If you would like to use the RS485 interface e.g. for modbus, you have to set some settings. Please log in to the SmartPi via SSH.

sudo nano /boot/cmdline.txt

remove
console=/dev/serial1
and
console=serial0,115200

save STRG+O and close STRG+C

sudo nano /boot/config.txt

insert the following lines at the end

dtoverlay=pi3-miniuart-bt
enable_uart=1
core_freq=250

save STRG+O and close STRG+C

sync
sudo reboot

Now you can use the RS485 bus via /dev/serial0