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Discover or Find the Process Using or Reserving a Serial Port

Updated Jan 14, 2019

Reported In


  • NI-Serial

Issue Details

I have a program that is returning VI_ERROR_RSRC_BUSY or other resource is busy error when trying to access a Serial (COM) port through VISA. How do I figure out what process is using the COM port?


To find the process which is using the Serial (COM) Port, follow the steps below:
  1. Download the Process Explorer tool.
  2. Navigate to Control Panel » Device Manager. In Device Manager navigate to Ports (COM & LPT) and right click and select Properties on the target serial port.
  3. Select the Details tab and click the Property field and change the value to Service. Write down the value that is listed here. In the example below (a NI USB-232), the service value is NiSerU. Note the value of this field for searching in Process Explorer.
  4. Open Process Explorer and you will see a screen similar to the image below.
  5. Select Find » Find Handle or DLL... in the menu. A new window will pop up and in the field Handle or DLL substring type in the value that was obtained from Device Manager, in this case NiSerU. 
  6. Click Search and the results will populate to show which process is using a the particular handle. For example in the search below, the USB-232 shows as \Device\NiSerU1 being used by NIMax.exe. From here you can tell which process is using the serial port resource.


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