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churst

Why is the ISBT128 ISO Mod37 calculation Wrong?

What is the method to calculate the K Code (i.e. Check Digit) for an ISBT 128 Barcode wrong? Please review and correct the source code from the following link:
https://www.barcodefaq.com/1d/isbt-128/#ISO_7064_MOD_37-2_Algorithm

Below is a reference to the correct formula for the calculation. See Appendix A.1.
The specific issues in the method above are the following
* The WeightedSum formula is wrong. It's not a multiplication by 2 and a modulo, rather it's a 2 ^n Power where n is the reverse "position" of the character within the string (i.e. 13-1). The Modulo is also not calculated inside the for loop, but rather outside of the for loop ==> WeightedSum = WeightedSum + (CharValue * 2^n)
* The CheckDigitAscValue is wrong as the Modulo needs to be taken on the complete sum total of WeightedSum ==> CheckDigitAscVal = (38 - (WeightedSum % 37)) % 37

http://www.iccbba.org/uploads/a3/56/a35603c2b626a45573cd98663086625f/ST-001-ISBT-128-Standard-Technical-Specification-v5.10.0.pdf

Operating System: Windows

Application: Visual Studio

02-06-19 5.7 year(s) ago

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Brant	Best Answer Referring to the ‘C’ language function CalculateMod37_2 on page 168 of the specification, the code we have appears correct and I have tested it with a few values, such as A9999 15 000001 = M and that also appears correct. Posted 5.7 year(s) ago (0) (0) Report Abuse

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Other Answers (1)

churst

For clarification, I'm not using your formula, and as such the info provided above is untested sudo code. I was mainly attempting to help others that may stumble upon this sample code and think the functionality is correct as is. The ICCBBA reference above is the standards body that specifies the ISBT 128 checksum formula and is where the calculation steps were pulled from.

My request would be to either correct the method or pull it down so others don't come across it and use it

Posted 5.7 year(s) ago

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