就是个重启一下电脑的问题，没必要搞这么复杂。

回以上各楼层：
修改串口号我明白，有问题的电脑的所有COM16之前的串口都显示在使用，所以也改不了低于16的，通过注册表可以把多出来的串口号删除，但不期望使用上位机软件的人会正确操作。
所以，一劳永逸的方法是更新上位机软件，跳过这个坑。。
不用控件用API倒是可以试一下。或许可以避开这个BUG

可以直接调用Windows API（CreateFile()...），本来控件只是API的一层封装而已，直接使用API也不会多复杂，不过既然你有这个提问，说明你不想用API。
还有一个简单的方法，修改串口控件OCX，将控件OCX判断串口号是否小于等于16的语句修改为小于等于255即可，这样就可以用到串口号1-255：


简明教程：
1. 找到mscomm32.ocx，如果是64位Windows7，则目录为：C:\Windows\SysWOW64\mscomm32.ocx
2. 使用HEX文本编辑器（比如UltraEdit之类的）打开
3. 查找HEX字符串“66 3D 10 00”
4. 将其修改为“66 3D FF 00”

如果你的程序需要在其他机器上运行，则需要将这个文件拷贝过去，或制作一个安装程序，里面包含这个文件。

通常这样写  \\.\COM23  。 不然，去设备管理器中修改到较小的串口号

你可以用 VB6 Serial API class

1. 
   
2. Option Explicit
   
3. '// WIN32API Function
   
4. '//Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As Long, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
   
5. 
   
6. Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As Any, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
   
7. 
   
8. Private Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, lpOverlapped As Any) As Long
   
9. Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
   
10. Private Declare Function SetCommState Lib "kernel32" (ByVal hCommDev As Long, lpDCB As DCB) As Long
    
11. Private Declare Function GetCommState Lib "kernel32" (ByVal nCid As Long, lpDCB As DCB) As Long
    
12. 
    
13. '// WIN32API Structure
    
14. Private Type DCB
    
15.         DCBlength As Long
    
16.         BaudRate As Long
    
17.         fBitFields As Long 'See Comments in Win32API.Txt
    
18.         wReserved As Integer
    
19.         XonLim As Integer
    
20.         XoffLim As Integer
    
21.         ByteSize As Byte
    
22.         Parity As Byte
    
23.         StopBits As Byte
    
24.         XonChar As Byte
    
25.         XoffChar As Byte
    
26.         ErrorChar As Byte
    
27.         EofChar As Byte
    
28.         EvtChar As Byte
    
29.         wReserved1 As Integer 'Reserved; Do Not Use
    
30. End Type
    
31. 
    
32. '// WIN32API Constant
    
33. Private Const GENERIC_READ = &H80000000
    
34. Private Const GENERIC_WRITE = &H40000000
    
35. Private Const OPEN_EXISTING = 3
    
36. Private Const FILE_FLAG_OVERLAPPED = &H40000000
    
37. Private Const INVALID_HANDLE_VALUE = -1
    
38. Private Const NOPARITY = 0
    
39. Private Const ONESTOPBIT = 0
    
40. 
    
41. Private Const FILE_FLAG_NO_BUFFERING = &H20000000
    
42. 
    
43. '// Comm Port Handle
    
44. Private hComm As Long
    
45. 
    
46. Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer)
    
47.     '// Close the opened Comm
    
48.     If hComm <> 0 Then CloseHandle (hComm)
    
49. End Sub
    
50. 
    
51. Private Sub CmdAction_Click(Index As Integer)
    
52.     Dim Idx As Integer
    
53.     Select Case Index
    
54.     Case 0 '// Open/Close
    
55.         If CmdAction(0).Caption = "&Open" Then
    
56.             If OpenPort(txtCOMM(0).Text, CLng(txtCOMM(1).Text), CLng(txtCOMM(2).Text)) <> 0 Then
    
57.                 CmdAction(0).Caption = "&Cancel"
    
58.                 For Idx = 0 To 2: txtCOMM(Idx).Enabled = False: Next
    
59.                 txtData.Enabled = True
    
60.                 CmdAction(1).Enabled = True
    
61.                 lblStatus.Caption = "Open Port Successful: Hanlde -> " & hComm
    
62.             Else
    
63.                 lblStatus.Caption = "Fail to open port!!!"
    
64.             End If
    
65.         Else
    
66.             CloseHandle (hComm)
    
67.             CmdAction(0).Caption = "&Open"
    
68.             For Idx = 0 To 2: txtCOMM(Idx).Enabled = True: Next
    
69.             txtData.Enabled = False
    
70.             CmdAction(1).Enabled = False
    
71.             lblStatus.Caption = "Port Closed"
    
72.         End If
    
73.     Case 1 '// Send
    
74.         Write2Port txtData.Text
    
75.     End Select
    
76. 
    
