Private Sub frmMain_Load(ByVal sender As System.Object, _
                     ByVal e As System.EventArgs) Handles MyBase.Load
        OpenComPort()

        SendRequestCommand("L0" & ChrW(13))  
    End Sub
    Private Sub OnDataReceived(ByVal sender As Object, _
                    ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) _
                    Handles comPort.DataReceived
        Try
         txtDataReceived.Invoke(New myDelegate(AddressOf UpdateTextBox), New Object() {})
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub


    Public Delegate Sub myDelegate()

    Public Sub UpdateTextBox()
        Dim strInData As String = ""

        While comPort.BytesToRead > 0 
            strInData &= ChrW(comPort.ReadByte())
        End While

        txtDataReceived.Text = strInData
        txtDataReceived.Refresh()
        System.Threading.Thread.Sleep(100)
        SendRequestCommand("L0" & ChrW(13)) 
    End Sub
   
    Private Sub OpenComPort()
            CloseComPort()            
            Try
                
                With comPort
                    .PortName = "COM1"
                    .BaudRate = 9600
                    .Parity = IO.Ports.Parity.Odd
                    .DataBits = 7
                    .StopBits = IO.Ports.StopBits.Two
                    .ReadBufferSize = 64
                    .ReadTimeout = 500
                    .ReceivedBytesThreshold = 5 
                    .Handshake = IO.Ports.Handshake.None
                    .RtsEnable = True
                    .DtrEnable = True
                    .Encoding = System.Text.Encoding.ASCII
                End With

                comPort.Open()
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try
     End Sub
        
     Private Sub CloseComPort()
        If comPort.IsOpen() Then
            comPort.Close()
        End If
    End Sub
    
    Private Sub SendRequestCommand(ByVal cmd As String)
        If comPort.IsOpen() Then
            comPort.DiscardInBuffer()
            comPort.Write(cmd)
        Else
            MsgBox("El puerto COM1 está cerrado")
        End If
    End Sub