How to Determine Which Process Is Reserving My Serial Port

Updated Dec 11, 2023


Operating System

  • Windows

In this article, it will be explained how to find which process is reserving a Serial port (COM). This procedure can be helpful to troubleshoot issues like the Error -1073807246, where a resource is valid, but VISA can't access it.

To find the process which is using the Serial port (COM), follow the steps below.
  1. Download the Process Explorer tool from Microsoft.
  2. Open Device Manager and expand Ports (COM & LPT). Right click on the target serial port and select Properties.
  1. Select the Details tab and change the value in the Property field to Service, as shown in the image below, and note the service value for searching in Process Explorer.
It is worth mentioning that NI-Serial devices use the service NiSerU, while built-in ports use the service Serial.
  1. Open Process Explorer. You will see a screen similar to the image below.
  1. Select Find » Find Handle or DLL... in the menu.
  2. In the Handle or DLL substring field, type in the value that was obtained from Device Manager. In this example, we type in NiSerU
  3. Click Search and the results will populate to show which process is using a the particular handle. In this example, the USB-232 shows as \Device\NiSerU1 being used by NIMax.exe.