Created attachment 308213 [details]
Picture of connections between UART adapter and board

I attach the UART CH430G USB device:

[ 1766.381342] usb 1-1: new full-speed USB device number 3 using xhci_hcd
[ 1766.505929] usb 1-1: New USB device found, idVendor=0403, idProduct=6001, bcdDevice= 6.00
[ 1766.505948] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1766.505954] usb 1-1: Product: FT232R USB UART
[ 1766.505958] usb 1-1: Manufacturer: FTDI
[ 1766.505962] usb 1-1: SerialNumber: A5069RR4
[ 1767.178959] usbcore: registered new interface driver usbserial_generic
[ 1767.178979] usbserial: USB Serial support registered for generic
[ 1767.183414] usbcore: registered new interface driver ftdi_sio
[ 1767.183442] usbserial: USB Serial support registered for FTDI USB Serial Device
[ 1767.183512] ftdi_sio 1-1:1.0: FTDI USB Serial Device converter detected
[ 1767.183566] usb 1-1: Detected FT232R
[ 1767.183765] ftdi_sio ttyUSB0: Unable to read latency timer: -32
[ 1767.184448] usb 1-1: FTDI USB Serial Device converter now attached to ttyUSB0

But 

  screen /dev/ttyUSB0 115200

remains empty. No output is received through it, nor does my input get reflected in the boards I connect to.

I cannot determine whether this is a bug in the driver, but I tried my best to rule out defects elsewhere:

* I confirmed continuity between the board and the UART adapter pins.
* Access through SSH shows that the board is working fine.

Boards tried:

* MilkV Mars V1.21 with Ubuntu image, as described in https://canonical-ubuntu-boards.readthedocs-hosted.com/en/latest/how-to/milk-v-mars.
* Raspberry Pi 5 with Raspbian, UART interface activated.