Having spent an very frustrating afternoon trying to get a simple loop back serial test to work on a Rpi 3 I finally managed to succeed.
The basic problem is that over time things have changed in both Pi s/w and h/w, particularly with the Pi3. The net result is that nearly all the instructions you can find are wrong in some way or another. I’m sure in time the follow will become obsolete but as of Sunday 8th Jan 2017 they worked for me !
Note these assume you don’t need the built in Bluetooth interface, they are largely based on the following post on Raspberrypi .stackexchange http://raspberrypi.stackexchange.com/questions/45570/how-do-i-make-serial-work-on-the-raspberry-pi3 – thanks naseer mohamad
And assume you have worked for me using an up to date 2.1 version of Noobs
from a terminal enable serial communications using
sudo raspi-configin /boot/config.txt disable Bluetooth
sudo nano /boot/config.txt
Add at the end of the file
dtoverlay=pi3-disable-bt
then save and exit
Disable the Serial Console edit the file using
sudo nano /boot/cmdline.txt
remove the word phase “console=serial0,115200” or “console=ttyAMA0,115200”
Exit and save your changes
Connection example to Arduino – not pi is 3.3 most Arduino’s are 5 hence some form of level shift is needed on the Pi Rx input [ credit toto Oscar Liang for pic]
Installing ‘minicom’ terminal App – to test serial link
sudo apt-get install minicom
to run – assuming Arduino is set to 9600
minicom -b 9600 -o -D /dev/ttyAMA0
The Meandering Pi and Friends 