77. End Sub
    
78. 
    
79. Private Function OpenPort(ByVal strPort As String, ByVal lngBaudRate As String, ByVal lngDataBit As Long) As Long
    
80.     Dim pDCB As DCB
    
81.     Dim lpPort As String
    
82.    
    
83.     '// Create Comm Name Buffer
    
84.     '//lpPort = String(6, Chr(0))
    
85.     '//Mid$(lpPort, 1, 6) = "COM" & strPort & ":"
    
86.        
    
87.     lpPort = "\\.\COM" + strPort + vbNullChar
    
88.        
    
89.     '// Close the current opened Comm Port (If any)
    
90.     If hComm > 0 Then CloseHandle (hComm)
    
91.    
    
92.     '// Open selected comm port
    
93.     '//hComm = CreateFile(lpPort, _
    
94.     '//                    GENERIC_READ Or GENERIC_WRITE, _
    
95.     '//                    0, _
    
96.     '//                    vbNull, _
    
97.     '//                    OPEN_EXISTING, _
    
98.     '//                    0, _
    
99.     '//                    vbNull)
    
100. 
     
101.         hComm = CreateFile(lpPort, _
     
102.                                                 GENERIC_READ Or GENERIC_WRITE, _
     
103.                                                 0, _
     
104.                                                 ByVal 0, _
     
105.                                                 OPEN_EXISTING, _
     
106.                                                 FILE_FLAG_NO_BUFFERING, _
     
107.                                                 0)
     
108. 
     
109.    
     
110.     If hComm <> INVALID_HANDLE_VALUE Then
     
111.         pDCB.DCBlength = Len(pDCB)
     
112.         
     
113.         '// Retrieve default Comm port settings
     
114.         GetCommState hComm, pDCB
     
115.       
     
116.         '// Configure new Comm port settings
     
117.         With pDCB
     
118.             .BaudRate = lngBaudRate
     
119.             .Parity = NOPARITY
     
120.             .ByteSize = lngDataBit
     
121.             .StopBits = ONESTOPBIT
     
122.             .EofChar = 0
     
123.             .ErrorChar = 0
     
124.             .EvtChar = 0
     
125.             .fBitFields = 20625
     
126.             .XoffChar = 19
     
127.             .XoffLim = 512
     
128.             .XonChar = 17
     
129.             .XonLim = 2048
     
130.         End With
     
131.         
     
132.         '// Set new configure Comm port settings
     
133.         If SetCommState(hComm, pDCB) = 0 Then
     
134.             CloseHandle (hComm)
     
135.             OpenPort = 0
     
136. 
     
137.             MsgBox "Fail to configure serial port!", vbExclamation + vbOKOnly, "Error"
     
138.         Else
     
139.             OpenPort = hComm
     
140.         End If
     
141.     Else
     
142.         CloseHandle (hComm)
     
143.         OpenPort = 0
     
144.     End If
     
145. End Function
     
146. 
     
147. Private Sub Write2Port(ByVal strData As String)
     
148.     Dim dwByteWrite As Long
     
149.     Dim Sz As Long, Idx As Long
     
150.     Dim Bytes() As Byte
     
151.    
     
152.     '// Create & Convert str into array of Byte
     
153.     Sz = Len(strData)
     
154.     ReDim Bytes(Sz) As Byte
     
155.     For Idx = 1 To Sz
     
156.         Bytes(Idx) = Asc(Mid$(strData, Idx, 1))
     
157.     Next
     
158.    
     
159.     '// Write data into Open Comm Port
     
160.     If hComm <> INVALID_HANDLE_VALUE Then
     
161.         WriteFile hComm, _
     
162.                     Bytes(1), _
     
163.                     UBound(Bytes), _
     
164.                     dwByteWrite, _
     
165.                     ByVal 0&
     
166.     Else
     
167.         MsgBox "Invalid port handle", vbExclamation + vbOKOnly, "Error"
     
168.     End If
     
169.    
     
170.     Erase Bytes
     
171.    
     
172. End Sub
     
173. 
     

复制代码

这个应该经常初始化，否则再多串口号也不够用。

系统的串口号升到了COM17，并不是有17串口在使用吧，
你可以进到设备管理器里面，把你现在用的串口号改小

开启命令提示视窗(win键+R>CMD)
执行指令：set devmgr_show_nonpresent_devices=1
同视窗继续执行指令：devmgmt.msc
你应该会看到装置管理员
选择 "检视"->"显示隐藏装置"
把右方 "+" 点开看看，你应该可以看到平时看不见的设备
呈现淡灰色的就是离线设备，原则上可以删掉

端口号在系统里可以改