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Comment by ohazi | Hacker News Reader

Okay, let's talk about that USB switch [1], because I was looking for one a few days ago and discovered something ghastly about the majority of the ones available on Amazon:

Most of them use a USB Type-A to Type-A cable for the link between the switch and PC.

This cable violates the USB spec, and is the moral equivalent of an electrical cord with prongs on both ends. It should never, ever be made or used, because plugging one of the ends into the wrong receptacle can destroy both USB ports.

The correct cable to use here would be a standard Type-A to Type-B or Type-A to Micro-B cable, but for some reason a very small minority of these switches do that [2].

WTF is going on here?

[1] https://www.amazon.com/gp/product/B01N6GD9JO/

[2] https://www.amazon.com/gp/product/B083JKDNRJ/

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I'm generally not a fan of supporting trademarks, but with respect to USB specifically, if the product doesn't have a trademarked USB logo[1], then the default consumer assumption should be that it's dubious wank which hasn't passed independent compliance testing.

And if the product under consideration does brandish a trademarked USB logo but clearly doesn't satisfy some observable aspect of the spec, then it's liable to be a counterfeit and you should seriously be questioning the integrity of your source.

[1] https://www.usb.org/logo-license

I've used this one [1] for the last four years. The cables from device to the PCs are hard wired, so I have no idea if it conforms to spec. I do know I have had zero problems with it.

[1] https://www.amazon.com/gp/product/B00BD8I3EI

I have the same switch that you have, but I have a bit of trouble with mine. Not sure if it’s an issue with the switch or with the way my computer boots or with my mouse or something else but when I boot my desktop computer which runs FreeBSD, the keyboard and mouse work during the initial stage of the boot process but then still during boot I think FreeBSD disconnects and reconnects all USB devices or something as part of the boot process, and when it does that the USB mouse which is connected via the switch stops working and then I have to press the button on the switch twice, if the other computer is on, or I have to unplug the switch from the desktop computer and plug it back in.

I once had a wall wart style power adapter whose output side was a standard coaxial cable (like for TV).

The reason was that the signal amplifier it came with was built into the housing of a 2-way coaxial cable splitter, presumably because said housing was inexpensive and readily available. One of the ports that would normally have a second TV attached was instead repurposed to provide power to the amplification circuits.

I shudder to think what it would have done if I'd run this directly to a TV, though.

I've actually seen a cable company deploy these in a commercial installation.

The Coaxial cable that comes from the pole has a voltage differential on it already, 96 Volts, IIRC, and it powers the demarcation device. (which is kinda a modem/voip device breaking out into a phone line or two)

> plugging one of the ends into the wrong receptacle can destroy both USB ports.

How would this happen?

If the two devices are wired for power and they don’t share a common ground one could imagine some unexpected current running through both devices if the USB ports weren’t properly isolated.

If you were to for example plug each end into a computer hoping to transfer files between them, both would try to act as the USB host.

The problem is rather that the GNDs on both computers might be at a different potential and substantial amounts of current might start to flow once connected (read: something between computer A and computer B catches fire).

That is why all serious inter-computer USB devices use galvanically isolated connection (e.g. using optocouplers)

Most laptops and desktops will disable the USB port/controller or at least cut power to it.

It won't stop you from sending 200V into it and frying something, but it will stop short-circuits and faulty/non-spec USB devices from doing serious damage.

I guess it's a cheap safeguard that works pretty well for consumer products.

This won’t help you if the GNDs on both devices are different.

If the GNDs are different then neither device will be “providing power” in the traditional sense. Rather current will flow from the GND of one device to the GND of the other, and GNDs are normally not protected.

You will probably also see power flow on the V+ rails as well because they will be referenced against the GND in each device. In that case the devices can cut the power, but that only protects the V+ rail, not your GND rail that could still be transporting enough current to melt something.

While modern PSUs are switch mode, surely there is still galvanic isolation from mains? Meaning both would have ‘zero equivalent’ ground, no?

