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From what I understand, USB has 4 lines just like an audio port. So why can't we have a USB port which is circular? I feel it can be more convenient to plug in and also would have a smaller form factor. May be, we can have the same 3.5 mm jack port switch between analog audio output and a digital USB function. What am I missing?

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    \$\begingroup\$ consider a USB type C plug, as a round connector which has been hammered to get a lower height \$\endgroup\$ – Tirdad Sadri Nejad Oct 23 at 11:47
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    \$\begingroup\$ Shorting things in and out, needing a deep connector, the list goes on... (already covered in the good Answers you've received). Have you noticed connectors have long since evolved away from 1/4" and 3.5 mm 'stacked contact' style and into side-by-side contacts. No-one's trying to go in the opposite direction, which you're proposing. \$\endgroup\$ – TonyM Oct 23 at 12:20
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    \$\begingroup\$ USB-3 has way more than 4 lines. \$\endgroup\$ – brhans Oct 23 at 14:13
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    \$\begingroup\$ There is nothing new about Apple inventing pointless, non-standard hardware interfaces simply because (1) it can and (2) it means that Apple aficionados have to to buy more merch. \$\endgroup\$ – alephzero Oct 23 at 20:39
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    \$\begingroup\$ That would never work. With a round connector it could be inserted in any orientation, and part of the USB spec is the first 2 attempts to insert the connector must have a chance of failing... \$\endgroup\$ – Harper - Reinstate Monica Oct 25 at 2:41
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Rotatability while the USB connection is already established is a feature you don't want:

While for audio frequencies, the electric ringing caused by imperfect contact are benign enough to not matter, I wouldn't trust that to be true for 1 Mbd (low-speed USB1), and certainly not at 12 Mbd (Full-Speed USB1.1), 480 Mb (High Speed USB2), or the gigahertzes of bandwidth that USB3.0 and later generations use.

In fact, for the latter, you need pretty well-matched connectors that are designed to conduct an RF signal instead of a current. Building such connectors even in rotatable form is, if at all possible (never saw a rotatable connector that has more than one waveguide, e.g. coax connection), extremely expensive. Think of > 20€ per connector, even in larger quantities, and, due to physics, probably not very compact.

I was surprised to find out there's actually USB3 slip rings! They look like this:

UM0155

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    \$\begingroup\$ They do make twinax cable, connector, etc. They are a bit on the expensive side though. For one (admittedly somewhat extreme) example: milestek.com/… \$\endgroup\$ – Jerry Coffin Oct 24 at 20:54
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    \$\begingroup\$ nice! That'd be at least one of two differential superspeed pairs for USB3 :) I love how literally every part of this connector (see the drawing PDF) screams "defense/aerospace" \$\endgroup\$ – Marcus Müller Oct 24 at 20:58
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    \$\begingroup\$ Yes--my experience with them is with MIL-STD-1553 and NATO STANAG 7221 communication (where 7221 is defined specifically to run over existing 1553 wiring). The price tag alone seems like a pretty solid military/aerospace-only indication, at least to me. \$\endgroup\$ – Jerry Coffin Oct 24 at 21:09
  • \$\begingroup\$ I wasn't even aware of STANAG 7221; pretty new (read: twice as expensive) stuff indeed. (pretty low specs in terms of data rate by modern communication standards, though) \$\endgroup\$ – Marcus Müller Oct 24 at 21:17
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    \$\begingroup\$ @MarcusMüller no it wouldn't \$\endgroup\$ – user253751 Oct 26 at 10:32
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USB connector is specifically designed to allow for safe sequence of pins contacting firmly for hot plugging and disconnecting, passing the rated amount of current, and not being able to connect incorrect cables or devices.

None of this applies to TRRS connectors. It will short out various pins during insertion and removal. It will rattle and crackle after some amount of mating cycles. And since 3.5mm connectors have been used for headphones, mics, line level audio, IR transmitters, IR receivers, and even for power supplies, there is a good chance two incompatible devices are plugged together and it does not work or causes damage.

