Right to the point: On a BPI-R3, should connecting the UART debug pins directly to the appropriate pins on a DB9 USB serial adapter (rx->tx, tx->rx, gnd->gnd) work? Because I’m just getting a garbled mess in my serial console. I’ve tried all different baud rates. I’m using a high quality serial adapter (Keyspan Tripp Lite). I’ve tried on Linux and Windows. I assume I’m missing something stupid.
To make matters more annoying, I have a $5 almost certainly counterfeit USB serial adapter with USB on one end and DuPont connectors on the other end and it works fine, so at this point my problem is mostly academic. I would like to know what I’m doing wrong, though.
It’s probably just the converter… something misconfigured in the drivers or… who knows. Try and see if you get the same garbled data in Windows and Linux (binary check). If they match, it’s something hardware wise.
Back in the day, yes, the Chinese USB to Serial/Prallel converters were terrible, no doubt there. But, over the years, they’ve gotten surprisingly good. In fact, I picked one up a few weeks ago (as you said, about $5), works like charm.
I suspect your serial converter is expecting a ‘real’ RS232 signal, with its old-school now-weird voltage levels.
Your USB serial adapter is probably using 5V or 3V so-called ‘TTL’ signaling (which in the real world is mostly cross-compatible between 5V and 3V)
Good explanation here: https://www.sparkfun.com/tutorials/215