So I decided to open up my HD dock today to determine if there might be possible for 2 shortcomings of the HD dock. First, I wanted to see if I could get the audio to output to my bluetooth headphones while in the dock for when my GF is watching TV and I'm studying. Currently, unless you plug in via HDMI, it will always default to the dock's built in speakers. Secondly, I wanted to see about adding USB functionality while docked. Here's what I found.
My first assumption was that the audio was output to the dock via the HDMI connection. This is not the case as the cable is merely a small extension from the port on the back of the dock to the plug on the front. There is no connection between the HDMI cable and the dock's board.
By a simple process of elimination, its clear the audio is output to the dock through the USB connection. So I thought a possible solution might be to add a switch on which ever connections on the USB plug signal that it is plugged into the dock which should then cause the audio not to output to the dock's speakers.
But to make sure that wouldn't cause problems I removed the Micro-USB and HDMI plugs and tried to charge it and ran into my third discovery. The unit will not charge without the micro-USB plug from the cradle plugged into the Xoom. With some quick testing it would appear the micro-USB plug provides a signal to the Xoom to let it know it can accept a charge from the 2 contacts on the bottom of the Xoom. Its likely to prevent accidental shorts. I confirmed the switching occurs inside the Xoom as the dock provides 13 volts through the contacts even when the Xoom is not docked.
This was the extent of my testing for now (as my knowledge of the USB protocol is limited), but the conclusion here is that the Micro-USB connection (with a proper signal to the Xoom) is required to charge via the contacts. I suspect the signal may possibly be as simple as the one for dedicated charging devices (shorting the data pins), but it may provide a different signal from the HD dock than the standard dock so the Xoom knows if it needs to output the audio through the micro-USB or not. This may limit the use of the data pins so it may not be possible to connect the usb to a computer while trying to charge on a dock (again, I don't know enough about the USB protocol). And since the Xoom does not charge via USB, its likely the 3 leftover pins are used to carry the audio signal to the dock. Its possible that we may never be able to see a USB dock that simultaneously charges unless it uses the standard charging port instead of the exposed contacts. Perhaps this will help understand why neither dock has a USB connection. Doesn't make us feel any better about it, but it at least provides some reasoning...
And of course, I'm still at square one with the bluetooth audio issue. Perhaps it needs a software solution.