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About Checkdigit
Multiply each number in the sequence of numbers
by the value called weight from specified algorithm. Add the product of these numbers
together give the sum, and divide it by the modulus from in the specified algorithm.
- DR checkdigit ... the remainder after the division.
- DSR checkdigit ... subtract the DR checkdigit from modulus.
Sometimes errors can occur during the process of printing a barcode.
Checkdigit
is a control that catches these errors.
That is,
when the sequence of numbers written in the barcode
doesn't match with the checkdigit we had read,
the computer refuses the sequence of numbers to input.
Some barcode types can have several checkdigit algorithms applied to it.
Following is a brief list of barcode types that require and do not require
checkdigit.
Barcode types and checkdigit:
- checkdigit required:
UPC/EAN/JAN, CODE-93, CODE-128, etc.
- checkdigit not always required: CODE-39, NW-7, ITF, etc.
Certain data indicates that addition of checkdigit can decrease the error
ratio to under 1/1000.
Using the information above, an examination of the popular checkdigit algorithm
will be conducted. We will take the frequently employed JAN(EAN)
barcode (Modulus 10, Weight 3:1)
to illustrate this example.
| sequence |
4 | 9 | 0 |
1 | 2 | 3 |
4 | 5 | 6 |
7 | 8 | 9 |
|
|
x | x | x |
x | x | x |
x | x | x |
x | x | x |
|
| weight |
1 | 3 | 1 |
3 | 1 | 3 |
1 | 3 | 1 |
3 | 1 | 3 |
|
|
| sum |
4 |
+27 |
+0 |
+3 |
+2 |
+9 |
+4 |
+15 |
+6 |
+21 |
+8 |
+27 |
=126 |
|
| 126 (sum) / 10 (modulus) =
12, remainder 6 |
| 10 - 6 (remainder) = 4 (check digit) |
By
dividing directly the sequence of numbers with the modulus, then
- the remainder is
the checkdigit of the type 7DR and 9DR; or
- (modulus - remainder) is
the type 7DSR and 9DSR.
The above example only illustrates a couple of checkdigit possibilities and
no where near covers all of the checkdigit algorithms in use.
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