熟悉VB和三菱PLC串行通讯的朋友请进！

最近用VB作了  一个上位机与FX2nPLC串行通讯的程序，不知什么原因总是不能返回需要的信息，请各位朋友指点一二，部分代码如下：

Private Sub Form_Load()
MSComm1.Settings = "9600,e,7,1"
MSComm1.InBufferSize = 1024
MSComm1.CommPort = 1
MSComm1.OutBufferSize = 1024
MSComm1.OutBufferCount = 0
MSComm1.PortOpen = True
MSComm1.InputLen = 0
End Sub

Private Sub send_Click()
Dim inf As String
inf = CmdRead(Text2.Text)
If inf <> "error" Then
  Text1.Text = Text1.Text + inf
Else
  MsgBox ("通讯错误")
End If
End Sub

Function CmdRead(Rd As String) As String
Dim x As Integer
Dim Y As Integer
Dim Z As Integer
Dim waittime, starttime As Long
Dim insr As Integer

MSComm1.InBufferCount = 0
read$ = Chr$(5) & Rd & CheckSum(Rd)
'读数据帧由ENQ、命令串和和校验码组成
MSComm1.Output = read$
waittime = 1
starttime = Timer
Do
   DoEvents
Loop Until Timer > starttime + waittime
Data$ = MSComm1.Input
If Data$ <> "" Then
insr = Asc(Data$)
CmdRead = Hex$(insr)
Else
CmdRead = "error"
End If

End Function
用ENQ00FFBRAX00000740和ENQ00FFTT204ABCD34等正确指令发送，只返回06H代码，请问有谁知道是怎么回事，PLC内D8120已作了相应设置，用网上下载的软件通讯没问题