UART output is enabled by flipping the UART switch to the ON position (item 9). To do so you need to remove the Pinebook Pro’s bottom cover - please follow proper disassembly and reassembly protocol. The OFF position is towards the touchpad, the ON position is towards the display hinges.
With the UART switch in the ON position, console is relayed via the audiojack and the laptop’s sound is turned OFF. Please ensure that you are using a 3.3 V interface (such as the CH340, FTDI-232R, or PL2303, which are sold in both 3.3 V and 5 V variants) to avoid damage to the CPU. Older version of the serial console cable sold by Pine64 uses wrong voltage level and should not be used, see this forum thread for further information. Recent version of the same cable uses the right voltage level.
Insert the USB plug of the cable into an open USB port on the machine which will monitor, ensuring that the audio jack of the serial cable is be fully inserted into the Pinebook Pro audio port. Run the following in a terminal:
you should find a line similar to this:
Bus 001 Device 058: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
Serial output should now be accessible using screen, picocom or minicom (and others). Examples:
screen /dev/ttyUSB0 1500000
picocom /dev/ttyUSB0 -b 1500000
minicom -D /dev/ttyUSB0 -b 1500000
Old versions of U-Boot do not use the UART for console output. <strike>The console function is activated by the Linux kernel. Thus, if you use a non-Pinebook Pro Linux distribution and want the UART as a console, you have to manually enable it.</strike>