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I am using a USB type C 24-pin connector for normal USB and power.

Can I use the remaining 6 signals for TTL inputs and outputs? Note, my TTL signals are not translated to/from USB, but rather are real-time TTL, and have no latency.

Using just the type C connector for all connections seems simple and robust, thus not requiring any other connector - wonderful!

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    \$\begingroup\$ Which signals you already use and which remaining signals you mean? \$\endgroup\$
    – Justme
    Jul 28, 2022 at 16:00
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    \$\begingroup\$ Consider that the risk associated with plugging your cable into the wrong device depends not only on whether you are generating large enough voltages to damage USB devices, but also whether your device would be damaged by connection to low-impedance loads (trying to drive the same data lines in an opposite direction) \$\endgroup\$
    – Ben Voigt
    Jul 28, 2022 at 16:01
  • \$\begingroup\$ hi, thanks for lively answers. Yes, my 6 signals are mirrored, so can plug in either way, no problem. my 6 signals are all low speed/power 3.3v TTL. also, I know using the 12 (6 pairs) unused pins are available for any signal, and will work fine. you mentioned compatibility with other usb cables, and the issue of plugging my special usb cable into a "standard" usb. My cable will be USB compatible for the gnd, +V, and D+, D-. but rest will not be, fine, likely will a label on my usb cable saying Don't plug into any USB except Excitron motors. I will have +48v max on the Vbus wires. \$\endgroup\$
    – vern
    Jul 29, 2022 at 18:15

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Absolutely, IF you are using a pcb in which you know or can control whether or not those pins are connected to a USB controller or hub on both ends.

Keep in mind the layout of your cable as well and try to select lines/pairs that are as isolated as possible to avoid cross talk. Especial if your TTL signals are high frequency or amplitude.

Connector Layout Connector Layout

Cable Example Cable Example

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  • \$\begingroup\$ thanks! yes, using CC1/2, SBU1/SBU2, Tx1+/Tx2+, Rx1+/Rx2+, Tx1-/Tx2-, Rx1-/Rx2-. my signals are not differential, so volts ref to GND. \$\endgroup\$
    – vern
    Jul 29, 2022 at 18:36
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    \$\begingroup\$ thanks! yes, using CC1/2, SBU1/SBU2, Tx1+/Tx2+, Rx1+/Rx2+, Tx1-/Tx2-, Rx1-/Rx2-. my signals are not differential, so volts ref to GND. Some internet people pointed out that some "standard" USB cables can damage other USB devices, if plugged into a non-recommended USB device. so 100% compatibility does not exist. example: some type C cables have no electronics, just wires. \$\endgroup\$
    – vern
    Jul 29, 2022 at 18:44
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If there's the slightest chance of a real USB-C device being plugged in, it'd be worth evaluating the risks to your board and to the device.

Also, note that USB-C cables aren't polarized, and you'd normally expect to plug them in either way up. I'd use some color marker pens on that nice white connector to make it really obvious that a) it's a non-standard configuration, and b) which way up the cable should go.

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    \$\begingroup\$ Good point on the polarization and adding a marker. Otherwise you would also need to make your custom connections mirrored so they line up in either configuration. \$\endgroup\$
    – Josh
    Jul 29, 2022 at 1:03
  • \$\begingroup\$ @Josh Yep - actually, if it could be made rotationally symmetrical, that'd be best, but if not, at least making sure that the positions of inputs and outputs worked either way so you didn't end up with output<->output. I guess you could add a signal so it could detect if it was wrong - that'd be cool! Might be able to do that anyway using USB-C's own polarity detection... \$\endgroup\$
    – SusanW
    Jul 29, 2022 at 5:38

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