What exactly cause adb devices => "device offline"

Question

I have connected 4 devices to my laptop with adb. Recently I started seeing one of my android device goes "offline", while others run file.

My question is, what exactly offline means and what cause it?

Answer 1

Every time the adb server sends a command to the adbd daemon on a device it expects a response. If it does not get the response within allotted time limit it marks the device offline.

The timeouts can be caused by many different software and or hardware problems on the device and the host system itself.

From my experience the most popular reasons are broken USB cables and worn out USB connectors on the devices. So try swapping cables between two devices (one working and another having problems). If the problem moves with the cable - it's the cable. If it stays with the device - you might want to take a closer look at it.