I have a Wi-Fi Motorola Xoom that I am connecting via ADB for development. I am seeing an intermittent problem where the device (with ID number) shows up in the adb devices list, but will present as being offline.
The device and computer both show the tablet as being connected. I've tried a combination of rebooting each device and swapping USB ports, but the only thing that solves the issue is switching out the cable. Furthermore, a cable that I used yesterday doesn't work today -- I am having to use a previously non-functioning cable to connect.
Is this a driver issue? Device port issue? I have seen this issue with several versions of the Motorola drivers as well as the PdaNet drivers. I am currently using Motorola Mobile Drivers 4.7.1.
WARNING: this question referred to an older device and as such the answers may no longer be relevant to current ADB connection issues.
I met the same problems like yours, and I found out that the USB cable has mal-functions in the PIN of head USB. So I switch to the new USB cable, it just works fine and detect device as online.