arduino yun picocom to see /dev/ttyATH0 message is ??????

Hello:

I got a problem about serial output.

I use serial.print("message"); in sketch,and I hope to see the message in ssh terminal.

I find http://forum.arduino.cc/index.php?topic=219602.0

and try ' picocom -b 115200 /dev/ttyACM0 ' command

the picocom from opkg version is 1.7

but my terminal output is "?????????"

I find it a day, does anyone have the same problem or method ?

Thank you .