It is probably much simpler to use a separate USB to serial adapter.
Those work out of the box with Ubuntu and you only have to change the
serial port name in your patch.
Yes, that works fine but it mixes up the port number of the arduino and the
serial adapter. I can't get the two to be assigned to the same port number
each time Pd starts. They both show up as /dev/ttyUSB0 or /dev/ttyUSB1 on
the [comport].
It also would be cleaner with less cables to have the arduino taking care of
the serial data transfer. If I can't get that to work I'll have to use a
serial adapter and find a good way to distinguish the two automatically.
Ingo