USB 3.0 specifies 900mA (0.9A) per port.
What happens when you plug a non-powered (no AC adapter) hub into an USB 3.0 port? Do all of the items plugged into the port need to use 900mA maximum total combined, or can they safely exceed that amount?
For example, on a 4-port non-powered USB 3.0 hub, could you have:
- Port 1: 400mA device
- Port 2: 400mA device
- Port 3: 400mA device
- Port 4: 200mA device
Or would it stop working when trying to plug in the device into Port 3?
Best Answer
This example is ambiguous. There are two kinds of "a non-powered" hubs, aka "bus-powered" hubs.
(1) The certified ones would have a special logical switch if plugged without external AC-DC adapter, and report to host as being "bus-powered", with an information on how much the hub silicon consumes itself.
(2) Hubs that fake their descriptors and falsely report themselves as "self-powered" hub, no matter with AC adapter or without.
In case (1) the host will make note of hub being bus-powered, and of its own power needs. Then, when sequentially enumerating devices from Port1 and so on, the host will read the device nameplate requirements, and check it against available port power budget of 900 mA. If the total doesn't exceed 900 mA, first device will be allowed to connect and operate. The same will happen with a device connected to Port2, etc, until the upstream (host) port budget is exhausted. The next attached device won't be allowed to operate. (Note: this USB 3.0 power budget schema is more elaborate than in USB 2.0, where a bus-powered hub would allow only under 100 mA devices per each port, no matter if all other ports may consume nothing).
In case (2) all devices will be allowed to connect, but the delivered power will likely sag (and cause hub malfunction and/or unstable operation), or host port would trigger its overcurrent protection, or cable might smoke out. You choose.