Windows 11 Serial Monitor COM Busy after upload

Development Tools IDE 1.x
1

I am able to successfully upload to my board on COM7. The board after upload resets and is now on COM8. I select COM8 from tools (shows the board) and Serial Monitor comes back with the COM busy error.

Arduino IDE 1.8.19

Here is the output:

processing.app.SerialException: Error opening serial port 'COM8'.
	at processing.app.Serial.<init>(Serial.java:152)
	at processing.app.Serial.<init>(Serial.java:82)
	at processing.app.SerialMonitor$2.<init>(SerialMonitor.java:132)
	at processing.app.SerialMonitor.open(SerialMonitor.java:132)
	at processing.app.AbstractMonitor.resume(AbstractMonitor.java:132)
	at processing.app.Editor.resumeOrCloseSerialMonitor(Editor.java:2126)
	at processing.app.Editor.access$1300(Editor.java:116)
	at processing.app.Editor$UploadHandler.run(Editor.java:2095)
	at java.lang.Thread.run(Thread.java:748)
Caused by: jssc.SerialPortException: Port name - COM8; Method name - openPort(); Exception type - Port busy.
	at jssc.SerialPort.openPort(SerialPort.java:164)
	at processing.app.Serial.<init>(Serial.java:141)
	... 8 more
2

Sounds like you're using a board with native USB where the change to (in your case) COM8 is the switching to the bootloader which is as expected.

So which board are you using?

3

I am using the adafruit feather M0 with 900MHz radio. It seems that the serial monitor is very inconsistent. Sometimes it works, other times is gives the error above. I have unplugged and plugged in the board multiple times as well as reset it when connected. There seems to be no rhyme or reason why it works intermittently.

4

I'm not familiar with the Feather M0, only with 32U4 based boards. When I encounter a problem like that, it happens when the Serial Monitor is open during upload so a first step would be to close serial monitor before the upload. Sometimes the port changes completely after the upload, I have not found a reason for that, you will have to ask Microsoft :wink:

You can follow what happens in Windows device manager; below for a SparkFun ProMicro

Normal operation
image

Upload
image

After the upload, it will / should fall back to normal operation (COM23 in the first screen shot). What happens in your case with serial monitor open while doing the upload and what happens if serial monitor is closed when you do the upload?

5

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