I have connected two USB-TTL converters and opened two terminal programs on the pc. One connected to the pc side, one to the nano side. I send one char on the nano terminal, then I get the ‘Nano is here!’ msg, after two more chars the ‘i got c line:’ is printed.
There is another uart3 on PB_10 / PB_11, maybe you can try this for the pc? PA_2 / PA_3 is also used for stdio, maybe it is locked? Although, I’m using a printf and pc.printf and both are working.