Akcofolb wrote:... When I plug the RPi into the lapdock using those two USB cables, I get the unpleasant odor of something overheating and the Pi doesn't boot. The Pi is mounted in an Adafruit Pi Box (top off) and I can't identify what is getting hot - I haven't left it plugged in long enough to see smoke. I tried just connecting the HDMI cable and powering the Pi with the Adafruit power supply and it boots fine and displays on the lapdock.
Any ideas what could be going wrong here? Does this sound like a defective USB cable problem? My Pi doesn't need any more baking and I don't want to stress it too much troubleshooting.
Sounds like a defective USB cable or adapter, perhaps shorting Vbus to GND somewhere. Lapdock's Micro USB is designed to charge a smart phone, so it can deliver a lot of current.
The trick is tracking down the short if it is indeed the case. Here's some suggestions:
1. With an Ohmmeter, check whether the USB cable or adapter has Vbus shorted to ground. Check both the ground pin and the shield. I attach safety pins to my probe tips for checking Micro USB pins. You can find USB pinouts at Wikipedia: http://en.wikipedia.org/wiki/USB#Connector_types
2. With RasPi power off, plug your USB cable+adapter into a RasPi full-size USB A port (the other end disconnected) and see if this shorts RasPi's 5V to ground using an Ohmmeter between TP1 and TP2. If this does short, try it with just the adapter. Try both RasPi USB A ports.
3. Disconnect HDMI: this makes Lapdock turn off all USB ports. Connect your USB cable+adapter between a RasPi USB A port and Lapdock's Micro USB plug. Use an Ohmmeter to see if this shorts RasPi's 5V to ground as in suggestion #2.
4. What happens if you have your USB cable+adapter plugged into the Lapdock's Micro USB jack and NOT connected to RasPi, and then connect the Micro HDMI to RasPi to turn on Lapdock? Do you get the overheating smell? This suggests a short in the USB cable or adapter. Try it with just the cable.
5. See if you can power RasPi from one of Lapdock's full-size USB A ports using a USB A male to Micro USB male cable plugged into RasPi's Micro USB power jack. You must have HDMI connected to turn on RasPi's internal 5V power and open Lapdock to power its USB A ports. This will tell you whether Lapdock's 5V is sound. Powering a RasPi this way doesn't give you the data connection you need for keyboard and track pad, but you can at least see if it boots.
Someone upstream in this thread has a cable that connects the ID pin to Vbus. I had never heard of that before. Usually ID is connected to GND at the master end of an OTG cable or left floating. But it's conceivable that your cable attaches ID to Vbus and your adapter connects ID to GND, which causes the short. This would be detected by suggestion #1.