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USB5 - How to reset the COM port assignments on Win XP & 2000?

*** If you read all the related articles you will learn more on how to confiq and check ports ***

Why does does windows assign so many numbers?

  • Each time a USB device is plugged into a USB port, Windows "rediscovers" the device.
  • If the device was plugged into a different USB port the device may be assigned 1 port number per USB plug (hence 3 ports for each device if 3 USB).
  • This can lead to having many different COM port assignments, and cause hardware compatibility issues when software was programmed only to allow you to select up to COM4 or COM8.

*** Bluetooth Devices also assign SERIAL PORTS (usually2) ***
*** It is recommended you UNPAIR ALL Bluetooth Devices from the Laptop ***
*** You can re-pair your devices later ***
*** This works also to a lower COM PORT for a Bluetooth Device ***

ALWAYS use the same USB plug to reduce the number of times a device is installed.

If you use a lot of USB (Serial Devices) you port list can be large.  After resetting the Assignment plug in the devices requiring low port numbers first then the devices that don't care even if the port is COM99 or anything high.

Further the devices that don't care about high port numbers you CAN change right away after installation if you are trying to RESERVE availability or low port numbers.  Read USB4 Info to find out how.


  • Plug in a device using a LOW port number.
  • In the device manager move the port to a high number to free up the low port number.
  • Plug in the device needin the LOW port number and change it to the free slot you just made.

After repeated installations of various USB-serial converters, the assigned com ports continue to increase with the operating system reserving a COM number for every USB device that was ever installed. Even though it is possible to reassign a com number in the Device Manager, some vestige of the old assignments remain and may be polled as part of COM interaction with the application and the operating system.

Clearing the PORT Allocation Table.
Performing these actions resulted in modifying the system registry.  As this does work it is possible to cause software damage to the computer requiring professional assistace if an unanticipated group of software or confiruration is present.


  • Uninstall any USB-serial converter drivers or other com port add-in PC cards or PCI cards in the Device Manager. Do this by right clicking on the device in the Ports branch of Device Manager. If you have any kind of multiport serial adapters, uninstall those as well. After they are uninstalled, unplug the USB-serial converter(s)
  • Click Start -> Run, type RegEdit and then click ok or hit enter.
  • Navigate to (Find) HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\COM Name Arbiter key

*** Backup step: Select COM Name Arbiter -> Select File | Export & Save***

  • Right-click on the ComDB entry on the right side of the window, and then click Modify
  • On the Edit Binary Data window, highlight all the values and delete them (set them to 0) by hitting the delete key on the keyboard. Do not delete the ComDB key, just clear the value data (set it to zero) and then click ok.
  • Click File -> Exit to exit the Registry Editor and restart the computer.
  • All of your native COM ports will be discovered automatically.
  • Any serial-USB converters will need to be reinstalled but they should start with the first COM number after your native COM ports.
  • Remember you can set them higher after they are detected if the program allows for high port use to reserve availability on low ports for later use.


USB4 - Setting or Checking your USB to Serial COM Port Number
USB1 - What is a Serial to USB Connection?
USB2 - Extracting Your Driver (Windows XP) for Downloaded Software
USB3 - Installing Your USB -> Serial Driver XP