[Today I found a very nice tutorial to enable UART (GPIO) serial on Raspberry Pi 3 and Pi Zero W.]
To summarize, here are the commands I did to enable serial (/dev/ttyS0) on my Raspberry Pi 3:
$ sudo -s $ echo "enable_uart=1" >> /boot/config.txt $ systemctl stop serial-getty@ttyS0.service $ systemctl disable serial-getty@ttyS0.service $ nano /boot/cmdline.txt #Remove console=serial0,115200
After a reboot you should be able to use
$ sudo apt install screen # Install screen if you don't have it already $ sudo screen /dev/ttyS0 # Open the port for viewing
Now you should be able to connect any other device to the Raspberry Pi using serial.