Port monitor error: command 'open' failed: Invalid serial port. Could not connect to COM serial port

[lucy08062]

Hi i am trying to connect a Lollin (wemos) r2 d1 mini to my computer to transfer data registered by my Arduino but when i connect the Lollin to computer its doesnt connect, it did the first time i connected it and then it wouldnt connect even though i changed the port.

[ptillisch]

Hi @lucy08062. This "Port monitor error: command 'open' failed: Invalid serial port. Could not connect to COM_n_ serial port." error occurs when uploading to the boards that use specific batches of the WCH CH340 USB chip. The problem only occurs when using the latest version of the CH340 driver.

You might also notice that attempting to upload a sketch to the board fails with a "can't set com-state for ..." error message.

The affected users have reported that doing a "roll back" to an older version of the driver was an effective workaround. You can give that a try.

I'll provide instructions you can follow to do that:

1. Click the following link to download the previous version of the driver from the chip manufacturer's website:
   https://www.wch-ic.com/downloads/file/65.html?time=2023-03-16%2022:57:59
2. Wait for the download to finish.
3. Run the downloaded CH341SER.EXE file.
4. A "User Account Control" dialog may now appear asking "Do you want to allow this app to make changes to your device?". Click the "Yes" button.
5. A "DriverSetup(X64)" dialog will open. Click the "INSTALL" button in the dialog.
6. Wait for the driver installation to finish, as indicated by the appearance of a "Driver install success!" dialog.
7. Click the "OK" button in the "Driver install success!" dialog.
   The dialog will close.
8. Click the X icon at the top right of the "DriverSetup(X64)" dialog.
   The dialog will close.
9. Close Arduino IDE if it is running.
10. Connect the Arduino board to your computer with a USB cable.
11. Open the Windows Device Manager.
12. Select View > Devices by type from the Device Manager menus.
13. Open the "View" menu.
14. If there is a ✓ to the left of the "Show hidden devices" menu item, click on "Show hidden devices" to disable it.
15. Open the "Ports (COM & LPT)" section of the Device Manager tree.
16. You should see a port identified as "USB-SERIAL CH340 (COMn)" under the "Ports (COM & LPT)" section (where "COMn" is some serial port e.g., COM4). Double click on that item.
    The "USB-SERIAL CH340 (COMn) Properties" dialog will open.
17. Select the "Drivers" tab of the "USB-SERIAL CH340 (COMn) Properties" dialog.
18. You will see a "Roll Back Driver" button in the dialog. If it is not clickable, perform the following instructions:
    1. Click the "Update driver" button.
       An "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog will open.
    2. Click on "Search automatically for drivers" in the "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog.
    3. You should now see the driver installation wizard update the driver. Wait for the update to finish, as indicated by the message "Windows has successfully updated your drivers" in the dialog.
       It is possible you will instead see the message "The best drivers are already installed". If so, please stop following the instructions here and reply on the forum thread to let me know. I'll provide alternative instructions you can follow.
    4. Click the "Close" button in the "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog.
       The dialog will close.
19. Click the "Roll Back Driver" button in the "USB-SERIAL CH340 (COMn) Properties" dialog.
    The "Driver Package rollback" dialog will open.
20. Click the radio button next to "My apps don't work with this driver" in the "Driver Package rollback" dialog .
21. Click the "Yes" button.
    The "Driver Package rollback" dialog will close.
22. Click the "Close" button in the "USB-SERIAL CH340 (COMn) Properties" dialog.

Now start Arduino IDE again and repeat whatever you were doing before when you encountered that error before. Hopefully this time you will not encounter that "Port monitor error: command 'open' failed: Invalid serial port. Could not connect to COM_n_ serial port." error or a "can't set com-state" error during the sketch upload.

[lucy08062]

As you said i could only follow the instructions until 18.3

[ptillisch]

I have found that when the driver installation gets into this state, it can be solved by uninstalling and then reinstalling the drivers. After that, the "roll back" option should be enabled.

