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Comment by danjoc | Hacker News Reader

USB-C is not backward compatible to USB-A. There's no way to plug a USB-C key into a computer with only USB-A ports. If you need to work with both ports, USB-A is the only game in town. The C to A adapters are not compliant with USB-C spec (see Benson Leung) and are dangerous to use with your USB-C devices.

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Except if you have a usb-c laptop and that’s the only place you’re going to plug it in, then you probably want to be dongle-less.

I can see why a passive adapter could be dangerous.

I don't see, though, why a self-powered hub could not be built that connects to the host over USB-A and provides USB-C ports for peripherals. But I can't find anyone selling such a thing.

Wrong! I use the following:

Yubikey 4C https://www.yubico.com/product/yubikey-4-series/#yubikey-4c

USB C-A Adapter https://www.amazon.ca/gp/product/B01C43FUIW/ref=oh_aui_searc...

>Wrong!

No, sorry, you are wrong.

https://plus.google.com/+BensonLeung/posts/UFCHbSDRa2o

99% of people do not care what the USB spec says is allowed, if the thing works they will buy and use it.

See: every phone charger that outputs more than 500mA over a USB-A port. Doesn't comply with the spec, nobody cares, everyone does it.

Even the post says the only reason C-A adapters aren't allowed in the spec is that they can be chained with a C-C cable to make an A-A cable. They work fine if you don't do stupid things with them.

Isn't current drawn? Meaning that the device doesn't have to use 500mA but could draw up to that much if need? So it would not hurt the device.

Buyers may not know or care, but retailers do. Anyone can report that Amazon link shared earlier and Amazon will remove it as it is not spec compliant. Those type adapters are also non-existent at basically every electronics brick-and-mortar store I've visited.

https://www.androidpolice.com/2016/03/29/amazon-updates-its-...

Amazon has had this policy for years but unfortunately enforcement is a complete joke and they remain full of noncompliant hardware.

I do find it interesting that they ban noncompliant Type C devices but not noncompliant Type A devices. Probably because the Type A current spec has got to be among the world's most-violated standards.

>If C receptacle to A plug adapters exist, they allow the user to plug one in on both ends of the cable, creating an invalid cable that devolves into a USB A-to-A cable. This is why the specification specifically forbids all legacy adapters that have a C receptacle on one end.

I don't understand this statement; If an adapter for USB-C to USB-A exists, then the usage of it would force the USB-C operations to be limited to USB-A specs?

Isn't that exactly what you want? USB 2.0 is slower than 3.0, so interaction between the two devolves to USB 2.0; USB-A is slower than USB-C, so interaction devolves, and now you're backwards compatible (omitting whatever benefits of USB-C, but still a working setup nonetheless)

In fact, I'd imagine thats what adapters do in general.. if they're between non-equivalent things, it naturally devolves to the maximum commonly supported

Doesn't that mean that 99.99% of all USB chargers for phones violate the spec? Since the spec says USB A only outputs 500mA ? Holy shit!

IIRC since at least 2.0 the USB spec is very liberal in what current can A port source and the 500mA is relevant only as maximum that can device with B port negotiate as it's sink current.

The spec is not liberal. See Section 7.2.1 for power delivery information. "A unit load is defined to be 100 mA. The number of unit loads a device can draw is an absolute maximum, not an average over time. A device may be either low-power at one unit load or high- power, consuming up to five unit loads."

http://sdphca.ucsd.edu/Lab_Equip_Manuals/usb_20.pdf

Liberal in exactly the sense that you describe, that is it describes and limits behavior of the current sink (ie. USB function) and does not constrain the current that downstream port (in traditional USB topolgy an A port, ie. host or hub's downstream port) can source apart from specifing minimum of one load unit and maximum as something IIRC safety related (and not exactly defined).

Originally (1.0, 1.1...) downstream USB ports were supposed to measure and limit downstream VBUS with the unit load precission, but this idea was shelved very early on and replaced with recommendation of placing fuses or polyswitches and sensing their state.

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