example for VB.NET to calculate CRC16 of an string or Byte Array - vb.net

Question

With reference to this link
Calculate CRC32 of an String or Byte Array
I modified the code in order to calculate the CRC16 instead of CRC32, however I am getting wrong result, can some one point me where is the mistake?
Private Sub Main()
Crc16.ComputeChecksum(Encoding.UTF8.GetBytes("Some string"))
End Sub
Public Class CRC16
Shared table As UShort()
Shared Sub New()
Dim poly As UShort = &HA001US 'calculates CRC-16 using A001 polynomial (modbus)
table = New UShort(255) {}
Dim temp As UShort = 0
For i As UShort = 0 To table.Length - 1
temp = i
For j As Integer = 8 To 1 Step -1
If (temp And 1) = 1 Then
temp = CUShort((temp >> 1) Xor poly)
Else
temp >>= 1
End If
Next
table(i) = temp
Next
End Sub
Public Shared Function ComputeChecksum(ByVal bytes As Byte()) As UShort
Dim crc As UShort = &H0US ' The calculation start with 0x00
For i As Integer = 0 To bytes.Length - 1
Dim index As Byte = CByte(((crc) And &HFF) Xor bytes(i))
crc = CUShort((crc >> 8) Xor table(index))
Next
Return Not crc
End Function
End Class

Answer 1

Try this, it's working VB6 code for Instrument control. (sCommand is a temp string which contains all Bytes, Result is added to sCommand, Modbus is using LSB first, TextToString and StringToAscii are functions to convert a readable string "FF EE" into ASCII and back, thus they are not of interest here.):
Private Sub cmdCRC16_Click()
Dim sCommand As String
Dim x As Long
Dim y As Long
Dim lCRC As Long
sCommand = TextToString(txtASCII)
'Initial value
lCRC = 65535 '(&HFFFF results in Integer -1)
For x = 1 To Len(sCommand)
lCRC = lCRC Xor Asc(Mid(sCommand, x, 1))
For y = 1 To 8
If (lCRC Mod 2) > 0 Then
lCRC = (lCRC And 65534) / 2
lCRC = lCRC Xor 40961 '(&HA001 results in whatever negative integer)
Else
lCRC = (lCRC And 65534) / 2
End If
Next y
Next x
'Add CRC with LSB first
sCommand = sCommand + Chr(lCRC And 255)
sCommand = sCommand + Chr((lCRC And 65280) / 256)
txtASCII = StringToASCII(sCommand)
End Sub

Answer 2

I just came accross the same issue. Simple solution is to omit negation at the end, so just change your "Return Not crc" to "Return crc" and you be fine.
There are various variants of CRC-16, where "CRC-16" normally refers to the IBM variant, also called "ARC". It uses an XorOut value of zero. See Catalogue of parametrised CRC algorithms with 16 bits.