specied in many military

and government specications Code 39 bar codes are self-checking and are not prone to substitution errors A checksum is not

required, but is recommended

Interleaved 2 of 5

Also known as I 2 of 5 or ITF, this is a numeric-only bar code that prints out a little larger

than the UPC-A bar code when 10 digits are encoded This symbology encodes any even

number of digits; if you have an odd number, a leading zero is added

This bar code is a candidate for numeric-only applications Because Interleaved 2 of 5 is

prone to substitution errors, you should always use a checksum

Figure 6 - Code 128 Symbology

Figure 7 - Code 39 Symbology

Figure 8 - Interleaved 2 of 5 Symbology

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