vb.net Serial Port show only last character

Question

My problem is then I read first time BAR Code with BarCode Scanner ds457 I get full text for e.g. 02405529, but if i read code again i get only last character 9. If i read other code for e.g. 02405530 ill get this code last char 0. I need to close my program, to read full text again. DC Button from my program dosent help as Clear Listbox too. Any idea?

Public Class frmMain
    Dim myPort As Array 'COM Ports detected on the system will be stored here
    Delegate Sub SetTextCallback(ByVal [text] As String) 'Added to prevent threading errors during receiveing of data

    Private Sub frmMain_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Timer1.Enabled = True
        Timer1.Interval = 100
        'When our form loads, auto detect all serial ports in the system and populate the cmbPort Combo box.
        myPort = IO.Ports.SerialPort.GetPortNames() 'Get all com ports available
        cmbBaud.Items.Add(9600) 'Populate the cmbBaud Combo box to common baud rates used
        cmbBaud.Items.Add(19200)
        cmbBaud.Items.Add(38400)
        cmbBaud.Items.Add(57600)
        cmbBaud.Items.Add(115200)

        For i = 0 To UBound(myPort)
            cmbPort.Items.Add(myPort(i))
        Next
        cmbPort.Text = cmbPort.Items.Item(0) 'Set cmbPort text to the first COM port detected
        cmbBaud.Text = cmbBaud.Items.Item(0) 'Set cmbBaud text to the first Baud rate on the list

        btnDisconnect.Enabled = False 'Initially Disconnect Button is Disabled

    End Sub

    Private Sub btnConnect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnConnect.Click
        SerialPort1.PortName = cmbPort.Text 'Set SerialPort1 to the selected COM port at startup
        SerialPort1.BaudRate = cmbBaud.Text 'Set Baud rate to the selected value on

        'Other Serial Port Property
        SerialPort1.Parity = IO.Ports.Parity.None
        SerialPort1.StopBits = IO.Ports.StopBits.One
        SerialPort1.Handshake = IO.Ports.Handshake.XOnXOff
        SerialPort1.RtsEnable = True
        SerialPort1.DataBits = 8 'Open our serial port
        SerialPort1.Open()

        btnConnect.Enabled = False 'Disable Connect button
        btnDisconnect.Enabled = True 'and Enable Disconnect button

    End Sub

    Private Sub btnDisconnect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisconnect.Click
        SerialPort1.Close() 'Close our Serial Port

        btnConnect.Enabled = True
        btnDisconnect.Enabled = False
    End Sub

    Private Sub btnSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSend.Click
        SerialPort1.Write(txtTransmit.Text & vbCr)
        'The text contained in the txtText will be sent to the serial port as ascii
        'plus the carriage return (Enter Key) the carriage return can be ommitted if the other end does not need it
    End Sub

    Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived
        ReceivedText(SerialPort1.ReadExisting()) 'Automatically called every time a data is received at the serialPort
    End Sub
    Private Sub ReceivedText(ByVal [text] As String)
        'compares the ID of the creating Thread to the ID of the calling Thread
        If Me.rtbReceived.InvokeRequired Then
            Dim x As New SetTextCallback(AddressOf ReceivedText)
            Me.Invoke(x, New Object() {(text)})
        Else
            ' If [text] <> Me.rtbReceived.Text Then

            Me.rtbReceived.Text = [text]
            ' Me.rtbReceived.Text &= [text]
            'End If

        End If
    End Sub

    Private Sub cmbPort_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbPort.SelectedIndexChanged
        If SerialPort1.IsOpen = False Then
            SerialPort1.PortName = cmbPort.Text 'pop a message box to user if he is changing ports
        Else 'without disconnecting first.
            MsgBox("Valid only if port is Closed", vbCritical)
        End If
    End Sub

    Private Sub cmbBaud_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbBaud.SelectedIndexChanged
        If SerialPort1.IsOpen = False Then
            SerialPort1.BaudRate = cmbBaud.Text 'pop a message box to user if he is changing baud rate
        Else 'without disconnecting first.
            MsgBox("Valid only if port is Closed", vbCritical)
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisconnect.Click
        rtbReceived.Text = " "

    End Sub



    Private Sub txtTransmit_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtTransmit.TextChanged

    End Sub

    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        'SerialPort1.Write(txtTransmit.Text & vbCr)
    End Sub


    Private Sub Isvalyti_Click(sender As Object, e As EventArgs) Handles Isvalyti.Click
        rtbReceived.Clear()
    End Sub
End Class

Answer 1

Thank you Hans Passant and DMAN for your help. Both answers helped me a lot. Also i found a bug with scanner and my code. Scanner cant scan to quickly so i added threading.Thread.Sleep(100), now scanner take full part of code, not one piece.

Answer 2

  ReceivedText(SerialPort1.ReadExisting())

Serial ports are very slow, your ReadExisting() call normally only ever retrieves one or two characters. Your ReceivedText() method does not append the text so you only see those few characters.

You need to configure your barcode scanner to send a special character that indicates the end of the string. If it doesn't already do that, most do. Set the SerialPort.NewLine property to that special character, now you can use SerialPort.ReadLine() instead and be sure you get the entire scanner response.