Private Sub MSC_OnComm()
Select Case
       Case comEvReceive
        Do
         buf = MSC.Input
         T = T + 1
         If T > 10000 Then Exit Do
       Loop Until (Left(buf, 1) = "=" And Len(buf) = 9)                                    
                                ‘保证buf是一个以  “=“开始的 9位数据，用do…loop进行循环等待。

ReDim ary(Len(buf))                              
        For i = 1 To Len(buf)
        ary(i - 1) = Mid(buf, i, 1)
        Next                                       ‘将buf重新定义为数组型
     If ary(Len(buf) - 1) = "-" Then              ‘判断buf的最高位是否为“-“
        For i = 0 To Len(buf) - 1
        buf1 = buf1 + ary(i)
        Next
        txtReceive1.Text = buf1              ‘如果为“-”，将赋值给txtReceive1.Text
        buf1 = ""
     Else
        For i = 0 To Len(buf) - 1
        buf1 = buf1 + ary(i)
        Next
        txtReceive2.Text = buf1                   ‘否则将赋值给txtReceive2.Text
        buf1 = ""
    End If
     Case comEvSend
End Select
End Sub

为什么在运行时，有的时候正常，有的时候在If ary(Len(buf) - 1) = "-" Then  这一句会出现错误提示：下标越界，

真的不知哪儿错了，想请高手帮帮忙！