CRC16 generation

[consciouspnm]

I've wanting to generate a CRC16-X25 checksum in the form HiByte and LowByte.

All the examples I've found by searching are either VB6 which uses functions and data types that are no longer supported or C, C++ etc.

Does anyone have a simple VB.Net version? Speed is not an issue.

[cicatrix]

Do you have a C or C++ code?

[Niya]

Post the VB6 version. Ill help you convert it if I have time.

[consciouspnm]

This is a C example with a VB conversion, but the solution is using non vb functions:

http://forums.codeguru.com/showthread.php?t=475191

[Niya]

This is what they have there:-

'
Private Function CRC16(ByVal crc As Integer, d As Byte) As Integer
  Dim carry As Integer, i As Byte
  For i = 0 To 7
    carry = (crc And 1) Xor IIf(d And (2 ^ i), 1, 0)
    crc = (crc And &HFFFF&) \ 2
    If carry <> 0 Then crc = crc Xor &H8408
  Next i
  CRC16 = crc
End Function
 
Public Function CalcCrc16(ByVal buf As String) As String
  Dim crc As Integer, t As Integer
  crc = &H8408
  For t = 1 To Len(buf)
    crc = CRC16(crc, Asc(Mid$(buf, t, 1)))
  Next t
  CalcCrc16 = Chr$(HiByte(crc)) & Chr$(LoByte(crc))
End Function

What functions do you see that dont belong to VB ? Are you referring to the HiByte/LoByte functions ? Even so this is rather trivial to convert to VB.Net do you need help to convert it or do you think you can manage ?

[cicatrix]

Here's a good C# class

http://www.codeproject.com/script/Ar....aspx?aid=5597

[Niya]

Here, I made two functions so you can extract the high and low bytes should you still want to convert that VB6 one:-

'
    Private Declare Function RtlMoveMemory Lib "Kernel32" (ByRef dest As Byte, ByRef src As Short, ByVal cb As Integer) As Integer
 
    Private Function HiByte(ByVal num As Short) As Byte
 
        Dim arNum(1) As Byte
 
        RtlMoveMemory(arNum(0), num, 2)
 
        Return arNum(1)
 
    End Function
 
 
    Private Function LoByte(ByVal num As Short) As Byte
 
        Dim arNum(1) As Byte
 
        RtlMoveMemory(arNum(0), num, 2)
 
        Return arNum(0)
 
    End Function

Also if you are going to convert from VB6, remember to change all those Integers to Shorts. You could also opt to use an online converter to convert the C# code in cicatrix's link. Whatever you choose, good luck. I'm off to bed....wayyyy too sleepy atm.

[consciouspnm]

Did you actually try the VB code in Visual Studio? if you try and calculate the checksum for "123456789" it should be 29B1 but it isn't!

[cicatrix]

To get your result you should initialize the CRC16CCITT class with &HFFFF (NonZero1)

'
    ' CRC16CCIT.ComputeCheckSum will return ushort with CRC16
    ' CRC16CCIT.ComputeCheckSumBytes will return 2 bytes (hi and low)
    
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
        Dim checksumcalculator = New CRC16CCITT(CRC16CCITT.InitialCRCValue.NonZero1)
        Dim value() As Byte = System.Text.Encoding.ASCII.GetBytes("123456789")
        MsgBox(Format(checksumcalculator.ComputeCheckSum(value), "X2")) ' 29B1
    End Sub
 
    Public Class CRC16CCITT
        Public Enum InitialCRCValue
            Zeroes = 0
            NonZero1 = &HFFFF
            NonZero2 = &H1D0F
        End Enum
 
        Private Const poly As UShort = &H1021
        Dim table(255) As UShort
        Dim intValue As UShort = 0
 
        Public Function ComputeCheckSum(ByVal bytes As Byte()) As UShort
            Dim crc As UShort = Me.intValue
            For i As Integer = 0 To bytes.Length - 1
                crc = CUShort(((crc << 8) Xor table(((crc >> 8) Xor (&HFF And bytes(i))))))
            Next
            Return crc
        End Function
 
        Public Function ComputeCheckSumBytes(ByVal bytes As Byte()) As Byte()
            Dim crc As UShort = ComputeCheckSum(bytes)
            Return BitConverter.GetBytes(crc)
        End Function
 
        Public Sub New(ByVal initialvalue As InitialCRCValue)
            Me.intValue = CUShort(initialvalue)
            Dim temp, a As UShort
            For i As Integer = 0 To table.Length - 1
                temp = 0
                a = CUShort(i << 8)
                For j As Integer = 0 To 7
                    If ((temp Xor a) And &H8000) <> 0 Then
                        temp = CUShort((temp << 1) Xor poly)
                    Else
                        temp <<= 1
                    End If
                    a <<= 1
                Next
                table(i) = temp
            Next
        End Sub
    End Class

[consciouspnm]

Thanks, that worked fine, though copying and pasting the code out of your answer seemed to lose all the CR!

[calvinpower]

i have a calculator that calculate crc-16/-x25 "123456789" and i get '9B2E' then how do i do it in vb.net?