July 25, 2008 03:44pm pdt

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	[x] Attachment Details

I need a checksum calculation written in C converted to VB

Zones: Microsoft Visual Basic.Net, Miscellaneous Programming, C Programming Language

I have been given some C code which calculates a checksum value for a Hex string. It looks like it could be a CRC-8 checksum, but I'm not positive. I need to create a VB version of this but I'm afraid I don't know C very well and the second function is confusing me. Can anyone help?

unsigned char crc_buff(unsigned char *p, unsigned char I)
{
      unsigned char x;
      unsigned char crc;
            crc = 0xff;
            for ( x = 0; x < I; x++)
                  crc = crc8( crc, *p++);
            return crc;
}
// ---
unsigned char crc8(unsigned char crc; unsigned char data)
{
unsigned char i;
      crc = crc ^ data;
      for (i=0; i<8; i++){
            if (crc & 0x01)
                  crc = (crc >> 1) ^ 0x8C
            else
                  crc >>= 1;
      }
      return crc;
}

Any help would be appreciated by this newbie.

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8.6

Question Stats

Zone: Programming

Question Asked By: CONTRACTOR4SITE

Solution Provided By: PaulCaswell

Participating Experts: 3

Solution Grade: B

Translate:

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Comment by PaulCaswell

Thanks for the points.

I don't think you need:

CRC = CRC - 1

as it is odd at this point and you are just about to divide by 2 anyway.

Paul

null

Comment by CONTRACTOR4SITE

Hi Paul

Intersting point. I have tested it extensively and it works so I am reluctant to change it.
But I will bear your comment in mind should there be any further problems.
Thanks for the ongoing interest

Mike

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Comment by PaulCaswell

Hi Mike,

If yours works reliably then this should too:

Public Function checksum8(HexString As String) As String

Dim buff As Byte
Dim CRC As Byte
Dim x As Integer
Dim y As Integer

CRC = 255

For x = 1 To Len(HexString) Step 2

buff = hex2dec(Mid$(HexString, x, 2))

CRC = CRC Xor buff
For y = 1 To 8
If CRC And 1 Then
CRC = CRC / 2 ' Shift Right 1 bit
CRC = CRC Xor 140 ' 0x8C
Else
CRC = CRC / 2 ' Shift Right 1 bit
End If
Next y
Next x

checksum8 = CRC

End Function

Note that you also don't need buff as an array as you only process it one byte at a time. Sorry to bang on but I just don't feel comfortable with code that is just 'space'.

Paul

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Comment by CONTRACTOR4SITE

Now, that is an improvement. Can't argue with that. I will incorporate your version - interestingly I was concerned about the array, I didn't get it so I'm glad you've explained that it was redundant.
Mind you, if I have any problems you're now my Level 1 support! Hah!

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Comment by CONTRACTOR4SITE

Paul
Thought you'd want to know. Buff doesn't need to be an array - as you rightly pointed out.
But removing the CRC = CRC -1 stopped it from working!
Re-instating this line restored functionality.
Interesting?!
Mike

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Comment by PaulCaswell

>>But removing the CRC = CRC -1 stopped it from working!
Extraordinary!! I wonder why.

If CRC is odd then subtracting one and then dividing by two should have exactly the same effect as just dividing by 2 with integer types.

Perhaps VB recognises the bit loss and turns it into a float. Yukk!!! Glad I don't work in VB.

Paul
.

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Comment by PaulCaswell

Cracked it!!

See Wikipedia on VB:

http://en.wikipedia.org/wiki/Visual_basic#Language_features

quote: ... Integers are automatically promoted to reals in expressions involving the normal division operator (/) so that division of an odd integer by an even integer produces the intuitively correct result. There is a specific integer divide operator (\) which does truncate.

So this should work using the '\' integer divide operator:

Public Function checksum8(HexString As String) As String

Dim buff As Byte
Dim CRC As Byte
Dim x As Integer
Dim y As Integer

CRC = 255

For x = 1 To Len(HexString) Step 2

buff = hex2dec(Mid$(HexString, x, 2))

CRC = CRC Xor buff
For y = 1 To 8
If CRC And 1 Then
CRC = CRC \ 2 ' Shift Right 1 bit
CRC = CRC Xor 140 ' 0x8C
Else
CRC = CRC \ 2 ' Shift Right 1 bit
End If
Next y
Next x

checksum8 = CRC

End Function

Paul

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Comment by CONTRACTOR4SITE

I see you have the bit between your teeth.
Well, I'm off home (it's 10 PM here) but I promise to try this version when I'm back in the office on Thursday. You'll be the first to know! Thanks for your continuing interest.
M

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Comment by PaulCaswell

10:00 pm here too. ;-) Haven't you got a home to go to. !!! :)

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