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Ask HN: Why do laptop chargers have data wires?

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Re: Ask HN: Why do laptop chargers have data wires?

#21

Post by nucleardog »

gnabgib wrote: post #6
PrimaryAlibi wrote: post #5

Earlier quoted context omitted.

So the charger won't work without the data wire and it could destroy the laptop. It's so crazy because I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it. I guess that's another popular misconception going around that it's generally fine to cut a data wire.

The charger will work, just at a low/slow power. No destruction, unless it's non-conformant (the source should only increase the volts/amps if it detects the correct signalling from the drain... and this should is defined in the USB certification specs). You may want to charge without a data-wire, or use a cable with a correct power-negotiation chip if you don't know/trust the source (eg a charging nook in a library/…

> You may want to charge without a data-wire, or use a cable with a correct power-negotiation chip if you don't know/trust the source (eg a charging nook in a library/school/bar/airport.. anywhere public).

There's an alternate charging interface you can use that's pretty widely available and I'd highly recommend--the 120/240VAC outlets all over the place!

Yeah yeah, I'm only half kidding. If you're going out to the bar you're probably not gonna shove a USB charger in your pocket. But in most of the rest of those situations (library, school, airport) and more you _probably_ have a few things you're carrying with you. Just leave a small adapter and cable rolling around the bottom of your bag and you don't have to worry about this. (Or at least you're into the realm of _wildly_ theoretical attacks.)

This doesn't just avoid the potential security issues... A lot of those charging lockers and things are not exactly well designed or well engineered. If you use your own charger you also know some weird cheap out-of-spec setup isn't going to damage your phone and there won't be any incompatibilities with the charger/cable/device that leave you charging at 7.5W.

Re: Ask HN: Why do laptop chargers have data wires?

#22

Post by throawayonthe »

PrimaryAlibi wrote: post #5
gnabgib wrote: post #3

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature. Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip. You may very well have experienced this with a very b…

So the charger won't work without the data wire and it could destroy the laptop. It's so crazy because I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it. I guess that's another popular misconception going around that it's generally fine to cut a data wire.

[dead]

Re: Ask HN: Why do laptop chargers have data wires?

#25

Post by Kirby64 »

yread wrote: post #18
Faaak wrote: post #14

Earlier quoted context omitted.

USB PD without signaling won't work. It wont supply the needed voltage (e.g. 19V), and the laptop won't charge

Is that true? I have a cable here that I use to charge my laptop with 65W PD but it doesn't make a data connection. Does it do some black magic?

There’s data, but then there’s also the “CC” pins. CC is mandatory for USB-C. It is what does the communication for PD. So, it’s data, but a very specific type of data.

Re: Ask HN: Why do laptop chargers have data wires?

#26

Post by cesarb »

yread wrote: post #18
Faaak wrote: post #14

Earlier quoted context omitted.

USB PD without signaling won't work. It wont supply the needed voltage (e.g. 19V), and the laptop won't charge

Is that true? I have a cable here that I use to charge my laptop with 65W PD but it doesn't make a data connection. Does it do some black magic?

> I have a cable here that I use to charge my laptop with 65W PD but it doesn't make a data connection. Does it do some black magic?

The magic is that USB-C has not one, but _several_ mostly independent "data" connection wires. Chargers normally do not use or care about the USB 2.0 data channel (or the separate USB 3.0 data channel), they only care about the separate "configuration" channel used for USB-PD negotiation; IIRC, according to the standard pure chargers are even supposed to short together the USB 2.0 wires, to signal to older USB B or micro-B devices "I'm a dumb passive charger which can provide more than just 2.5W of power".

So, if you have a broken cable which does not have the USB 2.0 wires connected (which AFAIK is not allowed by the standard), but has the power and configuration wires correctly connected, it might (or might not) work as a charge-only cable.

Re: Ask HN: Why do laptop chargers have data wires?

#27

Post by reaperducer »

andrei_says_ wrote: post #10
VoodooJuJu wrote: post #8

Earlier quoted context omitted.

>TANSTAAFL what?

“There ain’t such thing as a free lunch” I think? Coined by Heinlein in The Moon is a Harsh Mistress.

Coined by Heinlein in The Moon is a Harsh Mistress.

It's far older than that. At least a 150 years old.

Bars used to provide free lunches to encourage people to buy more drinks. Often very cheap and salty lunches, like stews or corned beef. This was very common in the 1920's-1940's, decades before Heinlein. It later evolved into just bowls of peanuts or pretzels on bars, though I haven't seen a bar with complimentary beer nuts in years.

The stew looked like a free lunch, but there's no free lunch because you paid for it in drinks.

Skimming this link: https://quoteinvestigator.com/2016/08/27/free-lunch/ shows it was in print at least back in 1886.

Re: Ask HN: Why do laptop chargers have data wires?

#30

Post by mrb »

gnabgib wrote: post #3

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature. Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip. You may very well have experienced this with a very b…

This is incorrect. USB power delivery does not need the data lines. Negotiation happens over the CC line and Vbus line:

"A power-only receptacle Upstream-Facing-Port might only have VBus, GND, and CC pins populated, because they do not need the data transfer capabilities" source: https://acroname.com/blog/breakdown-all-power-delivery-types...

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