Comment on Some basic info about USB

• tal@lemmy.today ⁨2⁩ ⁨months⁩ ago

  Why can’t I tether my phone to my laptop with two out of three of my cables?

  I have an Android phone with a USB-C port and a laptop with (several) USB-C ports.

  I have three cables that I carry with me: a USB-C-to-USB-C cable, a second USB-C-to-USB-C cable, and a USB-A-to-USB-C cable. None of these are charging-only power cables, and I’ve used them for data connections.

  One of the USB-C-to-USB-C – an unmarked cable – doesn’t permit for USB tethering to be used.

  The other two do not.

  The other USB-C-to-USB-C cable even has USB 3.1 symbol.

  I don’t know why.

  Looking more-closely, it looks like the other two don’t have a data connection established between the Android phone and the laptop from the laptop’s perspective. They’ve let me do so with other devices.

  Checking what data transfer rates a given cable supports electrically

  As far as I can tell, there isn’t a way to query the “e-marker” on a USB cable from Linux today; I found a comment from someone saying that kernel support is still being worked on. You can use lsusb -t to show the negotiated speed between two devices, so can use them to infer the speed.

  lemmy.world/post/18014298

  What USB PD rates does a USB cable or power consumer or charger support?

  I don’t know of a good way to determine this from a user standpoint. Note that this is a matrix of voltages and currents, so it isn’t just “I support up to rate X”. Also, not all devices display the rate of power that they are providing or consuming – in fact, most don’t. My Android phone, a reasonably-sophisticated device and one with a display and capable of both providing or consuming power, doesn’t show the rate of power consumption or provision, just “slow” or “fast”, without additional software.

  I have – had – a laptop that just won’t charge if a charger doesn’t support a certain USB PD profile, which its provided charger did but not all charging devices did.

  When I plug in two devices that both support USB PD, which is the consumer and which the provider?

  When I’m in my car, I typically I have three devices that have USB PD ports and can either provide or consume power – a large powerstation, a laptop, and a phone. I eventually learned a few facts:

  First, the direction in which power is being provided via USB PD is independent of which device is operating as a USB host or device using USB OTG ports; it’s possible for the direction to be different from the direction of power provision.

  Second, apparently the direction of host/device order is random, and devices just remember the host/device direction for a certain amount of time, so that if you plug two USB OTG devices into each other and the direction is not what you want, the idea is that you can figure it out from one or more of the devices indicating this and then plug them in in the other direction.

  Third, as best I can tell empirically, USB PD does the same random thing.

  This creates all kinds of fun if one device powers off and then on again or something; my laptop can start draining its power to my powerstation (generally not what I want), or my phone to my laptop, since all the USB PD ports in question support USB PD in both directions.

  USB does have reasonably good fallback, so most cables and most devices tend to sort of do something to some degree. But it definitely could be a lot better from my standpoint.

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  • tal@lemmy.today ⁨2⁩ ⁨months⁩ ago

    I’d also add that while I have rarely had problems with it, – only with one USB-powered mixer that had less-than-amazing power circuitry and bled noise from dirty power being provided by USB through into the audio signal, and where I put it on a dedicated charger – USB power can be stupendously dirty. I was watching some guy with an oscilloscope investigate various devices, and all those sensitive devices are accepting all kinds of craziness in terms of power. I’m surprised that USB power sources aren’t required to provide some hard guarantees on what they can do in terms of load and response.

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