me thinks there is a small bug in the A3 beaglebone spec:
(which I believe to be the latest available version).
In the cape section it says (page 61):
8.1.2 I2C Bus
The EEPROMs on each expansion board is connected to I2C2. For this
reason I2C2 must
always be left connected and should not be changed by SW to remove it from the
expansion header pin mux. The I2C signals require pullup resistors.
Each board must
have a 5.6K resistor on these signals. With four resistors this will
be an affective
resistance of 1.4K if all Capes were installed.
I interpret the sentence as it is required for cape developers to
include 5.6K resistor on a single cape. The schematics on page 60 show
1.5K pull up resistors for the I2C signal lines.