Calculate Checksum VB 6

[fabianuat]

Could I have some help calculating a checksum for receive data from a Video server?

The data is in Hex and the values and here is an example:
20 24 49 4C 30 31 37 38 37 39 CS
CS=Checksum

the cheksum is the 2's complement of the sum of the values.

If anyone have an algoritm or knows how to do it, please help me, thanks

[Killer42]

Could I have some help calculating a checksum for receive data from a Video server?

The data is in Hex and the values and here is an example:
20 24 49 4C 30 31 37 38 37 39 CS
CS=Checksum

the cheksum is the 2's complement of the sum of the values.

If anyone have an algoritm or knows how to do it, please help me, thanks

The algorithm would be simple enough. Convert each three bytes (two if you inserted the spaces for readability) into a number, add them up, calculate the two's complement, and see whether it matches.

Dim sLine As String, I As Long
Dim lValue As Long, lSum As Long, lChecksum As Long
sLine = "20 24 49 4C 30 31 37 38 37 39 CS"
For I = 0 To 9
  lValue = Val( "&h" & Mid$(sLine, I * 4, 3) )
  lSum = lSum + lValue
Next
lChecksum = val( "&h" & Right$(sLine, 2) )
 

You now have the sum and the checksum. Do the two's complement thing (obviously I haven't read up on how, yet) and see whether they match.

[Killer42]

Once you have each byte value, you could call this routine to calculate the two's complement...

Public Function TwosComplement(ByVal src As Long) As Byte
  Dim Bit As Long, Result As Long, Power As Long, Value As Long
  For Power = 7 To 0 Step -1
    Value = 2 ^ Power
    Bit = src \ Value
    src = src - Bit * Value
    Bit = Abs(Bit - 1)
    Result = Result + Bit * Value
  Next
  TwosComplement = Result
End Function

However, once I saw the results I realised you can simplify this just a tiny bit...

TwosComplement = 255 - ByteValue

:D

You should now have all the necessary parts to do the job.

[fabianuat]

thanks killer i'll try with that...thanks for your time