Debug UART does not work with FTDI and PL2302 cables
From forum post: BeagleV-Ahead - debug terminal problems
The Debug UART does not work with some UART to USB cables. I found it does not work with my official FTDI 3.3V cable nor does it work with a variety of PL2302 cables that I have. It does however work with CP2102 adapters.
User peter.nguyen noted:
I believe the V_Low is not "low" enough for TTL. This might explain the garbage we are seeing on most of USB-to-UART TTL. Not sure UM3304 was a good choice?