VB6与FX2N通讯无数据返回

小弟是新手，在FX2N中编入一个最简单的电机正反转的程序，在VB6中编制程序来读取Y0和Y1的状态，结果是向FX2N发出的读取指令发出了，但却没有数据返回
附VB程序
Public y0, y1 As Boolean
Public lim_left, lim_right As Integer
Public Function ReadPlc()
Dim recstr$, cmdstr$
Dim jym As Integer
cmdstr = Chr$(2) + "0000502" + Chr(3)
For n = 2 To 7
   jym = Asc(Mid(cmdstr, n, 1)) + jym
Next
MSComm1.Output = cmdstr + Right(Str(Hex(jym + 3)), 2) + Chr(13)
t1 = Timer
Do While (Timer - t1 < 0.1) '延时准备接收
DoEvents
Loop
recstr = MSComm1.Input
Text1.Text = recstr
If Asc(Left(recstr, 1)) = 6 Then
  If Mid(recstr, 3, 1) = "1" Then
       y0 = True
   Else
       y0 = False
   End If
   If Mid(recstr, 2, 1) = "1" Then
       y1 = True
   Else
       y1 = False
   End If
Else
   Label1.Caption = "读数据错误"
End If
End Function
Private Sub Form_Load()
lim_left = 830
lim_right = 5200
MSComm1.CommPort = 4
MSComm1.PortOpen = True
End Sub
Private Sub Form_Unload(Cancel As Integer)
MSComm1.PortOpen = False
End Sub

Private Sub Timer1_Timer()
ReadPlc
DoEvents
If y0 = True And y1 = False Then
   car.Left = car.Left + 15
   Label1.Caption = "zz"
ElseIf y0 = False And y1 = True Then
   car.Left = car.Left - 15
   Label1.Caption = "fz"
ElseIf y0 = False And y1 = False Then
   Label1.Caption = "stop"
Else
   Label1.Caption = "error"
End If
End Sub

我用的是SC09的USB-232转接头，请各位大侠给看看问题在什么地方，谢谢

里面的car是一个图片框

我重做了一个更简单的程序，发现不开PLC时没事，一开PLC就会无响应，程序挂了，这是什么原因啊，高手们帮下吧