Disabling the beaglebone black HDMI Cape

mount /dev/mmcblk0p1  /mnt/card

Edit the uEnv.txt on the mounted partition:

nano /mnt/card/uEnv.txt

To disable the HDMI Cape, change the contents of uEnv.txt to:

optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN

Save the file:

Ctrl-X, Y

Unmount the partition:

umount /mnt/card

Reboot the board:

shutdown -r now

Wait about 10 seconds and reconnect to the BeagleBone Black through SSH. To see what capes are enabled:

cat /sys/devices/bone_capemgr.*/slots

Every line shows something like “P-O-L” or “P-O–”. The letter “L” means 
the Cape is enabled; no letter “L” means that it is disabled. You can 
see here that the HDMI Cape has been disabled, so pin 27 to 46 on header
 P8 are now available to use

http://www.logicsupply.com/blog/2013/07/18/disabling-the-beaglebone-black-hdmi-cape/