They can still be plugged into different ground supplies. If they have different potential, you could get a large amount of current flow. If they're plugged into sockets in the same house, this is unlikely to happen.

This is usually more of a problem with audio equipment that has analogue signals with very high sensitivity. A ground loop can convert any nearby magnetic fluctuations (like say from the electricity flowing through the mains cables in the walls) into a nasty bit of noise.

Way back when computer to computer serial connections were a thing, you could get a cable that erroneously connected the power as well as the signal. Even if the two computers had the same ground, if the voltage regulator of one computer's power supply was slightly higher voltage than the other one, then it would try to power both computers, with the power flowing through the cable. This would tend to break either the cable, or the power supply. This particular thing shouldn't be a problem with USB, as the power supply has protection circuitry.

That wouldn't destroy the ports, though.

If you did this each device would provide +5V to the same line in the cable and nothing would happen (as there is no load). Same as if you had 2 batteries connected in parallel. If this was an issue than those ‘Y’ shaped USB cables that provide extra power for external HDD enclosures would also cause damage.

This is why I plug new USB devices to a USB Hub always and never directly to the USB port of the computer, if the device manufacturer pulls off some BS like this, it will be only the Hub's port which goes in smokes(i.e. if the Hub is done right).

I have a Hub to every port, some of them powered and advantage of using non-powered Hubs are that they can be easily used for isolating EMC(e.g. to prevent buzzing noise on speakers etc.).

It’s always about the money. USB A ports are either really cheap, or manufacturing the enclosure with the same port on both sides is cheaper. Could also be that the enclosure is a reused enclosure...

It's not just the receptacles, it's also 2-4 custom cables per device, with sleeves and overmolds and all that, because it's a non-standard, spec-violating monstrosity that necessarily needs to be included with the cursed switch.

I don't understand how that's cheaper than the alternative of paying a few cents more for the correct receptacles and then not even including cables, because the customer can use literally any standard off-the-shelf cable.

Could very well be mistaken on this, but if I’m right, at some point some USB host port conformance test specified such A-A rollover cable.

And also in more recent years, Intel DCI uses A-A 3.0 cable for CPU debugging on tablet PCs. Lesser known but Intel defines USB host controller spec.

So not allowed in spec for public use, but not custom cables per se.

I seem to remember some windows to windows networking that happened over an A to A USB cable, and I think it came as part of a PC upgrade kit, to allow you to move files from your old device to your new one.

So, I'm mildly confident you're right about A-A cables being valid, at least at some point.

Laplink cable?

https://images-na.ssl-images-amazon.com/images/I/817dK2e-q9L...

Those have a dongle inside the cable to actually do something useful, versus mindless delivering 5 volts to the other end.

Pretty sure those transfer cables were active devices, not just A-A cables.

I have a Pluggable brand switch which shares a single A via two B to A cables, I have one going to my gaming PC, and the other my Mac Mini, then I cheat a bit and have a compact 4-port hub plugged into the one shared A, which has mouse, kbd, webcam, and DAC in it. Works pretty smoothly, once in a while one machine or the other won't see one of the devices, but toggling the switch, or re-plugging the hub always perks things back up.

My monitor has a built-in USB hub that works exactly like this, 4 A inputs and 2 B outputs plus an extra USB-C output.

I breifly owned a usb -> IDE enclosure wired like this, but returned it because of the socket choice. B, regular, mini, or micro is clearly the proper choice.

My best guess is that the extra height of the B ports (quite a bit taller when you include the internal mount of the port) makes the device un-appealingly larger.

I have a keyboard from Cooler Master with one of those ghastly A to A cables :|

Given that non-conforming A-to-A cables have existed since very early in USB's life I'd be surprised if the vast majority of host devices didn't have protection against this.

I don't much about USB, but I've owned and used the same USB hub as the author for 4 years without issue for switching between my MacBook Pro and gaming PC.

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