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I wired up some USB-C 2.0 to USB-A 2.0 cables, and tested the maximum length I could get out of them when connecting a PC (host, USB-A) to an external hard drive (device, USB-C). I got stable power and data connections up to about 7 feet. Longer than that, I was able to make a power connection (light on external hard drive illuminated, hard drive spinning) but the PC would not recognize the hard drive. I am guessing the failure happens with the data connection? Or maybe there just wasn't enough power, I'm not sure. The USB-C connectors I use are these: https://i.stack.imgur.com/WNDgG.jpg

USB-C spec dictates I should be able to get about 12 feet of length from this assembly, so I am trying to figure out if there is something I can do to make a stable cable that reaches 10 or 12 feet. I am using 28AWG wire, which seems to fall within spec (though on the low end for power).

Any suggestions would be welcome! Thanks in advance.

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  • \$\begingroup\$ What do you mean "using 28AWG wire"? Do you use a special USB 2.0 cable with twisted and shielded green-white pair for data? \$\endgroup\$
    – Ale..chenski
    Oct 6, 2017 at 4:37
  • \$\begingroup\$ @alichen, yep...I use a USB 2.0 compliant cable that has two 28 AWG power conductors plus one 28 AWG twisted pair for the data lines. I have been experimenting with this tonight, and I ripped apart an iPhone charging cable (which has 24AWG power/28AWG data) and tried with that. I was able to get a stable connection at 10-feet (the length of the cable I had on-hand), so I'm guessing the problem is that the 28 AWG wires just aren't delivering enough power over the longer lengths? \$\endgroup\$
    – rockwell
    Oct 6, 2017 at 5:09
  • \$\begingroup\$ USB 2 uses relatively high frequency, when it's in high speed mode. It could be that you have problems with reflections inside of the cable. What test equipment do you have availble? It would be good uf you can see he eye diagram on the cable. \$\endgroup\$
    – AndrejaKo
    Oct 6, 2017 at 5:16
  • \$\begingroup\$ Consider using a "USB extension boost cable", as it boosts the data signal. \$\endgroup\$
    – Optionparty
    Oct 9, 2017 at 1:59

1 Answer 1

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The 28AWG wire has resistance of 65 mOhms/ft, see Wikipedia. With 7 feet two ways (ground and VBUS, = 14 feet) the total parasitic resistance added by the wire is about 1 Ohm, if account for connector contact resistance.

An external hard drive can take 800-900 mA of current, some up to 1.8 A when spinning up, which would degrade the input voltage by about 1 V (1A over 1 Ohm). This drop would be out of specs and is usually devastating for mechanical hard drives.

So yes, in this particular case the weak spot is power delivery over 28AWG wires. The next hurdle would be the impedance (mis)matching across the cable data wires and soldered connectors.

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  • \$\begingroup\$ Thanks for the detailed explanation. Very helpful as always! \$\endgroup\$
    – rockwell
    Oct 6, 2017 at 12:59

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