BeagleBone Black connects TRSTn to GND via 4.7K resistor, and thus keep TRSTn normally low.
My JTAG controller don't have TRSTn pin, so I connect BeagleBone Black TRSTn line directly to 3.3V VDD.
After powering up BeagleBone Black, I saw keep-alive LEDs flashing
I run my JTAG test controller few times, The TDO pin still get all 1's, even though I drive some user LED pins (gpmc_5 - gpmc_8, V15, ... V16) to 0.