Serial console

[zaius]

If having the RX pin connected is the problem, then the manufacturer of the cable should not make a difference.

It depends. Different cables have different circuits with different controller chips hidden in the USB end of the cable. If e.g. the RX line needs to be held low, but some adaptors allow it to float, or produce too much noise, this could be interpreted as data by the RK3399, or might interfere with it in some other way.

I haven't seen any reference to this problem with any of Pine64's A64 boards/laptops, only the RK3399 ones (RockPro64 and Pinebook Pro).

Then perhaps Dan's homemade adapter works because it uses a different chip?

The Wiki reads "you can make your own if you have a serial console USB 2.0 adapter and old/ broken headphones", which would lead one to believe that there are no significant differences between serial console USB 2.0 adapters.  The one Pine64 sells is "powered by CH340 chipset".  So it seems we need to figure out which chipsets work with the RK3399.

In the meanwhile, at exactly what point is it safe to insert the Pine64 cable?