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Q108314: Using More Than Four Communications Ports in Windows

Article: Q108314
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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SUMMARY
=======

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.

MORE INFORMATION
================

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

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

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