Picture of a US Robotics USB modem, connected and operating on laptop computer.

How to Change Assigned Windows COM Port Number on USB Modem




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One pesky problem with Windows 10, as well as the earlier versions of the Microsoft Windows operating system, is that the COM port assignments for devices that use them, often change.  A USB modem for example, may start off being assigned to COM3 when you plug it in right out of the box.  But that assignment can (and often does) change to COM4 or some other COM port, any time the modem is unplugged and reconnected, the computer restarts, a new version of a device driver is installed, a different USB port is used, certain Windows updates are installed, and a myriad of other possible port reassignment triggers.  Of course, when the modem COM port changes, any program that relies on the modem listening on the original COM port, no longer functions correctly.  The solution is to change the modem COM port back to the one it was originally using, and the procedure below demonstrates how to do this.

1. Plug the modem into a working USB port on your computer.  You won’t be able to change its COM port settings if it’s not connected to your machine when you try.

2. Bring up the -Device Manager- window.  You can find Device Manager in Control Panel, or enter the word -Device- into Cortana’s search box.  This brings up a link to -Device Manager- (usually the first one) in the results list.  Tap that link, and you’ll see a screen similar to the shot in the next picture.

Picture of the Windows 10 Device Manager screen, with the Modems entry highlighted.
Windows 10 Device Manager screen, with the Modems entry highlighted.

 

3. Find the Modems device group item, as pointed at by the purple arrow in the previous picture.  Then, tap / click the downward pointing arrow immediately to the left of that entry.  This expands the Modems device group, as shown in the next picture, revealing any modems currently connected to and operating correctly on your computer.  In this demo, we only have one USB modem connected, as listed.

Picture of the Windows 10 Device Manager screen, showing the Modems item expanded, with the desired USB modem highlighted.
Windows 10 Device Manager screen, showing the Modems item expanded, with the desired USB modem highlighted.

 

4. Double click the entry for the modem whose COM port assignment you wish to change.  In our demo here, we’re changing the COM port for the Agere Systems USB 2.0 Soft Modem.  So, we double click the entry for that modem, and this brought up the Windows 10 device configuration screen for this modem, as shown next.

Picture of the Agere Systems USB 2.0 Soft Modem Configuration screen on Windows 10, with the Advanced tab highlighted.
Agere Systems USB 2.0 Soft Modem Configuration screen on Windows 10, with the Advanced tab highlighted.

 

5. Then, click / tap the -Advanced- tab, as pointed at by the purple arrow in the previous picture.  The configuration window then redraws itself to show the advanced configuration options for this particular modem, as shown in the next picture.

Picture of the Agere Systems USB 2.0 Soft Modem configuration screen on Windows 10, showing the Advanced Settings tab, with the Advanced Port Settings button highlighted.
Agere Systems USB 2.0 Soft Modem configuration screen on Windows 10, showing the Advanced Settings tab, with the Advanced Port Settings button highlighted.

 

6. Tap / click the -Advanced Port Settings- button, pointed at by the purple arrow in the previous picture.  Doing so brings up the -Advanced Settings for COMx- window, as shown next.  The actual COM port number that is currently assigned to the modem replaces the x in COMx here.  In our demo here, this window is for COM4.  Thus, we see that our modem is currently assigned to COM4, although we wish to assign it to COM3.

Picture of the Advanced COM port settings screen on Windows 10, with the COM Port Number selector control highlighted.
Advanced COM port settings screen on Windows 10, with the COM Port Number selector control highlighted.
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7. Expand the  -COM Port Number- pop-up combo box  by tapping / clicking the downward pointing arrow immediately to the right of the port number.  We’ve drawn a purple arrow in the previous picture, pointing at this expander button in the previous picture.  When you expand the list, you get a complete list of available COM ports that this modem may use, as shown in the next picture.

Picture of the -Advanced COM Port Settings- screen on Windows 10, showing the -COM Port Number- control, expanded, with the current COM port (COM4) highlighted in blue, and the desired COM port (COM3) pointed at by the purple arrow.
-Advanced COM Port Settings- screen on Windows 10, showing the -COM Port Number- control, expanded, with the current COM port (COM4) highlighted in blue, and the desired COM port (COM3) pointed at by the purple arrow.

 

8. Tap / click the port number in the list that you wish to assign the modem to.  You may have to scroll up or down in the list to find the port you want.  In our demo, we’re assigning our modem to COM3.  So we tap COM3 in the list, as shown in the previous picture, pointed at by the purple arrow.  This then collapses the expanded port list back down to a single entry in the -COM Port Number- field, showing the new port number we chose.  We’ve changed our modem to COM3, as shown in the next picture.

Picture of the -Advanced COM Port Settings- screen, showing the -COM Port Number- control, changed to COM3, and the OK button highlighted.
-Advanced COM Port Settings- screen, showing the -COM Port Number- control, changed to COM3, and the OK button highlighted.

 

9. Tap / click the OK button on the -Advanced Settings for COMx- window.  Doing so closes that window, and reveals once again the device configuration window for our modem, as shown next.

Picture of the Agere Systems USB 2.0 Soft Modem configuration screen on Windows 10, with the OK button highlighted.
Agere Systems USB 2.0 Soft Modem configuration screen on Windows 10, with the OK button highlighted.

 

10. Press the OK button on the device configuration screen.  This applies the new COM port assignment to the modem, and closes that window, once again revealing the Windows 10 Device Manager window, as shown in the next picture.

Picture of the -Device Manager- screen on Windows 10, with the Exit option highlighted.
-Device Manager- screen on Windows 10, with the Exit option highlighted.

 

11. Exit the Device Manger window, either via its file->Exit menu item as shown in the previous picture, or by tapping / clicking its -Close- button, found in the top right corner of the window (the X button).  Your COM port reassignment for the modem is now complete.

With the COM port successfully changed, we recommend that you close and restart any program(s) that use the modem, and, if necessary, change the COM port settings in said program(s) to match the port number we just set up.

 

Revision History

  • 2017-12-04: Originally published.
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