Developers, looking for a quick Code 128 implementation? Check out the DLL for Windows and the UFL for Crystal Reports.

The following table and pseudo code is illustrated based on the free Code 128 Font found at Free Barcode Fonts. While the check digit calculation is the same with all Code 128 barcodes the location of characters within the fonts differ. There is some sample code available here.

The following table illustrates the Code 128 Value, the value of Variant B and C, and the ASCII location of the character within my Code 128 font set found at Free Barcode Fonts . If you are dealing with Unicode you will need to make the conversion between unicode and the ASCII value.

When dealing with Code 128 it is always advisable to think of a character as the Code 128 Value rather than the actual character. For instance, don't think of a capital A as a capital A. It's much easier to think of it as a Variant B code 33.

Converting between ASCII and Code 128

The font was written to allow an algorithmic conversion between Code 128 and ASCII. While it's possible to use a lookup table to find your information it's also quite easy to make the transformation algorithmically. Three steps are required.

Converting from Code 128 to ASCII:
1) Check for Code 128 value 0 [Variant B Space, Variant C 00] If Code 128 value is 0 than ASCII value equals 128
2) If Code 128 value is between 1 and 94 inclusive than ASCII value equals Code 128 value + 32
3) If Code 128 value is greater than 94 than ASCII value equals Code 128 Value + 50

For instance, Code 128 value 0 will equal ASCII value 128. Code 128 Value 1 [Variant B !, Variant C 01] will equal 1 + 32 equals ASCII value of 33. Code 128 value 98 [Variant B SHIFT, Variant C 98] will equal 98 + 50 equals ASCII Value of 148.

The same steps can be used in reverse to find a Code 128 Value from an ASCII character.

To begin encoding you will keep a running total of the checksum of each character. The checksum is weighted based on the location of each particular character. For instance, the first character is weighted by 1. The second by 2, etc, etc, etc. To start, lets assume that we are encoding the value 'Code 128' as a barcode in variant B. The structure and checksum is as follows:

As you can see calculation of the weighted values is quite easily accomplished. To calculate the actual check digit one needs to add the values together and apply modulos 103 to the sum of the weighted values. In the above case the total checksum value is 104 + 35 + 94 + 108 + 148 + 85 + 108 + 168 = 850. 850 MOD 103 = 26. Therefore, the check digit for this barcode would be code 26 of the Code 128 set, which in ASCII terms would be ASCII 58, or the character ':'

To complete the barcode we simply add the check digit and the stop character located at ASCII 156, Code 128 value 106, to result in:

[START B] C O D E 1 2 8 : [STOP]

Code 128 Structure:

As you can see in the example above Code 128 is structured by a start character, indicating the starting variant, the data itself, followed by the check digit, and ultimately ending up with the stop character:

[START VARIANT][DATA CHARACTERS][CHECK DIGIT][STOP]

Developer Notes:

Writing for code 128? This should get you on your way. It's not as confusing as it looks and remember, the Code128 DLL is available to make life much easier.