vb.net serialPort DataReceived事件控制有关问题

vb.net serialPort DataReceived事件控制问题
vb.net中如何控制serialPort DataReceived触发事件。
因为存在发送指令到串口，然后接收并对接收的数据进行判断，根据接收判断的情况再发送不同的指令。
发送的指令不同接收到的数据不同。
因为发送的指令不同接收需要的时间不同，为了保证接收数据完整，我想通过不同指令来控制触发事件。
例如如下代码中，有六种情况，第一种情况需要在接收到11个字符时触发接收时间，第二个在接收到46个字符时触发接收时间…………
Form1.SerialPort1.ReceivedBytesThreshold

VB.NET code

    Public Sub fasong(ByVal send As String)        Try            SerialPort1.Write(send & vbCrLf)            Select Case send                Case "LF%R1Q,2026:"                   '11个字符时触发接收事件                Case "LF%R1Q,2107:2"                    '46个字符时触发接收事件                Case Left(send, 15) = "LF%R1Q,9027,22:"                    '…………                Case Left(send, 16) = "LF%R1Q,9037,23:1"                    '…………                Case Left(send, 16) = "LF%R1Q,2008,27:1"                    '…………                Case Left(send, 16) = "LF%R1Q,2108,27:1"                    '…………            End Select        Catch ex As Exception            MessageBox.Show(ex.Message)        End Try    End Sub   Public Sub Sp_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived        Me.Invoke(New EventHandler(AddressOf Sp_Receiving)) '调用接收数据函数       End Sub    Private Sub Sp_Receiving(ByVal sender As Object, ByVal e As EventArgs)        Dim strIncoming As String        Try            If SerialPort1.BytesToRead > 0 Then                Threading.Thread.Sleep(100) '添加的延时                                strIncoming = SerialPort1.ReadExisting.ToString '读取缓冲区中的数据                                SerialPort1.DiscardInBuffer()                readdata = strIncoming            End If        Catch ex As Exception            MessageBox.Show(ex.Message)        End Try    End Sub