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Archived: Making unused COM Ports available in Windows

This document has been archived and is no longer updated by National Instruments

Primary Software: Driver Software>>NI-Serial
Primary Software Version: N/A
Primary Software Fixed Version: N/A
Secondary Software: N/A

I have a PCI-232/8 and want to change the COM ports assigned to the board. However, when I look in the Windows Device Manager, I see that many of my COM ports are listed as "in use". These ports are not being used. Can I use a port listed as "in use" when it really isn't?

If you have installed external serial devices several times in your machine, Windows will have assigned a new COM port each time. After the uninstallation, Windows fails to recognize that they are no longer in use. If you are using NI-Serial 1.6 and later, you are able to recover the use of the COM ports in Measurement & Automation Explorer. To do so, select Tools » NI-Serial » Recover Unused COM Numbers. For a general discussion of how Windows assigns COM ports, see Knowledgebase 4R69QEYW: How are the COM Ports for My USB Serial Device Assigned?

If you have an older version of NI-Serial, you will need to open Device Manager (Start»Settings»Control Panel»System»Hardware), select a port, and navigate to Action»Properties»Port Settings»Advanced»COM Port Number.  The previously reserved COM ports are identified as "in use." If you attempt to change your current port to a port listed as "in use," an error message says another device already is using the port. If you are certain another device or modem is not using the port, click Yes to that message, and the COM port should be changed without any problems.

Related Links:
Drivers and Updates: NI-Serial Version 1.6 for Windows 2000/XP--(Plug-in Boards)


Report Date: 06/28/2002
Last Updated: 05/17/2017
Document ID: 2MRAP77N

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