MUST USE CONTROL KEY FOR EXIT COMMANDS. You use the escape char to enter command mode, so to exit you would do ctrl+P, ctrl+Q for example.

Example:
picocom -b 9600 -e p -s "ascii-xfr -sv" /dev/ttyS0
-b = baud rate
-e = escape char (ctrl+__)
-s = send protocol
An alternative to using picocom if you just want to dump bytes only is something like this:
sudo cat /dev/ttyUSB0 or sudo screen /dev/ttyUSB0 115200 or sudo ttylog -b 115200 -d /dev/ttyUSB0

You can also send chars using:
echo "foobar" > /dev/ttyPS0

nano Terminal Shrinkage Problem

If you are connected to another Linux machine and you lose the bottom half of the terminal real estate after exiting nano, try reset; clear to recover.