I did some hacking last night on a live stream with a few other XDA members from this and other forums. The goal was to find the UART location on the AT&T Galaxy S3.
Why, you might ask, would this be useful? During kernel and bootloader development, sometimes the device won't boot to the point where you can obtain logs to determine the problem. UART can provide the realtime eyes-on that you need to troubleshoot such problems.
So the process was as follows... On a rooted device, pull the kernel. Extract it. Add command line parameters to enable UART.
su while [ 0 ]; do date| tee /dev/ttyHSL0; busybox sleep .5; done
The TX from the board (your RX lead] is placed 2nd from the bottom on the battery side of the board. RX is either the one above that or middle on the other side.
In the video, at about 5 minutes in, I said I didn't know what the 31 value was... and the kmesg logs were pretty thin.. Well, turns out they are the kernel message levels. For full logging, change that to 987654321. Samsung usually uses the 9 identifier to represent shell access .
So, I hope this helps. UART provides eyes before any other method of debugging (aside from JTAG) begins to work. UART is the first thing to do in order to make a device into a development board.