USB-C — the C is for confounding

November 13, 2018 6:29 PM

I use a 2018 MacBook Pro and a two-year-old Android phone, both of which have USB-C ports. I’d like to buy a rather short (about 1ft / 30cm) cable that will allow the phone to charge rapidly when connected to the MacBook and also to transfer files and tether (the HoRNDIS driver installed on the MacBook allows tethering to Android). I’ve tried a few different USB-C cables to connect the two and have gotten different results, none of which is exactly what I’m looking for.

The cable that came with the phone does let me tether and transfer files, but it’s quite long, it has USB-A at one end so I use this lousy Apple dongle, and it doesn’t do rapid charging when plugged into the dongle.

Two of the other cables I’ve tried were borrowed from external drives (one is USB 3.0 and the other is 3.1) — so I know they’re capable of transferring data — but all they do is charge the phone, though the rapid charging does work. I’ve also tried the cable that came with the MacBook Pro and it behaves very unpredictably. Sometimes it sends power from the computer to the phone, sometimes it does the reverse. Sometimes it triggers the USB connection menu on the phone that asks if I want to use it for transferring files, tethering, etc. but it doesn’t actually let me do those things. It sometimes seems this cable’s behavior depends on which of the 4 ports I plug into on the MacBook, even though I know they’re supposed to be identical.

I should probably note that USB debugging on the phone is enabled for the tethering hack to work, but I have it disabled at all other times.

So I have three questions:

1) What is going on here? I know there are a lot of issues with USB-C compatibility in general (still), but these behaviors don’t seem to make sense even with that caveat.

2) Can anyone recommend a short USB-C cable (at both ends) that does what I want?

3) I’ve seen reports of devices damaged by incompatible cables, but it seems to be only some with USB-A ports using dongles to connect to USB-C devices. Nevertheless, am I potentially endangering my MacBook by using any of these cables? (I’m a little less concerned about the phone)

posted by theory to Computers & Internet (1 answer total)

The cable that came with the Macbook would be equivalent to what I'd recommend, but it would rely on having the Android device properly negotiate a role swap when it's attached to the Mac. It turns out that the USB C design will come up with a random host/device orientation when two devices capable of acting as either hosts or devices connect to each other, so there's a mechanism built in to allow the devices to switch roles if the way it came up doesn't make sense. It sounds like your phone isn't doing that. The unpredictable behavior you describe is caused by the role coming up randomly.

If you use the Mac cable, you can unplug and plug in again if things come up backward; the roles get re-rolled each time you connect, so you can just attach repeatedly until the Android ends up as the "device" and the Mac as the "host."

However, your best bet is probably to attach the USB A to C and dongle and live with the charging being slow.
posted by doomsey at 7:24 PM on November 13, 2018