There are two UARTs accessible via J3A2, UART1 and UART2.
UART1 is accessible via /dev/ttyTHS0
UART2 is accessible via /dev/ttyTHS1.
Both UART1 and UART2 use 1.8V logic.
Here’s some code in a related thread that should help to get started:
https://devtalk.nvidia.com/default/topic/793384/tegra-k1-jetson-uart-on-expansion-header-does-not-work/#4608889, but change the port to /dev/ttyTHS1.
You can see a smashing video of the the UART in action (along with level shifting) here:
This thread has been open for a long time, unfortunately I just got around to playing with the UARTs. I’m sure the posters have long since solved the problem and moved on, this answer is here informational purposes.