I think you have to open the back and move a switch near the USB jack
on the same side as the headphone jack to change the headphone jack to
a serial port. Documentation for this thing is either not good or not
easy to find, can't tell which.
I found this:
...and when I did that from the pre-installed Linux on the eMMC, I got the terminal emulation on the frame buffer from u-boot ... but I don't know if that is a coincidence or not. I'll check for a physical switch later.
But I don't know about the switch for sure because the serial cable I
got with my Pinebook doesn't work. I don't get a tty device on my Mac
or NetBSD. On Linux I get a /dev/ttyUSB0 but as soon as I try to use
it with screen it exits suddenly. Haven't tried minicom yet.
I use Serial (available on the macOS App Store) ... it comes with non-kernel drivers for a bunch of USB serial interfaces, and works with the Pine64 serial dongle. The cost of this app is totally worth it if you use USB serial TTYs much.