I seem to have solved the issue by modifying my ClosePort sub to:

Public Sub ClosePort()
bAllowSndRcv = False
If IsNothing(comPort) Then Exit Sub
If comPort.IsOpen Then
comPort.DiscardOutBuffer()
RemoveHandler comPort.DataReceived, AddressOf comPort_DataReceived
Threading.Thread.Sleep(200)
comPort.Close()
End If
End Sub

Hi, Thanks for your answer. Ok, I will try the sleep. Interesting link you sent. What I am worried about is that once it's locked into the:

ERROR The I/O operation has been aborted because of either a thread exit or an application request.
Port Open Failed.

it seems to stay there even if I start and stop my application again (both in dev environment in debug config, and using a released exe). The only way to unlock it is by opening and closing the port with hyperterminal. I wonder how I might be able to do this programatically? Is there any ways to run a batch file to open and close a hyperterminal? It's not the best solution but will have to do for a quick-fix... Thanks, Charlie

Hi, I am getting a similar error. I am just trying to read from a serial COM port. It works fine. Then if I open and close the COM port a few times I get the following error when trying to open again:

The I/O operation has been aborted because of either a thread exit or an application request.

Starnagely enough the error is still tehre if I stop debugging (VisualStuio 2008, vb.net) and start again. The only way I can get it to work again is by opening and closing the port with hyperterminal! Here is my wrapper class:

Public Class clComPort
Private WithEvents comPort As System.IO.Ports.SerialPort
Private write As Boolean = True
Private bAllowSndRcv As Boolean = False
Private thRec As System.Threading.Thread

Public Event WroteData(ByVal msg As String)
Public Event DataRecieved(ByVal msg As String)
Public Event COMError(ByVal msg As String)

Public Function OpenPort(ByVal ComNum As Integer, ByVal baudRate As Integer, ByVal dataBits As Integer, _
ByVal parity As IO.Ports.Parity, ByVal stpBits As IO.Ports.StopBits) As Boolean
bAllowSndRcv = False
Try
'first check if the port is already open
'if its open then close it
If Not IsNothing(comPort) Then
Me.ClosePort()
End If
comPort = New System.IO.Ports.SerialPort()
'set the properties of our SerialPort Object
comPort.BaudRate = Integer.Parse(baudRate)
comPort.DataBits = Integer.Parse(dataBits)
comPort.StopBits = stpBits
comPort.Parity = parity
comPort.PortName = "COM" & ComNum
comPort.ReadTimeout = 500
comPort.WriteTimeout = 500

  'now open the port
  Dim b As Boolean = comPort.IsOpen
  comPort.Open()
  bAllowSndRcv = True
  'return true
  Return True
Catch ex As Exception
  bAllowSndRcv = False
  RaiseEvent COMError(ex.Message)
  Return False
End Try

End Function
Public Sub ClosePort()
bAllowSndRcv = False
If IsNothing(comPort) Then Exit Sub
If comPort.IsOpen Then
RemoveHandler comPort.DataReceived, AddressOf comPort_DataReceived
comPort.Close()
End If
comPort.Dispose()
End Sub
Private Sub comPort_DataReceived(ByVal sender As Object, _
ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles comPort.DataReceived
'This event will Receive the data from the selected COM port..
If Not bAllowSndRcv Then Exit Sub
If e.Eve