USB-C done cheap: when 2 ports become 1 (2020)

Also, a device’s settings that depend on the port to which it is connected is terrible UX.

By that I mean that when you leave, say, your upstream device and run the traces to downstream, you need to have them come out in the same order: if you start with _P on the left and _N on the right, you want to end that way. A swapover is annoying at USB 2 480Mbps speeds and utterly awful at 20Gbps. (There is also a >50% chance that you miswired the thing and are wiring _P to _N, which increases to >99% if the routing came out naturally neat, but that's just Murphy's Law in action.) It's so difficult and awful to deal with this stuff that USB 3 took a page from Gigabit Ethernet and usually features transceivers that just figure it out: they know you have eight wires in four pairs between eight pins and they can work with that.

Once you have that feature, and transceivers definitely do, there is no point whatsoever in trying to make superfluous changes at the connector bit.

Would the splits and merges definitely make the signal integrity worse to the point that it wouldn't work? Maybe not. But if you're designing a high speed cutting edge standard, you probably don't want to make decisions that limit the best signal integrity an implementer could achieve. Signal integrity is all about balancing the bad SI decisions you have to make because of some product goal or whatever with tighter tolerances elsewhere. The more "clean" the standard is, the more tolerance it has for the implementers.

What it comes down to is that once your signal rate is high enough, the speed that electrons move becomes a concern. Matching the length of your traces ensures that a signal sent down two wires at the same time arrives at the same time.

That's the general idea anyway. This topic gets very complex the further down you go

https://en.wikipedia.org/wiki/Transmission_line

It was surprisingly hard to gather all of this information in one place and actually implement it given the resources on the internet a few years ago. Even then, there were still some board revisions that got it a bit wrong, one where I left out the decoupling capacitors I'd later learn were a critical detail, and one where I put all the wiggles clumped together at the middle of the shorter trace and so despite both signals arriving at the end at the correct time, they were out of phase for nearly the whole trace run and caused (presumably) cross talk between each other. But I did get it working in the end :)

(* Digi-Key Marketplace is not as trustworthy as regular Digi-Key and has pissed off a great number of their customers. Engineers buy from Digi-Key to have guaranteed parts from guaranteed vendors shipped with guaranteed delivery rates and dates in the same box as everything else in the order. Most vendors are of of course perfectly good -- there are some huge names on there -- but in general Digi-Key Marketplace does not do any of this. It cuts against the Digi-Key brand and has not been a net positive for Digi-Key, in my and many others' opinion. I know I'm ordering a lot more from Mouser recently, for this and other reasons.)