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I'm designing a keyboard and I want to be able to daisy chain another USB device through it. I have a Drop ALT that does this and I really like it. It has two USB C ports and you can connect either one to the computer and use the other as a pass through. It uses a USB2422 as the hub and I've read the data sheet several times but I can't figure out how it's working. The USB2422 has one upstream port and two downstream. I thought the upstream would go to the computer but then why doesn't it matter which port you use?

Edit to add pictures.

PCB Back PCB Front USB 2422

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  • \$\begingroup\$ My guess is it's not the hub chip itself doing the host/device detection, but something else that figures it out by the USB-C standard magic (keyword: CC pins). One could then multiplex the USB lines to the hub chip based on that. There isn't really a way for us to tell you how it does that exactly without either someone from the design team willing to drop in, us guessing, or you showing some PCB pictures so we could guess more accurately :-) \$\endgroup\$ – Richard the Spacecat Mar 29 at 10:18
  • \$\begingroup\$ I added some pictures showing both sides of the PCB and the best image I could get of the chip. \$\endgroup\$ – CaseyB Mar 29 at 10:59
  • \$\begingroup\$ USB - unlike Ethernet - has a topology with a single host and multiple devices. The protocol is asymmetric with regards to these two roles. It also affects all connections and ports as it needs to be more or less fixed on which side the master is. \$\endgroup\$ – Codo Mar 29 at 11:51
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My guess (or how I would do it) is that there are multiplexers on the data lines of both USB ports, and then check which port is being connected to the computer using power sensing in order to switch the multiplexers from each USB port to the correct pins on the USB2422.

Those look like they could be multiplexers:

enter image description here

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    \$\begingroup\$ On one of the chips, the marking "LYV" is visible. They could be TI TS3USB30E USB mux/demux as they use that marking. \$\endgroup\$ – Codo Mar 29 at 11:48

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