Envios y recibos de caracteres VB .net

Imports System.IO.Ports
Imports System.Text
Public Class Form1
Dim recibidos As String
Dim stx As String = ASCIIEncoding.ASCII.GetString(New Byte() {2})
Dim etx As String = ASCIIEncoding.ASCII.GetString(New Byte() {3})
Dim eot As String = ASCIIEncoding.ASCII.GetString(New Byte() {4})
Dim enq As String = ASCIIEncoding.ASCII.GetString(New Byte() {5})
Dim ack As String = ASCIIEncoding.ASCII.GetString(New Byte() {6})
Public Sub New()
InitializeComponent()
If Not SerialPort1.IsOpen Then
Try
SerialPort1.Open()
Catch ex As Exception
MessageBox.Show(ex.ToString)
End Try
End If
AddHandler SerialPort1.DataReceived, AddressOf recepcion
End Sub
Private Sub Form1_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
If SerialPort1.IsOpen Then
SerialPort1.Close()
End If
End Sub
Private Sub recepcion(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs)
recibidos = Chr(SerialPort1.ReadChar)
If recibidos = stx Or recibidos = enq Then
SerialPort1.Write(ack)
Else
If recibidos <> stx And recibidos <> etx And recibidos <> enq And recibidos <> ack And recibidos <> eot Then
Me.Invoke(New EventHandler(AddressOf actualizar))
Else
If recibidos = eot Then
Me.Invoke(New EventHandler(AddressOf actualizarenter))
Else
If recibidos = etx Then
SerialPort1.Write(ack)
End If
End If
End If
End If
End Sub
Private Sub actualizar(ByVal s As Object, ByVal e As EventArgs)
textbox_visualizar_mensaje.Text = textbox_visualizar_mensaje.Text & recibidos
End Sub
Private Sub actualizarenter(ByVal s As Object, ByVal e As EventArgs)
textbox_visualizar_mensaje.Text = textbox_visualizar_mensaje.Text & vbLf
End Sub
Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick
StatusStrip1.Items(0).Text = DateTime.Now.ToLongTimeString
End Sub
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
SerialPort1.Encoding = System.Text.Encoding.Default
End Sub
End Class

uf, estás utilizando malas prácticas de programación en todos los rincones del código xD.

Prueba así (no lo he testeado, ni tampoco se si he entendido correctamente las condiciones que dices que se deben dar al recibir los datos, pero desde luego puedes simplificar ese IF anidado de tú código en un Switch/Select Case):

Código

#Region " Imports "
 
Imports System.IO.Ports
 
#End Region
 
Public NotInheritable Class Form1 : Inherits Form
 
#Region " Objects / Constants-ReadOnly / Properties "
 
    ' Estos objetos los inicializo en el constructor del Form solo para poder compilar este ejemplo.
    Private WithEvents sp As SerialPort
    Private ReadOnly ss As StatusStrip
    Private ReadOnly tb As TextBox
    ' ***********************************************
 
    Private WithEvents tmr As New Timer
 
    Private ReadOnly charStx As Char = Convert.ToChar(&H1)
    Private ReadOnly charEtx As Char = Convert.ToChar(&H3)
    Private ReadOnly charEot As Char = Convert.ToChar(&H4)
    Private ReadOnly charEnq As Char = Convert.ToChar(&H5)
    Private ReadOnly charAck As Char = Convert.ToChar(&H6)
 
    ''' <summary>
    ''' Gets the next character from the device's input buffer.
    ''' </summary>
    ''' <value>The next character from the device's input buffer.</value>
    Private ReadOnly Property NextChar As Char
        Get
            Return Convert.ToChar(Me.sp.ReadChar)
        End Get
    End Property
 
#End Region
 
#Region " Constructors "
 
    Public Sub New(ByVal sp As SerialPort, ByVal ss As StatusStrip, ByVal tb As TextBox)
 
        MyClass.InitializeComponent()
 
        Me.sp = sp
        Me.ss = ss
        Me.tb = tb
 
    End Sub
 
    Public Sub New()
        MyClass.InitializeComponent()
    End Sub
 
#End Region
 
#Region " Event-Handlers "
 
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) _
    Handles Me.Load
 
        Me.OpenPort(Me.sp)
 
    End Sub
 
    Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As FormClosingEventArgs) _
    Handles Me.FormClosing
 
        Me.ClosePort(Me.sp)
 
    End Sub
 
    Private Sub Serialport1_DataReceived(ByVal sender As Object, ByVal e As SerialDataReceivedEventArgs) _
    Handles sp.DataReceived
 
        Me.ProcessChar(Me.NextChar)
 
    End Sub
 
    Private Sub Timer1_Tick(ByVal sender As Object, ByVal e As EventArgs) _
    Handles tmr.Tick
 
        ss.Items(0).Text = DateTime.Now.ToLongTimeString
 
    End Sub
 
#End Region
 
#Region " Private Methods "
 
    Private Sub OpenPort(ByVal sp As SerialPort)
 
        If Not sp.IsOpen Then
 
            Try
                sp.Open()
 
            Catch ex As Exception
                MessageBox.Show(ex.ToString &
                                Environment.NewLine &
                                ex.StackTrace, "error", MessageBoxButtons.OK, MessageBoxIcon.Error)
 
            End Try
 
        End If
 
    End Sub
 
    Private Sub ClosePort(ByVal sp As SerialPort)
 
        If sp.IsOpen Then
 
            Try
                sp.Close()
 
            Catch ex As Exception
                MessageBox.Show(ex.ToString &
                                Environment.NewLine &
                                ex.StackTrace, "error", MessageBoxButtons.OK, MessageBoxIcon.Error)
 
            Finally
                If sp IsNot Nothing Then
                    sp.Dispose()
                End If
 
            End Try
 
        End If
 
    End Sub
 
    Private Sub ProcessChar(ByVal chr As Char)
 
        Select Case chr
 
            Case Me.charStx, Me.charEnq, Me.charEtx
                sp.Write(Me.charAck)
 
            Case Me.charAck
                ' Do Nothing?.
 
            Case Me.charEot
                Me.ThreadSafeAppendText(Me.tb, chr & ControlChars.Lf)
 
            Case Else
                Me.ThreadSafeAppendText(Me.tb, chr)
 
        End Select
 
    End Sub
 
    Private Sub ThreadSafeAppendText(ByVal tb As TextBox, ByVal str As String)
 
        SyncLock tb
 
            If tb.InvokeRequired Then
                tb.Invoke(Sub() tb.AppendText(str))
 
            Else
                tb.AppendText(str)
 
            End If
 
        End SyncLock
 
    End Sub
 
#End Region
 
End Class

Nota:
En mi modificación no modifico la codificación de entrada del dispositivo, ya que puedes hacerlo de la siguiente manera, al declarar e inicializar el puerto de serie:

Código

Dim sp As New SerialPort With 
    {
      .Encoding = Encoding.Default
    }

Saludos