serialport.ReadExisting

[husky_doggy]

hi,
i have the serial open for reading data from a RFID reader.
because the data comes in Hex ASCII, so I did a code snippet to decode them to characters. below code, I need to click a button in order to decode, but it can only decode once because the text in the textbox wont be HEX anymore after one decode (get error) and I dont want to clear the txtbox after the 1st decode.

2nd is that I cant do it without a button click event, I wanted to do automatically when the data exist. but I can not put anything else in the updateTextBox() function, otherwise i'll get an error...

any ideas how to decode without error?

Serial port code:
public class
Dim updatefromdevice As String

Private Sub DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles serialport.DataReceived

        txtReceivedMessage.BeginInvoke(New myDelegate(AddressOf updateTextBox), New Object() {})
        End Sub
    Public Delegate Sub myDelegate()
    Public Sub updateTextBox()

        txtReceivedMessage.Text = serialport.ReadExisting & vbCrLf & txtReceivedMessage.Text 'Display received data to textbox
    End Sub

decoder code:

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

        Dim hexnum As String
        Dim converttohex As String
        Dim hextonum As Integer
        Dim chara As Char

        updatefromdevice = txtReceivedMessage.Text
        hexnum = updatefromdevice.Trim()
        For i = 0 To Len(hexnum) - 1 Step 2

            converttohex = hexnum.Substring(i, 2)

            hextonum = Convert.ToInt32(converttohex, 16)
            chara = Chr(hextonum)
            txtReceivedMessage.Text += chara

        Next i
    End Sub