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USB C TO A Connector

By Jestine Yong on May 4, 2024

Have a 4-port unpowered USB hub that was connected to the computer with a USB A to A cable and did fine service.

However, I recently ordered a new iMac at work which has only USB C ports, while I still had need for USB A for my thumb drive, Yubikey etc. as well as wanting to use my old, wired mouse and keyboard. I imagined this would be a simple matter of using a USB C to A cable, but no matter what cable I tried, it didn’t work (my local IT was helpful in letting me also try some of their cables). So, I asked my workplace to get me a powered hub with its own USB C to A cable to go with the ordered iMac, which would have been unusable to me without USB A connectors.

Not only did the USB C to A cable that came with the new hub work on the new iMac, but it also worked with the original unpowered hub, so there was something ‘magic’ about this cable. That the cable in the picture was clearly pasted together made me wonder.

Now, USB C cables include resistors to indicate ability, so I got some cheap breakout boards for the C and A sides to try and figure out what was so special about the ‘magic’ cable.

With a multimeter I discovered that the many normal cables had a 56K ohm pull up resistor on pin A5, while the lone ‘magic’ cable that came with the new hub had a 5.1K ohm pull down resistor on pin A5. This is where referencing an old copy of the USB C specifications USB Type-C Spec R2.0 – August 2019.pdf helped and things now got interesting:

P77: a USB C to A cable should have a 56K ohm pull up resistor on A5
P86: a USB C to A adaptor should have a 5.1K ohm pull down resistor on A5

Seems the ‘magic’ cable is wired as an adaptor; so, I got an adaptor

to try with the original USB A to A cable and now even the old unpowered hub can be used from a USB C port.

The non-standard cable that came with the new hub had made me imagine that such cables were easy to obtain, but such was not the case, and my intent here is to help people avoid this rabbit hole, save all the time and money I put in before locating the rather simple solution of using an adaptor. Who would have imagined that USB C to A cables and adaptors were wired up so differently?

I guess this could be considered a repair or modification to the old hub so that it could work from a USB C port.

This article was written by Anwar (Andy) Shiekh originally from London, England; he repairs things to help make an income go further and presently teaches Physics in Colorado, U.S.A.

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Note: You can check his previous repair article on 92mm CPU Fan Replacement Project

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3 Comments

Albert van Bemmelen

May 4, 2024 at 8:46 pm

If I ever will buy an Apple or IMac computer, which I doubt heavily! (My Minisforum Ryzen 9 minicomputer with 64GB DDR5 and 1TB SSD is probably faster!), this info may come in handy! Thanks for sharing and the warning that comes with using these USB-C to A adapters.

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Parasuraman S

May 4, 2024 at 10:17 pm

The information contained in this article is very unique and many thanks for sharing this article, which otherwise would have kept me puzzled if and when such a case arises.

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Yogesh Panchal

May 5, 2024 at 12:42 am

Good Job! Anwar,
Thank!for sharing the Info.

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