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I have a schematic for a 4 port USB hub (4 output ports, 1 input port). I'd like to make two of these hubs and connect them together, thus making a 7 or 8 port USB hub on a single PCB. I know there are 7 port hub ICs, but I find them harder to use, plus I have this schematic already tested.

I can think of two ways to connect these 4 port hubs together. First way, which I'm pretty sure will work, is to connect one of the hub A's output ports to hub B's input port. This way however we get a total of 7 usable USB ports.

The second way, which I'm not sure if it will work, is to use only one input port, but clone its USB signals - they go both into hub A and hub B. This way we get a total of 8 usable ports. But would that work? I know the differential signals are a bit tricky, so I'm not sure if cloning them like this would be acceptable.

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  • \$\begingroup\$ "I have this schematic already tested" What schematic exactly..? \$\endgroup\$ – Matti Virkkunen Dec 4 '16 at 19:14
  • \$\begingroup\$ USB protocol is a master/slave relationship, you can go from one-to-multi or multi-to-one configuration. \$\endgroup\$ – Jack Dec 4 '16 at 21:02
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    \$\begingroup\$ If you could do that, you wouldn't need the hub ICs at all - you could just wire all 8 output ports to the input port! \$\endgroup\$ – immibis Dec 4 '16 at 22:44
  • \$\begingroup\$ @MattiVirkkunen that's the one: github.com/ltomov/4-port-usb-hub/blob/master/4port-usb-hub.pdf \$\endgroup\$ – Lacho Tomov Dec 6 '16 at 14:36
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You can't duplicate the signal because, ignoring the electrical challenges, the protocol does not support it. The USB host would not be able to understand who it is talking to if two slave devices were talking back to it at the same time.

To put it another way, why do you think we have USB hub ICs in the first place? If you could just share the signals between multiple devices there would be no need for a hub.

If you have two devices (e.g. two hubs) which need to share the same host port, you simply need another 2-port (or more) hub IC to connect them. So using 3 of the 4-port hub ICs you could make your 8 usable ports - in fact you would have 10 usable ports, 2 spare on the first hub, and then 4 on each of the other hubs.

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