How to find application COM port and list all COM ports on Windows

The task may not be as simple and obvious. The easiest way is to open Devices Manager, go to the Ports group, and view the list there. However, this does not always work:

• Your device may be in an unexpected place in Devices Manager (e.g., Modems).
• The port number may not be displayed next to the device name, requiring you to dig into the device settings (e.g., multi-port adapters or virtual devices).
• The device may create multiple ports for one physical device (e.g., Bluetooth devices).

To solve this problem, you can use Advanced Serial Port Monitor. The program can identify ports in several ways to obtain the most complete list with the port number and its description:

1. Download Serial Port Monitor (the first link) and install it on your computer. During installation, you can leave all options at their default settings.
2. Run ASPM, and the program will determine all ports upon startup (see image).

On Windows, each COM port can be used by only one program, since it is opened in the exclusive mode. If the port number is unknown, it can be found by simple trial and error, sequentially trying to select and open the port using a program like Advanced Serial Port Monitor. When opening the port, you should use the "Manual" mode (Main menu → Mode) and relatively standard communication parameters 9600 8N1.

1. Run Serial Port Monitor.
2. Select the desired port from the list.
3. Click the "Open" button.
4. If the text on the button changes to "Close" and no error message is displayed, the port has been successfully opened and is not in use by another program.
5. If the port could not be opened, it may be that another program is using it; ASPM will show the name of the process that is using that port. This is often accompanied by an error message: Port busy, Access denied.
6. If it is possible, stop the program using the port and try to open it again to confirm that it was indeed occupying the port.

The Advanced Serial Port Monitor program allows you not only to view data transmitted through the COM port, but also to determine communication parameters. This can be useful if you need to find out the settings of an application interacting with an external device via a serial interface. Communication parameters include: baud rate, number of data bits or stop bits, parity check, and others. To determine communication parameters, you need to do the following:

1. Close the application that interacts with the external device.
2. Run Serial Port Monitor.
3. Go to program settings "Options → Program settings → Data view → System events", and ensure that checkboxes are marked for all system events in the list. Click "OK" to save the changed settings.
4. Select the desired port from the list.
5. Select "Mode → Spy".
6. Click the "Open" button.
7. Run your application and initiate data exchange within it for a few seconds.
8. The main window of the program will display data exchange and system events highlighted in red. It is more convenient to use the "ASCII" data window for viewing events.
9. Close the port to stop data flow.
10. Scroll to the top of the data window.
11. Look for lines with "Baud rate" and "Data bits," which contain the communication parameters used by another application when opening the port.