So I rebuilt the test with the arduino-pico library under arduino ide, this defaults to pid 0x000A. This now generates the same write error so the problem is GUI-O usage of the CDC driver.

I then disabled flow control in the test:
Serial.ignoreFlowControl(true)

And GUI-O now works. So I think the problem is you are not asserting the DTR line when using the CDC driver.

[edit]
Since I can see no way to ignore flow control with the mbed core I will await you to change GUI-O to assert DTR. Incase it isn't obvious without DTR (DataTerminalReady) asserted no arduino will send data down a serial connection with flow control enabled, which it is when using the CDC driver.

@kl3m3n The other thing to check is you assert the virtual DTR line when using the CDC driver.

I've done some further investigation, the vid/pid is set in the core you build against (i'm was testing using arduino).

The official arduino-mbed core hardcodes pid 0x00C0. The unofficial (old) arduino-pico core defaults the pid to 0x000A.

What core did you test with or did you just compile some code with the pico sdk (i.e. not arduino) ?

Anyway I patched arduino-mbed to use 0x000A and I get the same problem, so maybe the problem is the way arduino-mbed configures the tinysub stack is somehow incompatible with your gui-o code?

I'm fairly sure you could replicate this by building a hello world app on platformio using the default arduino platform which is mbed.

FYI the board came from the RPI official shop.

@kl3m3n Hi thanks for doing this, the pico now connects ok however gui-o raises a "device write error" ?

Again this code works perfectly on nano's, esp32, etc.

Is it possible to get both press and release events for buttons? So a user can hold down a button to start and action and release the button to stop it?

@kl3m3n I have the official RPI pico, not a clone.

Serial USB Terminal reports the Pico as:

Serial device: CDC
Vendor: 2E8A Name: Arduino
Product: 00C0 Name: RaspberryPi Pico

Pico works fine with "Serial USB Terminal", I can send @init and get the correct response.

GUI-O tested on esp32 over usb serial and it works fine.

GUI-O error on plugging in Pico: "Usb serial device not found or not supported, retrying in 5 seconds"

Baud 115200 in all cases, also tried 9600, same error.

Please help.