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Comment by dntrkv | Hacker News Reader

Slightly related, but you can also charge MacBook to MacBook. The first device plugged in is the chargee.

WTH — the cable is stateful? :O

No, it is entirely a device thing.

it can't be. the cable has to be stateful for this to work. even if it's one bit that simply tells the device it is connected.

Not necessarily. It can just test the resistance of the port/cable to see if anything is there. The first device (power provider) to detect the cable can say "ok, I got plugged in first" and then tell the other device (the consumer) that when they try to negotiate power delivery. That doesn't require maintaining state in the cable itself, just that the devices be able to detect the cable when it's plugged in (even without the other end connected).

Cable detection: http://blog.teledynelecroy.com/2016/02/usb-type-c-cable-dete...

Overview of USB power delivery negotiation: https://www.embedded.com/usb-type-c-and-power-delivery-101-p...

And then after the initial connection, the devices can renegotiate their roles if you want to reverse the charge.

It's great when it actually works. In the real world there are so many incompatible implementations that USB-C is a real clusterfuck.

The cable has a few wires dedicated to directionality (CC1 & CC2) lines as well as sideband channels. My guess would be that the first device to be plugged in sees nothing on the other side, decides it's going to ask for power, and configures itself (and thus the cable) as such. The second device sees a USB-PD device requesting power and begins to provide it.

While I don't doubt the foresight of Apple HW engineers, the possibility strikes me that this is entirely accidental and derived from the design of the USBPD spec. If so, these "stars align (but only because the spec was functionally designed)" moments are some of my favorites.

There are cables with an IC connected to the CC pins. However this is AFAIK only used to determine if the cable can handle higher charging currents, still the MacBook could detect if the cable was plugged in before the device in the other side https://www.inviolabs.com/blogs/news/what-is-e-marker-and-wh...

If you think that's cool you should check out the OMG cable.

https://fossbytes.com/charging-cables-used-to-hack-devices/

What do you mean by "first device plugged in"? If I connect two devices with an USB-C cable, which one is the "first device" then?

I imagine you’d plug the cable into one of them and then the other. I assume the author means first and second as in temporal order.

A device can tell when one end of the cable is plugged in by testing the cable's control channel, even without the other end connected yet. It sees that a cable is connected, but there's nothing on the other end, and presumes it's first and should act as the provider. A second later, you connect the other end of the cable to a consumer, and the first device will try to negotiate its role with the second. It's up to the devices to respect each other's role requests.

http://blog.teledynelecroy.com/2016/02/usb-type-c-cable-dete...

Different device types (like a powered hub or desktop monitor) can request different roles, and they'll have to sort it out during negotiation if there's a conflict. Sometimes a user-facing popup will ask you if you want to provide or take power from the other device.

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