Cannot get serial monitor working with Teensy 4.1


Well-known member
I will start by saying I am new to using Teensy so apologies in advance for any stupid misunderstandings!

I am trying to get the serial monitor working with a Teensy 4.1 device

The device is working and I can upload sketches

I understand that the Teensy, when running, creates some sort of virtual serial ports

When I click on Tools-> Port I get the ports shown below


If I select the Teensy 4.1 option, run the sketch and then try to open the serial monitor, then I get the error message: "Error opening serial port 'usb:0/140000/0/3/4/4'. (Port busy)"

If I select the COM8 option, I can open the serial monitor but when I upload the sketch again, it says that COM8 does not exist

Not sure when I am doing wrong...

I have just selected COM8 (Teensy 3.2, 3.1), opened the serial monitor and then uploaded the sketch again and it has worked

Is this what I should be doing despite the fact that I have Teensy 4.1?
remove/uninstall the com 8 port (using device manager) as it seems locked for Teensy 3.2 (Windows problem, I guess)
Hi, the COM8 port appears in device manager when I plug in the Teensy 4.1 so firstly I cannot delete it but also it's clearly being created by the Teensy 4.1

I have never had a Teensy 3.2 or Teensy 3.1 BTW

The Teensy 4.1 port does notwork with Serial Monitor - port busy error
What seems to work sometimes is setting the sketch running with the 4.1 Teensy Port selected

The select the COM8 port (ignoring Tensy 3.2 comment) and then open serial monitor
Arduino IDE showing 1.8.15? Current version in use is IDE Version 1.8.19.

Also the current Teensyduino version is up to 1.57. Not sure of the version there, but a fresh install of that as well might clear up the issue where the Serial Monitor may have changed.
Which version of Teensyduino is installed? Click Help > About in Arduino to see. Or when the small Teensy Loader window opens, click Help > About in that window. Should show the same version in both.

Latest version is 1.57.

If using an old version, please first update to 1.57 so you're using the latest stable version.

Or you might even try the latest beta. It has several improvements to how USB serial is detected and used on Windows, but at least in theory those improvements are only supposed to affect usage with Arduino 2.0.x. They're not supposed to change anything regarding use of Arduino 1.8.x.

If none of this helps and you're stuck in a "Windows messed up" situation (it happens... Microsoft loves trusting stale Windows Registry data even though it has to fully detect everything about the device every time you connect) you might try clicking Tools > USB Type. As far as Windows is concerned, when your Teensy runs a freshly uploaded program with a different USB Type, it is as if you just obtained a completely different USB product and plugged it into your PC.

The USB Type options without "Serial" use a special HID interface to emulate serial, so Serial.print() can still work. Maybe try one like MIDI or RawHID. If the serial driver (which is built into all versions of Windows and loads automatically with Windows 10 & 11) is somehow broken, this can completely circumvent using that driver. Or if the driver in Windows is still working but not loading properly due to corrupt registry info which is specific to this detected USB vid/pid/ver info, just switching to one of the many other choices which has Serial will use the serial driver but cause Windows to associate it with a (as far as Windows knows) a brand new device.

How to actually fix those sorts of Windows problems (short of a complete reformat of hard drive and clean Windows re-install) I really do not know. But I do have these pretty solid tips about how to work around those problems.

Hopefully some combination of all this works. But please, if you're using older than 1.57, update the software first.
Thanks Paul.

I can't resolve the Arduino IDE 2.03 issue on my desktop so posted a query on Arduino forum with debug output. Looks like it is crashing trying to load discovery related information related to Teensy

I have installed IDE 2.03 on a laptop that has never had Teensy addin installed. IDE 2.03 loads fine and then I have installed Teensydino 1.57. That combination seems to work fine in terms of the serial monitor

That just leaves me with the main issue of not being able to get the ethernet connection to function as per othet thread on Project Guidance section :)

Many thanks for you help defragster and paul