USB Endpoint Limitation – what is it, and which Cambrionix products can fix it?

The ‘USB Endpoint Limitation’ is something which can cause issues with users who connect many USB devices to a host computer. Typically you will see an error relating to running out of USB resources, or similar, on the the host computer. Worst-case you will not see any warning or error message, your USB devices (or...
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The ‘USB Endpoint Limitation’ is something which can cause issues with users who connect many USB devices to a host computer. Typically you will see an error relating to running out of USB resources, or similar, on the the host computer. Worst-case you will not see any warning or error message, your USB devices (or some of them) may simply fail to operate correctly or become intermittent.

The USB Endpoint Limitation only applies to xHCI host controllers (USB3.0 Host Controllers) on the computer motherboard. These host controllers typically offer between 64 and 128 USB endpoints (not to be confused with the amount of USB devices which can be connected onto a single USB port which is 127).

The problem with the limited number of endpoints provided by the host controller is that many USB devices enumerate as more than one endpoint. A device could enumerate and request, for instance, 5 endpoints. At this point, if your host controller only has 64 available endpoints you would only be able to connect 64/5 devices to that host controller, or 12 devices. Do note that all USB devices, including USB hubs, require endpoints.

Because the limitation applies to the host controller on the computer motherboard, it is possible to avoid the limitation by using a different host interface such as Thunderbolt.

Our Thunderbolt products (both Thunderbolt 2 and 3) do not use the motherboard USB host controller since the connection is Thunderbolt rather than USB. We then use multiple PCIe USB host controllers within our Thunderbolt products to provide the downstream USB ports.

The hardware architecture within our Thunderbolt products is such that each downstream USB port has at least 16 USB endpoints. This effectively removes the USB Endpoint Limitation.

To find out how our Thunderbolt products can help you, please contact enquiries@cambrionix.com

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