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I am trying to connect to an Raspberry Pi Pico from an Raspberry Pi 3B+ running Raspian Bullseye. The Pico shows up in the File Manager when I connect with the BOOTSEL button pressed. When I try to install the MicroPython interpreter via the Thonny Interpreter Configuration dialogue it appears to work. I get the 'Done!' message and looking at the dialogue that appears when you click Done I see the following:

Waiting for the port
Found 2e8a:0005 at /dev/ttyACM0
Done!

That seems to indicate that the firmware install worked, but I don't see any REPL prompt in Thonny and I get:

Backend terminated or disconnected. Use 'Stop/Restart' to restart.

But 'Stop/Restart' doesn't do anything, nor do any of the other Thonny commands.

I have tried to flash both the MicroPython_RenameMainDotPy.uf2 file and the flash_nuke.uf2 file, but neither makes any difference, and I typically get an error saying An operation is already pending and I haven't found any way to clear this seemingly blocked operation.

I have tried two Picos, with the same results for both. Both Picos were working fine a few months ago, and the only thing I have done in the meantime is an apt update followed by an apt upgrade. Apt tells me that Thonny is the latest version when I try to upgrade it.

UPDATE: The above is incorrect, I did upgrade from Raspbian Buster to Bullseye some months ago on the Raspberry Pi host. That may well have something to do with it. I had forgotten that.

I also thought it might be a bad USB cable, but I have tried 3 or 4 different cables with exactly the same results.

This is extremely frustrating. Does anyone know what the problem might be?

NEW UPDATE: I tried the Pico on an RPi 4 also running bullseye and REPL in Thonny works fine on that host after flashing the MicroPython interpreter. The only difference between that and my RPi 3B+ is that I did an apt update followed by apt upgrade two days ago on the 3B+. It seems that quite a bit was upgraded as connman was installed on the 3B+, but it wasn't on the model 4B. So, something got updated/upgraded recently that is blocking proper connection with the Pico.

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SOLVED: Enabling the serial port in raspbi-config resolved the problem. I can now access the Pico through the new port marked

    Board in FS mode - Board CDC (/dev/ttyACM0)

However, on my RPi 4B the serial port was not enabled and I was still able to access the micropython REPL prompt.

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