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I plan to make debug probe that allows me analyze traffic on USB Type-C cable. I want to make blue middle board from the following schametic:

Block diagram of global device connection overview

From my point of view right port of the my board is upstream (UFP) and left is downstream (DFP). Do I need to cross CC1/2, SBU1/2 and SS differential channels wires on my board? Currently I plan do something like this:

Schematic of debug board

I did not crossed CC wires and I conencted them directly. But I crossed SBU lines because it is easier to route them on the PCB. Can I do this? I thouhgt about that in the way that because USB Type-C allows user to plug connector in two ways, crossing wires can occur naturaly. But can I cross only some wires and some dont (cross SBU and do not cross CC)? Will this connection work? I also crossed Super Speed diferential channels. Did I do this correctly?

Misaz
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  • Technically, your middle board is supposed to be a "piece of the cable", which it replaces. If anything crosses in the cable, you cross it there. If it doesn't, why would you? – Ilya Mar 09 '22 at 22:34
  • Yes but originally single cable turns into three cables and multiple crossings. So I am asking if my approach is correct. According to slide 10 [USB Type-C® System Overview by USB Implementers Forum](https://www.usb.org/sites/default/files/D1T1-2%20-%20USB%20Type-C%20System%20Overview.pdf) cables should do some crossings. It also behave differently when I choose to use one port as a plug instead of receptacle I think. – Misaz Mar 09 '22 at 22:52
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    You're not having plug or receptacle. You're replacing the middle of the cable. So you must emulate the cable. If there were a cable, how would the lines be connected? You do the same, doesn't matter if you have plug or socket there, you're replacing a piece of cable, so for the signal you need to "feel like" cable. Of course, except for twisting stuff, which may or may not introduce too much signal noise. I'd say if you go full USB 3.1 and faster, signal integrity should be a greater concern. Can't give much advise on that. – Ilya Mar 10 '22 at 07:20

2 Answers2

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You should cross signals exactly like a USB-C to C cable would. The first cable will cross the signals, but so will the second cable, so if you don't also cross the signals on your passthrough board (effectively 'resetting' the first cross by the first cable), you'll end up with signals crossed back to their original positions - and opposite what you need.

Your schematic looks correct, and you should be crossing the SBU wires. The CC wires should be connected directly as you have done and not be crossed. Basically, your schematic as given looks to be 100% correct.

Here is a table that tells you exactly which signals should be connected to which between the two connectors on your board:

enter image description here

metacollin
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Your idea is using C receptacles with passives in between. This is not a recommended way of working, because it comes with many pitfalls such as user errors. First, the user is not allowed to mix and match the types of cables, if the user uses an extended power rating cable at the charger side with a 3A cable at the device side, the 3A cable won't be happy when 48V and 5A is pushed over it, while it is designed for 20V and 3A (its VBUS plug capacitor might not be rated for 2.5 times the voltage) You also have problems with orientations, the user needs to plug both sides in with the same orientations, else the CC wire won't be connected between the sides. This solution also fails is active cables are used, as one of the active cables won't receive power, which means only USB 2.0 works. USB C is just not make with C receptible to C receptible cable extenders in mind

Because all these restrictions in connecting wires, I highly recommend, you change one of the sides to an USC plug, this makes many of the above points a non issue.

When wiring a receptible directly to plug, wire up all connections strait through, no flips needed like you did for the USB 3.0 or SBUS wires. Make sure all your components are rated for the current highest USB C standard, which is 5A and 48V, just to make sure your device survives all user connection ways

Ferrybig
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