Handling data from a Serial Port event

Versions:

Visual Basic 2005 Express Edition

DotNet 2.0

Windows XP SP 2

Problem:

In writing an app that communicates with a machine via the serial port I've encountered a conceptual problem, I use an event handler to catch response but the is port slow so I wrote the function CheckInput() to wait a few seconds for the machine to respond with the right string, checking each loop for matching input in a buffer filled by the event reciever.

Unfortunately I don't seem to get any result from the event handler while inside the loop unless i use Application.DoEvents which seems to create exactly the same problem, it lets the rest of the form carry on processing and doesn't write to the buffer until I exit the loop at which point I've already returned false.

Hope that makes sense, below is my code edited for clarity and brevity.

Thanks,

Nick.

Code:

Imports System.Text
Imports System.IO.Ports
Imports System.Xml
Public Class Form1
  Dim WithEvents Port As New SerialPort
  Delegate Sub SetTextCallback(ByVal [text] As String)

  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    'Port Configuration
    Port.PortName = Me.cmbSerialPort.SelectedItem
    Port.Encoding = ASCIIEncoding.ASCII
    Port.BaudRate = 38400
    Port.DataBits = 8
    Port.Parity = Parity.None
    Port.StopBits = StopBits.One
    Port.RtsEnable = True

  End Sub

  'Waits tWait seconds for the machine to respond with checkVal
  'either in the whole buffer or the last line.
  Function CheckInput(ByVal checkVal As String, ByVal tWait As Double, _
  ByVal type As String)
    Dim start, finish As Double
    start = Microsoft.VisualBasic.Timer
    finish = start + tWait
    Do While (Microsoft.VisualBasic.Timer < finish)
      Application.DoEvents()
      Dim Lines() As String = Me.txtStatus.Lines
      If Lines(Lines.Length - 1).Contains(checkVal) Then
        Return True
      End If
    Loop
    Return False
  End Function


  'Sends Data to a serial Port
  Private Sub SendData(ByVal data As String)
    If Not Port.IsOpen Then
      Try
        Port.Open()
      Catch ex As Exception
        MessageBox.Show("Error Opening Port:" & Port.PortName)
        Exit Sub
      End Try
    End If
    Try
      Port.Write(data)
    Catch ex As Exception
      MessageBox.Show("Error writing to Port: " & Port.PortName, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try
  End Sub


  'Threadsafe call to txtStatus
  Private Sub DisplayText(ByVal [text] As String)
    If Me.txtStatus.InvokeRequired Then
      Dim d As New SetTextCallback(AddressOf DisplayText)
      Me.Invoke(d, New Object() {[text]})
    Else
      Me.txtStatus.AppendText([text])
    End If
  End Sub

  'Event handler for Serial Port
  Private Sub port_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles Port.DataReceived
    If e.EventType <> SerialData.Chars Then Exit Sub
    Dim inData As String = Port.ReadExisting
    DisplayText(inData)
  End Sub

  Private Sub Form1_FormClosing(ByVal sender As System.Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles MyBase.FormClosing
    If Port.IsOpen Then
      SendData("quit" & vbLf)
      Port.Close()
      Port.Dispose()
    Else
      Port.Dispose()
    End If
  End Sub

End Class