Serial port reading is very slow

[wolf99]

Hi Folks

I have a simple synchronous function that write n bytes to the serial port and reads the same number of bytes back. In the case of the code below n = 4.

Now I have an oscilloscope connected on the serial line so I can see the actual real timings of the data on the lines and that the date is there.

The external device responds very promptly, the write and response takes a grand total of 5 milliseconds. However the actual vb.net code (2010) does not complete the reads until after between 15 - 25 ms have passed!!

I.E. The data is sent out, taking 2.08ms, the response starts 1.04ms later and takes 2.08ms to complete.

Timing the code using Systems.Diagnostics.Stopwatch shows that the writebyte functions take effectively 0ms while the reads take a total of 20ms.... even though my scope shows the whole caboodle is finished in 5ms!


eg.

Tx: ______[]_______________________________
Rx:_________[]____________________________
          ^tx 4 bytes
            ^rx 4 bytes
              ^should finish read here
                                  ^actually finishes read here

Is this just due to how the serial port drivers are implemented in Windows or can I do something to improve this situation?

Imports System.IO.Ports
Imports System.Diagnostics

Private _port As New SerialPort()

Public Sub OpenPort
    _port .PortName("COM14")
    _port.Parity = 0
    _port.BaudRate = 9600
    _port.StopBits = StopBits.One
    _port.DataBits = 8
    _port.ReadTimeout = 500
    _port.WriteTimeout = 500
    _port.Handshake = Handshake.None
    _port.Open()
End Sub

Public Overridable Overloads Function SendCommand(ByRef Data() As Byte, ByVal Write As Boolean) As Boolean
    Dim Length = Data.Length
    Dim Offset As Integer = 0
    Dim sw1 As New Stopwatch()

    While Length >= 4 'Send and read data in blocks of two bytes
        Try
            If Write = True Then 'Set or clear the R#/W bit
                Data(Offset) = Data(Offset) Or &H80
            Else
                Data(Offset) = Data(Offset) And Not &H80
            End If

            _port.Write(Data, Offset, 2) 'Write the data to the port.

            sw1 = Stopwatch.StartNew()
            Data(Offset) = _port.ReadByte() 'Save data that is read back
            Data(Offset + 1) = _port.ReadByte()
            Data(Offset + 2) = _port.ReadByte()
            Data(Offset + 3) = _port.ReadByte()
            sw1.Stop()
            Console.WriteLine("Read time: " & sw1.ElapsedMilliseconds.ToString)
        Catch ex As Exception
            _port.DiscardInBuffer()
            _port.DiscardOutBuffer()
            Throw
            Return False
        End Try

        Length = Length - 4 'Update indices
        Offset = Offset + 4
    End While
    Return True
End Function