Fastboot command returns a list of question marks I am trying to root an Acer Iconia Tab A200 tablet using a script. I have adb working and fastboot works too, however, command fastboot devices returns a list of question marks. Eg:

???????????? fastboot

Image ilustrates the steps followed and the result:

I execute command fastboot devices in the bootloader which is already unlocked. Using Ubuntu 14.04. udev settings are already set (the file with vendors IDs).

What can I do to get the correct result? How can I make fastboot work well?

Thanks in advance!
It can be a cable fault. Change the cable. In my Redmi 3 on a cable from old Xperia P i've had question marks, 
replaced the cable to a newer cable from Redmi Note 2 and voila - the correct number displays instead of the question marks.