I'll provide instructions you can follow to do that:

1. Close Arduino IDE if it is running.
2. Connect the Arduino board to your computer with a USB cable.
3. Open the Windows Device Manager.
4. Select View > Devices by type from the Device Manager menus.
5. Open the "View" menu.
6. If there is a ✓ to the left of the "Show hidden devices" menu item, click on "Show hidden devices" to disable it.
7. Open the "Ports (COM & LPT)" section of the Device Manager tree.
8. You should see a port identified as "USB-SERIAL CH340 (COMn)" under the "Ports (COM & LPT)" section (where "COMn" is some serial port e.g., COM4). Double click on that item.
   The "USB-SERIAL CH340 (COMn) Properties" dialog will open.
9. Select the "Driver" tab of the "USB-SERIAL CH340 (COMn) Properties" dialog.
10. Click the "Uninstall Device" button.
    The "Uninstall Device" dialog will open.
11. Check the box next to "☐ Attempt to remove the driver for this device.".
12. Click the "Uninstall" button.
    The "Uninstall Device" and "USB-SERIAL CH340 (COMn) Properties" dialogs will close.
13. Disconnect the USB cable of the Arduino board from your computer.
14. Reconnect the USB cable of the Arduino board to your computer.
15. Wait for the "USB-SERIAL CH340 (COMn)" device to appear under the "Ports (COM & LPT)" section of the Device Manager tree.
    This will take a couple of minutes as Windows must first automatically reinstall the driver.
    It might appear first as a "USB2.0-Serial" device under the "Other devices section of the tree. Just wait for a little longer and it will appear in its final place under the "Ports (COM & LPT)" section.
16. Click the following link to download the previous version of the driver from the chip manufacturer's website:
    https://www.wch-ic.com/downloads/file/65.html?time=2023-03-16%2022:57:59
17. Wait for the download to finish.
18. Run the downloaded CH341SER.EXE file.
19. A "User Account Control" dialog may now appear asking "Do you want to allow this app to make changes to your device?". Click the "Yes" button.
20. A "DriverSetup(X64)" dialog will open. Click the "INSTALL" button in the dialog.
21. Wait for the driver installation to finish, as indicated by the appearance of a "Driver install success!" dialog.
22. Click the "OK" button in the "Driver install success!" dialog.
    The dialog will close.
23. Click the X icon at the top right of the "DriverSetup(X64)" dialog.
    The dialog will close.
24. Switch back to the Windows "Device Manager".
25. Double click on the "USB-SERIAL CH340 (COMn)" device under the "Ports (COM & LPT)" section.
    The "USB-SERIAL CH340 (COMn) Properties" dialog will open.
26. Select the "Driver" tab of the "USB-SERIAL CH340 (COMn) Properties" dialog.
27. Click the "Update driver" button.
    An "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog will open.
28. Click on "Search automatically for drivers" in the "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog.
29. You should now see the driver installation wizard update the driver. Wait for the update to finish, as indicated by the message "Windows has successfully updated your drivers" in the dialog.
30. Click the "Close" button in the "Update Drivers Device - USB-SERIAL CH340 (COMn)" dialog.
    The dialog will close.
31. Click the "Roll Back Driver" button in the "USB-SERIAL CH340 (COMn) Properties" dialog.
    The "Driver Package rollback" dialog will open.
32. Click the radio button next to "My apps don't work with this driver" in the "Driver Package rollback" dialog .
33. Click the "Yes" button.
    The "Driver Package rollback" dialog will close.
34. Click the "Close" button in the "USB-SERIAL CH340 (COMn) Properties" dialog.

Now start Arduino IDE again and repeat whatever you were doing before when you encountered that error before. Hopefully this time you will not encounter that "can't set com-state" error during the sketch upload.

[lucy08062]

nevermind because now the lolin appears as connected thanks

However, when i try to use the AT commands i dont receive the OK message or any other response for the other AT commands, do you know how this can be solved?

[qubits-us]

Hi @lucy08062 ,

Welcome to the forum..
Glad you got your drivers sorted, been there too..

The pic you posted, check your baud rate, 921600 that's pretty fast..
Thinking that's the issue, should match what you got your Serial.begin(baud rate) at..
usually something like 115200 or 9600..

good luck.. ~q

[lucy08062]

I've been changing the baud rate between all the values options, but still there are no responses. Because, what our teacher tell us, if we set the wrong baud rate there should appear like a lot of weird characters until we set the correct one and shows something like this

Since i tried with 9600 and 115200 as recommended i tried with higher values to try if i would show me any other response, but still havent occurred.

[lucy08062]

what i mean with weird characters for a wrong baud rate, and mine is all blank space

[qubits-us]

@lucy08062

what i mean with weird characters for a wrong baud rate

yes, true..

can you post the code, use that little code button please??
does it in fact upload to the board??

~q