Question CRC-CCITT Calculation

[lukemeyers]

Hello everybody

I'm new to this forum so please be kind

I'm currently writing a small application for a serial device and need a CRC-CCITT (inverted) for the outgoing data.

The following code works fine:

    Public Function getCRC16(ByVal strInput As String)

        Dim lngCheck As Long
        Dim Power(7) As Integer
        Dim I As Integer
        Dim J As Integer
        Dim Poly As Long
        Dim CRC As Long
        Dim TestBit As Boolean
        Dim TestBit1 As Boolean
        Dim TestBit2 As Boolean

        Poly = &H1021
        CRC = &HFFFF

        For J = 0 To 7
            Power(J) = 2 ^ J
        Next J

        For I = 1 To Len(strInput)
            lngCheck = Asc(Mid$(strInput, I, 1))
            For J = 7 To 0 Step -1
                If (CRC And 32768) = 32768 Then
                    TestBit1 = True
                Else
                    TestBit1 = False
                End If

                If (lngCheck And Power(J)) = Power(J) Then
                    TestBit2 = True
                Else
                    TestBit2 = False
                End If

                TestBit = TestBit1 Xor TestBit2
                CRC = (CRC And 32767) * 2
                If TestBit = True Then
                    CRC = CRC Xor Poly
                End If

            Next J
        Next I

        Dim tmp As String
        tmp = Hex(CRC)
        getCRC16 = tmp

    End Function

Is it somehow possible to change the input from String to an Bytearray? I tried it several times, but the line

lngCheck = Asc(Mid$(strInput, I, 1))

is making problems.

And how would you invert all bytes after the checksum is built?

Thanks for your help.

Luke

 

[picoflop]

lngCheck = buf(i)

invert a byte: b = not b

 

[lukemeyers]

thx picoflop for your fast reply. unfortunately, your solution with lngcheck doesn't work for me.

when I debug the application, everything works fine, but the calculated CRC is now wrong. What could be the reason for this?

Thanks for your help.

 

[lukemeyers]

does anybone have an idea? i got really stuck with this little thing...

My code looks now like this:

Public Class Form1
    Public z As Integer = -1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Do
            Dim input As String
            Dim array As Object = Nothing
            Dim x As Integer = 0

            input = InputBox("Insert String")

            input = Replace(input, " ", "")
            input = Replace(input, "%", "")

            For i = 0 To input.Length - 1 Step 2
                ReDim array(x)
                array(x) = "&H" & input.Substring(i, 2)
                MsgBox(array(x))
                x = x + 1
                z = z + 1
            Next

            getCRC16(array)

        Loop

    End Sub

    Public Function getCRC16(ByVal input As Object)

        Dim lngCheck As Long
        Dim Power(7) As Integer
        Dim I As Integer
        Dim J As Integer
        Dim Poly As Long
        Dim CRC As Long
        Dim TestBit As Boolean
        Dim TestBit1 As Boolean
        Dim TestBit2 As Boolean

        Poly = &H1021
        CRC = &HFFFF

        For J = 0 To 7
            Power(J) = 2 ^ J
        Next J

        For I = 1 To z
            lngCheck = input(I - 1)
            For J = 7 To 0 Step -1
                If (CRC And 32768) = 32768 Then
                    TestBit1 = True
                Else
                    TestBit1 = False
                End If

                If (lngCheck And Power(J)) = Power(J) Then
                    TestBit2 = True
                Else
                    TestBit2 = False
                End If

                TestBit = TestBit1 Xor TestBit2
                CRC = (CRC And 32767) * 2
                If TestBit = True Then
                    CRC = CRC Xor Poly
                End If

            Next J
        Next I
        Dim tmp As String
        tmp = Hex(CRC)
        MsgBox("CRC: " & tmp)
        getCRC16 = tmp

    End Function

End Class

Unfortutanetly, the calculation is now wrong (the failure starts at the second place of the CRC) and I think the reason for this must be in one of my "for ... to" loops.

Luke