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Interleaved 2 of 5 Bar Code Page 

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Interleaved 2 of 5 (also called I 2 of 5) is a numbersonly bar code. The symbol can be as long as necessary to store the encoded data. The code is a high density code that can hold up to 18 digits per inch when printed using a 7.5 mil X dimension. A check digit is optional.
The "Interleaved" part of the name comes from the fact that a digit is encoded in the bars and the next digit is encoded in the spaces. The encoded digits are "Interleaved" together. There are five bars, two of which are wide and five spaces, two of which are wide.
The symbol includes a quiet zone, the start character (narrow barnarrow space narrow barnarrow space), the encoded data, the stop character (Wide barnarrow spacenarrow bar), and a trailing quiet zone.
Number  Pattern 
0  NNWWN 
1  WNNNW 
2  NWNNW 
3  WWNNN 
4  NNWNW 
5  WNWNN 
6  NWWNN 
7  NNNWW 
8  WNNWN 
9  NWNWN 
The Xdimension is the width of the smallest element in a bar code symbol. The minimum Xdimension for an "open system" (a bar code label that will be read by scanners from outside your company) is 7.5 mils (a mil is 1/1000 inch) or 0.19 mm. The "wide" element is a multiple of the "narrow" element and this multiple must remain the same throughout the symbol. This multiple can range between 2.0 and 3.0 if the narrow element is greater than 20 mils. If the narrow element is less than 20 mils, the ratio must exceed 2.2. Quiet zones must be at least 10X or at least .25 inches
The height of the bars must be at least .15 times the symbol's length or .25 inches, whichever is larger. The overall length of the symbol is given by the equation:
L = (C (2N + 3) + 6 + N)X
where
This calculator will show you the length of a Interleaved 2 of 5 bar code given the X dimension, widetonarrow ratio and total number of data characters.
Caution About Partial Scans
A partial scan of an I 2 of 5 symbol may decode and cause incorrect data
to be read. To prevent partial scans on long symbols, one should include bearer
bars. These are bars that run along the top and bottom edges of the symbol
in the scanning direction. If a partial scan of the symbol occurs, the scanning
beam will hit the bearer bar and will not decode. The bearer bar must touch
the top and bottom of all the bars and must be at least 3X wide.
Another solution for the short scanning problem is to fix all I 2 of 5 symbols to a set number of digits. Zeros can be used to pad the data to the set number of digits. The application program would then be set to only accept scans of the correct number of digits.
Finally, a check digit may be used. The I 2/5 symbology has an optional check character which uses a weighted Modulo 10 scheme. The check character is the last character in the symbol and should checked by the decoder and then transmitted with the data. Since I 2/5 must always have an even number of digits, the leftmost character may need to be a zero when the check character is added. The standard check digit is calculated by assigning alternating 3,1,3,1.. weighs to respective data digits. These weights are then multiplied by their respective data digits and the products are summed. The check digit is the digit needed to be added to the sum to make it an even multiple of 10. An example would be if the sum of the products was 37, the check digit would be 3.
Here is another explanation of how to calculate the check digit. Starting with the left most character, multiply every other character (odd positions) by 3 and sum the results with the values of the even positions. (The check character is always an even position so it is never multiplied by 3.) The sum should be a multiple of 10. To compute the check digit for printing a bar code, perform the summing operation above without the check character, do a mod 10 on the result, and subtract the value from 10 to yield the check digit. (If you start with an even number of digits, make sure you add a leading zero before you do the calculation.)
Other Resources
Altek Instruments based in the UK also has pages with I 2 of 5 specifications.
Barcode Mill from Altek Instruments allows you to issue unique, identifiable documents from your own web site, dynamically and in real time! This service can produce Interleaved 2 of 5 and other codes.
You can purchase complete, official specifications from AIM Global, Inc.