The "superspeed" data path consists of four pairs of balanced wires that can be used singly, in pairs, or all four together for USB 3.x, video, Thunderbolt, or some other alternate mode data. There's the power path, the USB 2.0 path, and the "superspeed" path. Going deeper into this we can see that the USB-C connection consists of three data paths. The dock is then a middleman in the power delivery portion of the USB-C connection, effectively serving two masters. The power input USB-C port then has no USB data connections, only the connections required for power delivery. ![]() The way this is worked around on a USB-C dock is that the USB-C functions are split inside the dock so that the power master is on one USB-C port, the power input only port, and the USB data master is on the other port, the one to the laptop. There can be only one master on a USB bus. Also what is a master is a USB-C power brick. A laptop is a master on a USB bus, typically. With USB-C there's a handshake protocol on determining who is the master. With "A" ports always being the master it always provides power. When a USB device is connected it's the master that is expected to provide power. The alternate modes doesn't play much into this but the roles of each connected device and the power delivery methods do. What USB-C also added was "alternate modes" that used non-USB protocols, and added a more complex power delivery. There are USB-C ports that can act in either role but to do this requires extra electronics. With USB-C this mechanical enforcement of roles was lost but the "upstream" and "downstream" roles remained. This was enforced by using type "A" ports on the master device, and any hub ports that faced "upstream" to the master, and "B" ports on slave devices, and the hub port that faced "downstream". USB was designed from the start as a master/slave protocol with one master device and many slave devices. The reason why USB-C power input ports can only be used for power input has a lot to do with how the USB protocol works. The power input port on USB-C docks can only be used for power input.
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