What is the connections configuration that I should implement to output a usb-C and usb-A 3.0 from a usb-C input? The input will be from a docking station capable of supplying 20V to charge my laptop. It can also be used with a usb-C to usb-A adapter to only use it as a docking station without supplying power to charge the laptop. However, since the usb-C port to my laptop doesn't work properly, it can only be used as a charger. Therefore I would like to design a PCB that outputs a usb-A to use as a docking station and a usb-C to use as a 20V supply to my laptop. So far I figured out the connections if I had to achieve this separately. But I'm not sure how I can achieve this combined when CC1, CC2 and VBus are common to both outputs.

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    \$\begingroup\$ Trust me, you will not be able to do this yourself. A USB dock requires very high speed data, and that means very particular PCB layout. \$\endgroup\$
    – Hearth
    Mar 7, 2021 at 23:41
  • \$\begingroup\$ I will be using the dock, just the output of the dock is usb C, i need to design an adapter to convert from this output to USB C and USB A \$\endgroup\$ Mar 8, 2021 at 0:31
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    \$\begingroup\$ Yes, and I'm saying you cannot do that yourself. This stuff operates at such a speed that the exact shape and size of the PCB and the traces on it matter, and if they're off by even just a little bit it won't work at all. \$\endgroup\$
    – Hearth
    Mar 8, 2021 at 0:36
  • \$\begingroup\$ Forget it. Role change on Type-C port requires heavy negotiation over Power Delivery protocol. It requires special ICs and neavy microprogramming. Normally a laptop's Type-C should take care about all this. And it is very unlikely that your laptop will take 20V charge over Type-A port. \$\endgroup\$ Mar 8, 2021 at 3:10
  • \$\begingroup\$ Exactly, the laptop does all the work. The dock has already a usb c to type a adapter which doesnt have a microcontroller in it, the dock does all the work too... \$\endgroup\$ Mar 8, 2021 at 7:54

1 Answer 1


I'll add to the pile of comments suggesting that this is outside the realm of something an electronics hobbyist could build. I believe the proper solution is a different dock.

I may be a bit confused on the dock description. Does the dock have a captive USB-C cable? Captive as in it has a short cable with a male USB-C jack on the end? And the dock came with an adapter to plug this USB-C cable into a USB-A port on the computer? An adapter that allows a USB-C captive cable to plug into a USB-A port violates the USB specs, anyone that included such an adapter with their dock is not taking good engineering practices to heart. I have a suspicion that the reason the USB-C port on your laptop is not working properly is because the dock damaged it. By connecting a poorly designed dock to another USB port on your laptop is just asking for another USB port to be damaged.

Perhaps I assume too much and the dock is fine. What remains is that an adapter with a female USB-C port and male USB-A jack violates the USB specs. This is an adapter that should not exist as the people that wrote the USB spec explicitly state that such an adapter should not exist. There is no knowing how such an adapter is wired because the spec gives no guidance on how to wire it. The way it should function is ambiguous, and there's several ways in which using this kind of adapter can damage anything connected to it.

Another thing that I'm a bit confused about is why you don't simply plug the dock into the USB-A port and the laptop charger into the USB-C port. If by chance you were able to build a functional adapter as you proposed then you'd still end up plugging two plugs into your laptop when it's set down to be connected to the dock. Why bother building this adapter when it's so easy to just plug the dock in one port and the laptop charger into the other?

  • \$\begingroup\$ The dock comes with its own power supply, if i use dock and charger i would be using two power supplies. The dock is Dell, and and so is my laptop. The dock works well with its provided adapter usb-c to usb-a \$\endgroup\$ Mar 11, 2021 at 13:14
  • \$\begingroup\$ @SimonSultana " if i use dock and charger i would be using two power supplies. " Why is this a concern? Are you running short on electric outlets? " The dock works well with its provided adapter usb-c to usb-a " I'm sure it does, that doesn't make it any less of a violation of the USB specifications and a hazard to your hardware if used with another device. I am suspicious of anyone willing to violate a clearly stated prohibition in a specification, it makes me wonder what other rules they decided to break. \$\endgroup\$
    – MacGuffin
    Mar 12, 2021 at 3:24

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