What happens if you plug two USB-C laptops into each other using a USB-C cable
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> (@m_ou_se) I just tried to charge my phone by connecting it to my laptop (with USB-C), but instead of my phone, my laptop started getting charged, quickly draining my phone's battery. Uh, what.
> (@Gankra_) in case you aren't familiar: there was a semantic function of asymmetric usb cables that symmetric usb-C has broken. to "fix" this, each device picks a number for how subby it is, and the less subby one is the dom and charges the sub. these numbers can be surprising.
> (@m_ou_se) After some experimenting, I concluded that Dell laptops are the most powerful USB-C PD doms followed by ThinkPads, while phones and MacBooks are (equally) subby. Connecting phones and MacBooks with each other results in a switch fight.
One does PD, one does not. One does video out, one does not. One charges fast, and you guessed it, one does not.
Is it supposed to be easy because the cables fit? Honestly, it's only made it harder and I know what I'm doing. "It fits but doesn't do what I was told it would" is probably more confusing to the average Joe.
I like the physical connection of usb-c but that’s it. The whole thing is an absolute mess.
I’m going to miss lightening port.
EU agrees with this.
Also, then hopefully I won't need to switch out cables every so often because of a shorting pin #4.
It would be super useful at times to charge one laptop (or phone) off the other - provided I have some way to specify which one should be charged.
Perhaps that could be harnessed into something more productive.
At least on Android, there is; it's in the same place where you choose which file sharing protocol should be used (it's in the Android notifications area, it says something like "Charging the device through USB; click here for more options").
Do you mean phones here or iPhones? I've been charging my LG phones (G6 & G8) from MacBook Pros for years and never noticed an unexpected switch.
That was disappointing I was expecting a write up and pictures. Should I know who this person is? The headline should be modified to lower expectations.
"Guy polls Twitter about the result of connecting usb c laptops with usb c cable." "Twitter poll on the outcome of connecting two usb c laptops with a usb c cable"
I am no good at headlines obviously.
Further it depends on the USB C cable being used, and the properties / capabilities of the ports being used.
Given all the different things a USB C port can do these days and the fun smallish fire that happens if you use a cheap usb c cable and abuse it to do something it was not intended for I think there could be multiple distinct outcomes.
It will show your photos.
What happens if you plug USB-C Thinkpad T480 into USB-microA Panasonic DC-FZ82?
It will show your photos.
What happens if you plug USB-C 2022 Macbook Air into USB-microA Panasonic DC-FZ82?
Nothing. It won't even show in `/dev/` and you need to use an annoying and shitty app over stupid Wi-Fi.
Secondly you'd think for devices of comparable battery capacity they'd prioritize having the one with AC power feed the one without it. If neither have AC power then have the one with a higher charge level feed the one with a lower charge level only when the one with the lower level reaches 20% and only as much as necessary to maintain 20%. Once the system providing power hits some threshold (40%?) stop feeding power altogether. This would do what most people want when you connect two laptops or two phones: share enough power to keep the other device running but don't completely drain them.
If a "battery bank" flag is set then it should always be a supplier unless the other end is a power supply.
You can have minor quibbles about the exact numbers here but this strategy is simple and does the right thing the vast majority of the time.
If I plug my laptop into the wall and my battery bank into my laptop I want the laptop to charge the battery bank.
Screw that. They just need a signal for one device to tell the other that it wants to charge or be charged. The rest can be left up to the UI. It's better if it isn't smart.
I'm mystified that we have this many USB C cable types, chargers and data transfer speeds and there isn't some protocol for explaining what is available when a cable is plugged in or dictating behavior based upon that.
This whole series of resistors in usb legacy and aux channel chat in usb-pd makes me more than a bit sad. I would have liked to have used longstanding usb hid specs to implememt usb-pd, rather than having assorted side-band negotiations.
It still leaves the question of how two laptops would work. The naive "wait to see if there's a device and if not become the device" strategy more or less mirrors what usb-pd already does, with all the weird uncontrollable quirks. Im not sure how usb4 figures out there is a host-to-host link & negotiates networking, that's another wrinkle that might give us a good extension point to build from to enable bidirectional control. Maybe we run usb-ip over the network link - expose our usb hid battery/charger devices over that! ;p
Source: I have a theoretical degree in physics.
In practice however ...
Then I plugged laptop B into it’s own USB-C brick (on the same surge protector), and nothing happened other than it took me a quick second to think if it was okay to do.
Very Disappointed. I was under the impression when you plug two thunderbolt devices into each other you got charging and a network connection.
Lenovo at 50% charges Macbook at 100% if and only if the right macbook port is used. If another macbook port is used, nothing happens. When using this one macbook port (the one right next to the charging port) lenovo also tried to set up a new "macbook pro device", I didn't let it finish. The Lenovo has just one USB-C port.
I now wonder what two macbook pros or two lenovos would do to each other.
I do know that macbooks only accept one (the highest rated) power input, so when one is plugged in, it will also charge from the plugged in macbook to the other one.
When trying to google for the answer, I noticed a lot of misinformation about this topic, and it doesn't seem well-known that thunderbolt can be used for networking data transfer instead of "target disk mode", which is not even support on Mx macs. Anyway, finally I found a convincing answer: It depends on the order of plugging in the cable and the first mac will plug in the second![1] This is great, the user gets to decide, and I am glad you asked this question!
IIRC the second one you plug in charges the first.
You can also charge from a single MacBook Pro that is plugged into a power adapter, in a daisy-chain or star topology.
I haven't tried a feedback loop however.
A possibly more useful scenario that I've seen when power outlets are scarce is daisy-chaining multiple laptops to the one that is plugged in.
Edit: whatever is in the post (poll?) won't load for me.
Aka, debug tracing.