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For a side project I was planning on using a connector with about 20 lines/pins and as small as possible. The USB-C popped into my mind since it contains many lines (24), it's very cheap nowadays and is readily available everywhere.

I created the breakout board below: enter image description here

And tested the conductivity between the different lines when using two of these breakout boards at both ends and a standard? USB-C cable (one used also for data transfer, so I know it is not only power). It seems that I get electrical conduction only between GND pins on both ends and between the VBUS pins. I also put a sine throug the VBUS line and it seems heavily filtered.

I was already aware of the existance of some sort of front-end conversion chip present in many? all? USB-C cables, so I suspect that without that chip being powered up thanks to the voltage usually present in VBUS, the rest of the lines are "blocked". Is that assumption correct?

If that is the case, the other question would be... is there any usb-c cable that would allow me to use it in the way I want? Some sort of "just-the-lines" USB-C cable.

Thanks.

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  • \$\begingroup\$ If the signals are high frequency, this may be susceptible to crosstalk. \$\endgroup\$
    – mr_js
    Mar 1, 2022 at 11:33
  • \$\begingroup\$ Yeah that's a problem I was expecting to see, but might be ok for my purpose. But at this point I'm not sure at all if it's possible since all lines seem "blocked", even for the DC conductivity test. \$\endgroup\$
    – Frilance
    Mar 1, 2022 at 11:41
  • \$\begingroup\$ Basic cables don't have any "blocking" capability or any active circuitry. Are you sure the lines are connected? \$\endgroup\$
    – Finbarr
    Mar 1, 2022 at 11:42
  • \$\begingroup\$ Also bear in mind the USB-C plug can be fitted in two different orientations so the connections may be swapped over. \$\endgroup\$
    – Finbarr
    Mar 1, 2022 at 11:56
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    \$\begingroup\$ Using standard connectors in non-standard applications is rarely a good idea - particularly if you're planning that anyone other than you will use your product. Will your device survive getting plugged into a USB 'charger'? Will some other USB device survive being plugged into your device? Even if it just does nothing (without letting any smoke out), it's still a poor user experience. \$\endgroup\$
    – brhans
    Mar 1, 2022 at 12:37

1 Answer 1

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The USB-C to USB-C cable can't be used as an arbitrary transmission line.

The VBUS bandwidth is limited because the cables are mandated to have bypass capacitors between VBUS and GND.

There are also different USB-C cables. A standard cable only connects the old USB 2.0 data pins as differential pair, in addition to the VBUS, GND, Vconn, and a single CC pins. A full featured cable will also connect the high speed data lanes. Still, two sets of USB 2.0 pins are reserved on the connector to allow cable flip but the cable itself uses single USB 2.0 data pair.

So no cable will connect all pins, and each type of connection uses different kind of cabling (single wire, differential pair).

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  • \$\begingroup\$ Thanks. I still don't understand why I did not see conduction in the 'Dp' 'Dn' pins of the cable I used if those USB 2.0 data pins should be present in all cables. I can't find 'Vconn' and 'CC' pins in the pin layout I'm using: we-online.com/catalog/datasheet/632723100011.pdf are there different naming conventions? \$\endgroup\$
    – Frilance
    Mar 1, 2022 at 13:20
  • \$\begingroup\$ I already said that the connections depend on cable orientation. DP1 will be connected to DP1 only if both ends of the cable have correct orientation to connect DP1 to cable, so plugging the connector random you have 25% chanse to see connection. \$\endgroup\$
    – Justme
    Mar 1, 2022 at 13:23
  • \$\begingroup\$ Oh yeah sure, maybe that's what happened. But I think I did try flipping the cable ends to get all 4 possibilities. I will try again just to be sure. But if I find the right orientation and get my hands on one of the "full-featured" usb-c cables... what's stopping me then from using it as an arbitrary transmission line then? Given I always stick to the same orientation of the cable ends, as I said. \$\endgroup\$
    – Frilance
    Mar 1, 2022 at 16:21
  • \$\begingroup\$ @Frilance Nobody stops you but you might think differently when you or someone else accidentally connects it wrong or to a standard computer, mobile phone or charger and something blows up. \$\endgroup\$
    – Justme
    Mar 1, 2022 at 18:07
  • \$\begingroup\$ it's a rather personal project. No one is going to touch it but me. Thanks a lot for the input! \$\endgroup\$
    – Frilance
    Mar 1, 2022 at 18:13

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