Check Sum

[kurtsimons]

I need some help understanding check sums and how to work with them in VB.

how do i validate the following line is "correct"... B4 is the check some value for the line

74 11 12 00 2E 22 74 13 12 00 2E 22 B4














Check Sum
Check Sum is a method where we use modulo summation to detect errors in a stream of data. Usually the sum operation is operated on package of bytes where the last byte is the Check Sum number. When you add all the numbers in the package the sum should be zero (in octets the addition is modulo 256, it means that if the sum reaches to 256 it turns to zero), if from some reason it isn't then we know that there is an error.

Check Sum has no ability to correct errors but only detect them.

Let's see an example:

12 40 05 80 FB 12 00 26 B4 BB 09 B4 12 28 74 11 BB
12 00 2E 22 12 00 26 75 00 00 FA 12 00 26 25 00 3A
F5 00 DA F7 12 00 26 B5 00 06 74 10 12 00 2E 22 F1
74 11 12 00 2E 22 74 13 12 00 2E 22 B4 In the example we can see a block of octets that are represented in Hexadecimal. The last number in every row is a number that if you sum the whole row, and there is no error, you should get a zero.

It's important to mention that you should build a reasonable row, that is, not very short where the overhead would be too much, and not very long because if you detect that there is an error you should send the whole message again.

[sparbag]

Hello Kurt,

I remember there being four methods of which there would be similarities in creating the check digit.
I was told the most common was the Mod 11 method.
You have chosen to add the numbers which gives you a total.
You next divide this total by 11 giving you a remainder of 1 to 10. This remainder would then become your check digit.
Other methods involved multiplication of each byte value against a previously decided value, and adding these up, but still Mod 11 would be used to create the check digit.

Steve
Is that what you wanted to know or have I got the wrong end of the stick.

[namrekka]

The most common method is however CRC. This is the most secure for checking data.

I don't thrust your examples. The first row gives me B0 and the second A0.

You add a value, if result is bigger then 255 subtract 256 (byte overflow). Thats modulo 256.

[kurtsimons]

this is actually what i am trying to solve

how is the check sum for ~CK` \ C8 ? when ~CK` \ are not hex values

~CK` \C8

[kurtsimons]

according to this calculator my examples are wrong.... frustrating

http://www.lammertbies.nl/comm/info/...+11&method=hex

[Al42]

Your examples aren't adding the bytes modulo 256. Maybe they're using a different checksum, maybe they're just random bytes as a non-working example.