Microsoft KB Archive/108314

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Using More Than Four Communications Ports in Windows


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) 3.1


In theory, it is possible to use more than four communications (COM) ports in a Windows-based application. In practice, however, due to the limitation imposed by the Windows 3.1 communications (comm) driver, only a maximum of four communications ports can be used.


The virtual communications device (VCD) under Windows can support up to nine communications ports. And, the Windows communications functions also support the use of nine communications ports in a Windows-based application. All this information might indicate that a maximum of nine COM ports can be used in a Windows-based application.

But the communications driver (COMM.DRV) that is shipped with Windows version 3.1 supports a maximum of only four communications ports. For example, you can use the following code in a Windows-based application:

   // Get the number of COM ports.
   nPorts = EscapeCommFunction(0,GETMAXCOM)+1; 

Note, nPorts is not an indication of the number of communications ports that the computer has; it indicates the maximum number of communications ports the communications driver supports. For the Windows version 3.1 standard communications driver, nPorts is always four.

This situation might change in the future. For example, the communications driver that ships with Windows for Workgroups version 3.11 does support more than four communications ports. Also, some of the third-party communications drivers, such as TurboCom by Pacific CommWare, do support up to nine communications ports. For more information, contact:

Pacific CommWare

180 Beacon Hill Lane
Ashland, OR 97520
Voice: (503) 482-2744
Fax: (503) 482-2627

BBS: (503) 482-2633

Note that Microsoft does not recommend any third-party applications for a given purpose. TurboCom is an example only.

Also, because the Windows Device Driver Kit (DDK) version 3.1 provides the source code to all the drivers including the Windows standard communications driver, one could modify the source code to provide support for more than four communications ports under Windows. However, note that modifying the communications driver source code is not a trivial operation. For more information on how to modify the Windows communications driver source code to provide support for nine communications ports, query the Knowledge Base on the words:

modifying comm nine

Additional query words: 3.10

Keywords : kb16bitonly
Issue type :
Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310

Last Reviewed: November 6, 1999
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