Problems sending hex code via Serial port to a Customer Display (CD7220) in VB.Net

If you use Encoding.ASCII.GetBytes on a "£", you get &H3F ("?" - it'll use that for anything above &H7F). So, you can get the bytes of the text to be displayed and change all occurrences of &H3F to &HBC before sending the data as long as you don't need to use "?" as "?". You can use the SerialPort.Write(Byte[], Int32, Int32) method to avoid having the data regarded as characters in the byte range 0-127.

I suggest using constants for things like the display size - it will make it much easier to change to, say, a four-line display.

Something like this:

Sub SendToDisplay(displayText As String)
    Const DISPLAYWIDTH As Integer = 20
    Const DISPLAYHEIGHT As Integer = 2
    Dim CRLF As Byte() = {13, 10}
    Dim blankText = New String(" ", DISPLAYWIDTH)
    Dim blankBytes = Encoding.ASCII.GetBytes(blankText)
    If displayText.Length < DISPLAYHEIGHT  DISPLAYWIDTH Then
        displayText = displayText.PadRight(DISPLAYHEIGHT  DISPLAYWIDTH)
    End If
    Dim displayBytes = Encoding.ASCII.GetBytes(displayText)
    ' Replace pound sign with &HBC for what the display uses.
    For i = 0 To displayBytes.Length - 1
        If displayBytes(i) = &H3F Then
            displayBytes(i) = &HBC
        End If
    Next
    Using ser As New IO.Ports.SerialPort("COM3", 9600, IO.Ports.Parity.None, 8, IO.Ports.StopBits.One)
        ser.Open()
        'TODO: Check if display has a code to clear it.
        For i = 1 To DISPLAYHEIGHT
            ser.Write(blankBytes, 0, 20)
            ser.Write(CRLF, 0, 2)
        Next
        For i = 0 To DISPLAYHEIGHT - 1
            ser.Write(displayBytes, i * DISPLAYWIDTH, DISPLAYWIDTH)
            ser.Write(CRLF, 0, 2)
        Next
        ser.Close()
    End Using
End Sub