Hooked up a bus pirate in transparent mode and it didn't work. Tried a link to 3v3 to prove the point. THAT didn't work. So now I've got a fault Carambola yes? Well no actually.
I'm SO embarrassed to admit that once I put a little pressure on the Carambola SO IT WAS PROPERLY PLUGGED INTO THE DEV BOARD, all started to work
The idea was right - poor voltage on the RXD pin, but the reason was just my complete incompetence.