'在窗體的Load事件中加入下列代碼對串口進行初始化:
Private Sub Form_Load()
  MSComm1.CommPort = 1                  ' 設置通信端口號為COM1
  MSComm1.InputMode = 1                 ' 接收二進制型數據
  MSComm1.RThreshold = 1                ' 設置并返回要接收的字符數
  MSComm1.SThreshold = 1                ' 設置并返回發送緩沖區中允許的最小字符數
  MSComm1.Settings = "9600,E,7,1"       ' 設置串口1通信參數
  MSComm1.PortOpen = True               ' 打開通信端口1
  Timer1.Enabled = False
End Sub
'向PLC發送指令：02 30 31 30 30 30 32 30 03 35 36，功能是從D0開始讀取32個字節數據
Private Sub Cmdsend_Click()
  MSComm1.Output = Chr(&H2) & Chr(&H30) & Chr(&H31) & Chr(&H30) & Chr(&H30) & Chr(&H30) & Chr(&H32) & Chr(&H30) & Chr(&H3) & Chr(&H35) & Chr(&H38)
  Timer1.Enabled = True
End Sub
'獲得各輸入端口狀態
Private Sub Timer1_Timer()
  Dim Inbyte() As Byte
  Dim buffer As String
  ReDim Inbyte(100)
  Inbyte = MSComm1.Input                         '返回的數據串
  For i = LBound(Inbyte) To UBound(Inbyte)
     buffer = buffer + Hex(Inbyte(i)) + Chr(32)
  Next i
  'Text1.Text = buffer                            '顯示返回的數據串，需自己添加TextBox
  If Hex(Inbyte(2)) = "31" Then                  '31表示觸點閉合
    Shape1(0).FillColor = QBColor(12)
  Else                                           '30表示觸點斷開
    Shape1(0).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(6)) = "31" Then
    Shape1(1).FillColor = QBColor(12)
  Else
    Shape1(1).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(10)) = "31" Then
    Shape1(2).FillColor = QBColor(12)
  Else
    Shape1(2).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(14)) = "31" Then
    Shape1(3).FillColor = QBColor(12)
  Else
    Shape1(3).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(18)) = "31" Then
    Shape1(4).FillColor = QBColor(12)
  Else
    Shape1(4).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(22)) = "31" Then
    Shape1(5).FillColor = QBColor(12)
  Else
    Shape1(5).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(26)) = "31" Then
    Shape1(6).FillColor = QBColor(12)
  Else
    Shape1(6).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(30)) = "31" Then
    Shape1(7).FillColor = QBColor(12)
  Else
    Shape1(7).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(34)) = "31" Then
    Shape1(8).FillColor = QBColor(12)
  Else
    Shape1(8).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(38)) = "31" Then
    Shape1(9).FillColor = QBColor(12)
  Else
    Shape1(9).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(42)) = "31" Then
    Shape1(10).FillColor = QBColor(12)
  Else
    Shape1(10).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(46)) = "31" Then
    Shape1(11).FillColor = QBColor(12)
  Else
    Shape1(11).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(50)) = "31" Then
    Shape1(12).FillColor = QBColor(12)
  Else
    Shape1(12).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(54)) = "31" Then
    Shape1(13).FillColor = QBColor(12)
  Else
    Shape1(13).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(58)) = "31" Then
    Shape1(14).FillColor = QBColor(12)
  Else
    Shape1(14).FillColor = QBColor(10)
  End If
  If Hex(Inbyte(62)) = "31" Then
    Shape1(15).FillColor = QBColor(12)
  Else
    Shape1(15).FillColor = QBColor(10)
  End If
  Timer1.Enabled = False
End Sub
'當退出程序時，關閉串行口
Private Sub Cmdquit_Click()
  MSComm1.PortOpen = False           '關閉串口
  Unload Me
End Sub
// 讀不到PLC 輸入點，下標越界，望高手指點修改

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為什么不用專用協議做呢？格式一或格式四都可。

不要這樣寫程序 給你推薦個官方使用的控件即可 使用方便
MX Component

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ASCII 跟 HEX 要先搞清楚。