บันทึกเวลาเข้า-ออกด้วย vb.net2010 ติดปัญหาตรงเวลาออกครับ

 Private Sub txtSN_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtSN.TextChanged
        If txtSN.Text <> "" Then
            Dim intNumRows As Integer
            Dim strSQL As String
            sqlcon = New SqlConnection("Server=TOSHIBA-PC\SQLEXPRESS;Database=parking;Trusted_Connection=True;")
            sqlcon.Open()
            strSQL = "SELECT COUNT(*) FROM member WHERE CardSN = '" & txtSN.Text & "'"
            sqlcmd = New SqlCommand(strSQL, sqlcon)
            intNumRows = sqlcmd.ExecuteScalar()
            If intNumRows > 0 Then
                Dim dt As String
                dt = DateTime.Now.ToString("dd-MM-yyyy HH:mm:ss")
                Dim dtin As String
                sqlcon = New SqlConnection("Server=TOSHIBA-PC\SQLEXPRESS;Database=parking;Trusted_Connection=True;")
                sqlcon.Open()
                dtin = "SELECT dtin FROM inout WHERE CardSN='" & txtSN.Text & "'AND dtout IS NULL"
                Dim sqlcmd As New SqlCommand(dtin, sqlcon)
                Dim Rs As SqlDataReader = sqlcmd.ExecuteReader
                If Rs.Read Then
                    txtIN.Text = Rs("dtin")
                End If
                sqlcon.Close()
                txtOUT.Text = dt
                Dim sdtin As Date = txtIN.Text
                Dim sdtout As Date = txtOUT.Text
                Dim timeDif As Long = DateDiff(DateInterval.Minute, sdtin, sdtout)
                Dim timeSpan As TimeSpan = sdtout.Subtract(sdtin)
                Dim difDays As Integer = timeSpan.Days
                Dim difHr As Integer = timeSpan.Hours
                Dim difMin As Integer = timeSpan.Minutes
                txtTime.Text = timeDif
                Select Case timeDif
                    Case 0 To 120
                        txtCash.Text = "20"
                        SerialPort1.Write("a")
                    Case 121 To 240
                        txtCash.Text = "40"
                        SerialPort1.Write("s")
                    Case Is > 241
                        txtCash.Text = "60"
                        SerialPort1.Write("d")
                End Select
                ds = New DataSet
                da = New SqlDataAdapter("ALTER TABLE inout ADD CONSTRAINT chk_Time CHECK (CardSN='" & txtSN.Text & "' AND dtout='')", sqlcon)
                da = New SqlDataAdapter("UPDATE inout SET dtout='" & dt & "', total='" & txtTime.Text & "',cash='" & txtCash.Text & "' WHERE CardSN='" & txtSN.Text & "'AND dtout IS NULL", sqlcon)
                da.Fill(ds, "inout")
                Call showdata2()
            Else
                SerialPort1.Write("g")
            End If
        End If
        Call refreshdata()
        System.Threading.Thread.Sleep(1500)
        Call cleardata()
    End Sub