USB Connection Cables

Hi All,

I’m certain that many members of this group have a quick answer to this question.

At the moment I find the numerous USB adaptors on Amazon just too confusing. I have a 2015 iMac (Retina 4K, 21.5-inch, Late 2015). I recently bought an iPad Pro (12.9) and I should soon receive an iPhone 13 Mini. What adaptor do I need to be able to connect these new devices to my iMac for backup and charging. I only rarely do this but it is one of my backup strategies for my portable Apple devices.


For the iPad Pro:

For the iPhone:

Or one of these to use the included cables with your Mac:


Thanks Simon. I knew you’d have the answer I needed and be willing to share.

Note that some of those A-to-C adapters are USB 2.0 and some are 3.0.

If you take a look at the USB-C pinouts, you will see that the data pins used for USB 1 and 2 traffic (pins A6, A7, B6 and B7) are separate from the pins used for USB 3 “superspeed” traffic (pins A2, A3, A10, A11, B2, B3, B10, B11).

An A-C adapter that only wires the USB 1/2 pins will only support USB 1/2 traffic (up to 480 Mbps). An adapter that only wires the USB 3 pins will only support superspeed traffic. You’ll need a cable that wires both sets if you want to use it with devices of either speed.

So be sure to look out for this in product descriptions when you buy your cable/adapter.

Note that (from Apple’s spec sheet) the current iPad Pro supports USB 3.1 gen 2. You’ll need a USB 3 adapter in order to reach this speed via an A-C cable/adapter.

Thanks, @Simon. I’m always surprised at how fast you can respond with links.

For the “USB C Female to USB 3.0 Male Cable Adapter 2 Pack,5Gbps GEN 1 Type A…” cable, the description reads, in part,

Turn your legacy USB-A devices (charger, power bank, computer) into a USB-C enabled platform with this cable adapter while adding an additional 8.5 inches.

I thought I had read (long ago) that there was a relatively short maximum length for a USB 3 cable. (I also seem to recall there was a discussion of active and passive cables, and I think the length restriction was for passive cables, which were much less expensive.) If my memory is correct, does the additional 8.5 inches come at a performance cost?

If I connected a USB 2 device using a cable with USB 3 pins, would it simply fail to work?

I would like to think that any USB 3 cable will also wire the USB 2 pins, but if there is one that doesn’t, then you’re right, it wouldn’t work at all because there would be no physical connection for the signal.

According to Cable Matters, the maximum length of a basic (passive) USB-C cable depends on the speed of the signals carried:

  • USB 2.0 data has a maximum length of 4m (about 13’)
  • USB 3.2 Gen1 / USB 4 Gen2 (5 Gbit/s) has a maximum length of 2m (about 6.5’)
  • USB 3.2 Gen2 / USB 4 Gen 2 (10 Gbit/s) has a maximum length of 1m (about 3.3’)
  • USB 4 Gen3 / Thunderbolt 3 (40 Gbit/s) has a maximum length of 0.8m (about 2.6’)

These lengths are (as far as I know) approximate. I don’t think the USB spec mandates a maximum - they only specify the data rate and signal quality required for a finished cable (of any length). So you may find longer cables that work fine, or you may find that they work at reduced performance, or perhaps not at all. Which is why it’s a good idea to look for USB-IF certification for cables - they should be guaranteed to support the speeds specified by its certification.

An active cable can support longer distances over copper cable. According to the article, you can get active USB 3.2 Gen2 (10 Gbit/s) cables up to about 3m (a little less than 10’) and active Thunderbolt 3 (40 Gbit/s) cables up to about 2m. But these cables can be expensive and even with active transceivers, you won’t be able to get more than about 5m without signal degradation and data loss.

For longer lengths (3m or more), you pretty much need optical cable. These are very expensive because they include electical-to-optical transceivers in each connector.

As for the adapter in question, I’m not sure what it actually is. The photos and description of containing a “USB 3.1 chip” would imply that it is an active cable, but the product description doesn’t actually use the word “active”, so it’s anybody’s guess. To be on the safe side, I’d add it’s 8.5" length to that of whatever you’re connecting it to and treat the combined cable as if it was a passive cable.