Endpoints

If you experience the endpoint limit, you are likely to see an error indicating you have “run out of USB resources” on your host computer, although you may not see any error message at all and (any of) your USB devices may randomly fail to operate correctly or become intermittent.

The USB endpoint limitation applies to xHCI (USB3) host controllers on recent motherboards only. These USB host controllers have limited memory and typically offer between 64 and 128 USB endpoints.

The problem with the limited number of endpoints provided by xHCI host controllers is that most USB devices, especially mobile phones and tablets, enumerate as more than one endpoint. A typical mobile device might enumerate as 5 endpoints. Consequently, if your host controller has 64 available endpoints you would only be able to connect 64/5 devices to that host controller, equivalent to 12 devices. Please be aware that all USB devices, including USB hubs, require endpoints, so when you are setting up your work flow it is important to factor in this limitation.