WhatCable: Know what your USB-C cable can do

We get it. A cheap tester will not tell me if a cable from a dodgy supplier is counterfeit. It will tell me what the capabilities are of the dozens of assorted cables I own from reputable manufacturers, which is otherwise difficult if not impossible to discern.

What the chip reports is itself inherently useful information, especially if you have cables from reliable manufacturers.

If that's the extent of the thing that they want to do, then there is nothing further to discuss: They already have everything they need to begin that kind of evaluation immediately.

It doesn't give them any viable forecast as to whether the cable will work or not with some other bits of gear, though, or on some other day. It just evaluates it as a binary pass/fail for right now.

"Digital" cables are often dismissed as simple things, wherein: A string of bits goes in one end of a random collection of copper wires or metal coat hangers or whatevers, and it always comes out the other end the same way. After all, it's digital.

But reality isn't that way at all, and there's a lot that goes into a reliable cable that connects things together. Characteristic impedance is only one of the things[1].

---

Interrogating the emarker doesn't provide further practical information, either. The information that can come from there is similar to what wording on the package on the shelf at the store is: Both things merely document what someone else once said about what the cable should be capable of doing.

Like a printed URL vs a QR code that each lead to the same web site: It's the same information, differing only in presentation. One of these presentations is human readable and other is not.

And neither thing neither thing tells the user whether that website is still reachable or whether it fell off the face of the earth, or whether it is friendly or evil.

It's all just a fixed string of information and that information can be wrong. It can even be deliberately wrong. The words on the box, the bits burned into the emarker chip, the URL, and the QR code: In the best case, they may or may not lead to a useful outcome.

In the worst case, they can be lies.

emarker chips set a hard not-to-exceed performance limit, but they don't provide a promise of performance below that limit.

---

[1]: Like the RG-6 cable connecting a cable modem to the wall: It functions with a very specific characteristic impedance of 75 Ohms. Or the SATA cable in the desktop computer of someone with some Linux ISOs: That's twinaxial cable, with its center pair of straight, parallel wires and the shielding all interacting with eachother at a controlled impedance. USB 4, meanwhile, uses cables with a characteristic impedance of 90 Ohms.

Characteristic impedance can be complex, and even dynamic. Like the ye olde POTS phone lines, for voice audio: That was designed and built very specifically in order to behave itself within those seemingly-minor constraints. 600 Ohms at 1 KHz was the target, and that was good enough for a very long time. But when frequencies increase into the MHz range with things like DSL, that characteristic impedance tends to drop to somewhere in the realm of 100 Ohms.

100 Ohms is, not coincidentally, around the same characteristic impedance that our Ethernet cables seek to provide at the frequencies we care about in that space. :)

Whatever that characteristic impedance is (at whatever frequencies are being used), when it matches what the termination impedance of the two endpoints are then we get an ideal transmission line. (That's always the target; an ideal transmission line is a glorious thing, indeed.)

But bad impedance match results in not just seemingly-obvious things like attenuation; it also introduces things like reflection -- signals bouncing back and forth, causing temporal distortions that obscure the desired signal. Bad matches also result in standing wave patterns where the length of the wire makes a difference, but shorter isn't better: The pattern of these standing waves within a wire are a function of the wavelength of the signal, not the overall length of the cable.

It's easy to make any of those things work within their designed uses and designated systems, where all of the effort to get this stuff right happens behind the scenes.

It's much harder to replace with bits of copper wire from the junk drawer and get reliably-good results.

For 1KHz in modern-ish POTS lines, that wavelength is around 40km. That's easy-enough to hand-wave away for lots of uses. The impedance matching can all be completely screwed and it really doesn't matter much for short runs.

But at 12.8GHz (the frequency maximum of USB 4) in a USB cable, the wavelength is only around 16mm. It's kind of a big deal to seek to have the little parts exactly-right in these seemingly-innocuous, hard-to-see ways. :)

And the impedance changes in-use, too. A kink in the wire produces an impedance aberration at that point that can be measured, and that can also be bad enough to produce practical issues that affect real-world performance.

Bad enough to fail the binary test you described today, even though the test was completely successful yesterday.