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USB Cheat Sheet

fabiensanglard.net

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Re: USB Cheat Sheet

#161

Post by Aachen »

cesarb wrote: post #125
kevin_thibedeau wrote: post #88

Earlier quoted context omitted.

I doubt any normal hosts will enumerate a USB device without the USB 2 data lines. PD will definitely not work. You might be able to get an alt mode running.

They probably would enumerate just fine, because the SuperSpeed enumeration is completely independent from the USB 2.x enumeration. USB-PD uses a separate pin/wire, not the USB 2.x D-/D+ pair, and you cannot get an alt mode running (except the special analog audio and debug accessory modes) if USB-PD doesn't work, since alt mode enumeration goes on top of USB-PD.

I'd actually be interested in a 3-only cable. Force it to use high speed or bust, no surprises, no guessing at which proto runs under the hood on that "universal" connector. Didn't know it was possible; I might make one once I've got a spare cable (just today I've got another spare but they always have just 4 cables).

More for geeky/testing purposes than to replace all my cables, but still

Re: USB Cheat Sheet

#162

Post by zamadatix »

can16358p wrote: post #20

So on the next versions of USB, the cable length will get shorter and shorter until the max gets to 5cm? While I get the technical reasoning about high frequency/attenuation etc that limits cable length as speeds go higher, there are obviously some practical limits to how short cables can be. How would that be solved, I don't know.

I'm confused what that section is supposed to represent. E.g. Apple has a 3 meter USB 4 3x2 (40 Gbps) cable but the "cable" value for that section is listed as 0.8m. The only hit I'm getting in the USB 4 spec for "0.8" is on page 59 referring to maximum receiver insertion loss in dB for a gen 3 connection including a 0.8m passive cable but that in itself isn't a hard limitation on cable length.

Re: USB Cheat Sheet

#163

Post by kingcharles »

CharlesW wrote: post #96
jbverschoor wrote: post #82

Earlier quoted context omitted.

Apple just released a €159 cable

FWIW, Monoprice's 1m USB4/TB4 cable (100W) is $25. https://www.monoprice.com/product?p_id=41946

Do you have any of these? The photos show no markings on the cable at all, which is a bummer. If I get some I'll have to put zip tie tags on them to label them, otherwise I'll never remember which ones are the good ones or not.

Re: USB Cheat Sheet

#164

Post by jsjohnst »

paulmd wrote: post #154
Dylan16807 wrote: post #140

Earlier quoted context omitted.

> 3m is beyond the max cable length specified by Thunderbolt, so it requires active extenders (they're hidden in the plugs) and tight manufacturing and shielding. You're paying extra for the ability to break that max length spec, and it's one of only a handful of products that do it. I'm pretty sure it's not breaking the spec. Are you sure about that claim? And the main factor is almost always decibels of signal loss…

> I'm pretty sure it's not breaking the spec. Are you sure about that claim? Actually we're both wrong... it appears max length for a passive cable is 18 inches for full performance. Passive cables technically max out at 18 inches for 40gbps and drop to 20gbps at 2 meters. Past that you need an active cable (which has signal repeaters). Active cables generally run up to 2 meters (the Apple is the first 3m active cabl…

> for example I looked up a 2 meter Belkin cable advertised as TB3 and it doesn't carry the DisplayPort channel

It’s not possible (I think?) for a USB-C cable to support TB3 but not DisplayPort. Both are alt modes on the USB protocol and use the same wires for transmission, so it doesn’t make sense to go the extra mile of TB certification and not support DP too when you already have the necessary wires.

Re: USB Cheat Sheet

#165

Post by Ldorigo »

fabiensanglard wrote: post #110
masklinn wrote: post #10

Some of the entries seem incorrect: "USB 3.2 (USB 3.2 Gen 2x2) and "USB 4" (USB 4 USB4 Gen 2×2) should have the same nominal data rate of 2500MB/s, they're 2 lanes (x2) of 10GB/s. Though they are apparently coded differently electrically, so they're distinct protocols. The tables would benefit from mentioning the coding (8/10 or 128/132) as IMO it's one of the most confusing bits when you see the effective data rates…

Thank you for noticing these issues, I have updated the table. I would be happy to improve it and add encoding. I am surprised by some of the summary entries on Wikipedia ( https://en.wikipedia.org/wiki/USB4 ). Looks like USB4 "reverted" to 128b/132b. It is accurate?

Fyi, last two columns in table 2 are a bit confusing: footnote c says "real life sequential speed", but then the last column title is "real life", so it's unclear what the difference is

Re: USB Cheat Sheet

#166

Post by masklinn »

fabiensanglard wrote: post #143
lxgr wrote: post #131

Earlier quoted context omitted.

128b/132b is the more efficient coding. The closer to 1 the fraction is, the less coding overhead it has, and 128/132 is larger than 8/10.

Actually, I just noticed that 128/132 is the same fraction as 66/64 so both scheme has the same encoding efficiency. So USB-4 did no "revert in terms of efficiency.

Indeed, according to the wiki page the subtetly is:

> USB 3.1 and DisplayPort 2.0 use 128b/132b encoding, which is identical to 64b/66b, but duplicates each of the preamble bits to reduce the risk of undetected errors there.

I guess that was found not to matter so they went back to the more normal 64/66 in USB4? I'm really weak on the hardware stuff so I really have no idea.

Re: USB Cheat Sheet

#167

Post by masklinn »

adrian_b wrote: post #128
fabiensanglard wrote: post #113

Earlier quoted context omitted.

I should add Nominal vs Raw vs Effective speed to the table. Can you confirm with the rule to be used. Raw Speed = Nominal / Encoding UMS Speed = Raw / UMS overhead In the case of 3.0 that would be: Nominal = 625 MiB/s Raw = 625 - 20% = 500 MiB/s UMS = 500 - 20% = 400 MiB/s

The names for the various bit rates vary between authors and standards. I believe that the least confusing names would be: Data bit rate = the rate at which the data bits provided by the user are sent Signaling bit rate = the rate at which bits are sent over the physical communication medium The 2 rates are not the same because the user data bits are encoded in some way before being sent. The signaling bit rate does…

>Data bit rate = the rate at which the data bits provided by the user are sent

> Signaling bit rate = the rate at which bits are sent over the physical communication medium

There is a third one: in addition to the line coding, there's the message framing (at the logical level) e.g. USB 3 has a signalling rate of 5Gb/s, it has a raw data rate of 4Gb/s, but it has a theoretical effective data rate of around 3.2Gb/s (400MB/s).

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