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    \$\begingroup\$ iPod Shuffle 2G did it \$\endgroup\$ – endolith Oct 23 at 17:21
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    \$\begingroup\$ @endolith that one used a docking station though, that connected via USB and (presumably) made solid contact with the pod before attempting to shove data through a 3.5, then performed a complex mating ritual to identify itself as an Apple USB docking port and then used now affixed 3.5 wire pair as a PHY for data transfer. If it was a dongle form, I presume the data rates would be abysmal to non-existent during times the device was moving. \$\endgroup\$ – John Dvorak Oct 24 at 7:13
  • \$\begingroup\$ @JohnDvorak all true \$\endgroup\$ – endolith Oct 24 at 10:13
  • \$\begingroup\$ Round doesn’t necessarily mean T(RR)S. Power supplies often use coaxial connectors to prevent shorting. A variation often found on laptop power supplies has a pin in the center of the power plug. One could build on that, giving that pin a tip and sleeve, with a wide-enough insulating ring in between to prevent shorts during insertion. Or one could put separate contacts on the inside and outside of the barrel in addition, or even have multiple concentric barrels. OK, this does not scale well above 4+ contacts, and there may still be signal quality issues if the plug is rotated while inserted. \$\endgroup\$ – user149408 Oct 25 at 15:59
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There isn't really any reason we couldn't have a round connector. But there are reasons why the existing connector designs are better:

  1. It's hard to have a round connector with shrouded contacts. On all USB connectors it is physically very difficult to touch or short out the actual electrical pins. Even if the cable brushes against metal surfaces it is unlikely to happen. You would need to intentionally jam something metallic into the connector to achieve this. This was of particular importance when USB was first designed as ESD protection wasn't very good, and the power supply for the port was not switched (as it is now for USB-C). Having an unshrouded connector like the 3.5mm jack would have required extra protection for the pins and this would have added cost. There can also be more corrosion problems when contamination can be more easily transferred to the pins (e.g. Ligthning connector) and the pins are more susceptible to mechanical damage.

  2. Hot-plugging is harder. This is not dissimilar to the first point, but in USB connectors the ground pins (and shield) are designed to connect before signal pins. If you have a 3.5mm type plug then the tip contact will run through the other contacts before it reaches the correct one. This can be dealt with, but it would require more protection circuitry and hence more cost.

  3. The width of the connector isn't that important. Most devices you'd connect to are not round. They are normally rectangular and in most cases the critical dimension is thickness (e.g. phone, flash drive). If you had a round connector you'd have to make it longer to hold all the contacts at the expense of the width. For most types of products this is not a desirable tradeoff.

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    \$\begingroup\$ refuting points one and two, consider my favorite connector: bnc. \$\endgroup\$ – hildred Oct 26 at 2:08
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    \$\begingroup\$ @hildred And just how many discrete contacts does your beloved BNC have? \$\endgroup\$ – T.J.L. Oct 26 at 12:56
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Why? Let us count the ways...

  • Ease of manufacture (stamped pieces, single-plane wiring)
  • robust (flat wiping contacts)
  • safe contact sequencing (ground/power first, signals last)
  • protected contacts
  • overall shielding
  • good signal integrity (defined impedance)
  • low cost
  • can be stacked (better backpanel density)
  • can be a circuit board (dongles, converters)

and perhaps most important...

  • USB adheres to a defined, open standard.
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  • \$\begingroup\$ I think that the power and ground pins (in a USB A connector pin 1 and 4) are longer and hence connect first so that the device is powered before the data lines are connected. Not sure whether this is to prevent voltage spikes on the data lines when connecting power later, or in order to give a simple controller the opportunity to pick up an initialization sequence right away. Either way, in order to power the device +5V as well as ground are connected first. \$\endgroup\$ – Peter - Reinstate Monica Oct 26 at 15:02
  • \$\begingroup\$ if you connect the positive power and any of the data cables before the ground, current will shoot through the ESD protection diodes and explode them, and maybe other things on your board \$\endgroup\$ – BeB00 Oct 30 at 0:04
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I have actually seen USB carried through a 3.5mm TRRS connector, which is also used for headphones, so it's possible. This is on a waterproof MP3 player, where adding another sealed socket would add cost and bulk. The other end of the cable is a standard USB-A, so both the device and the host computer must be able to handle hot-plugging. I have no idea of the data rate this device tries to obtain.

In general, running power over male T(R)(R)S connectors is a bad idea. I've got hardware in work that does, and is also designed to be used on metal tables (photodiode modules and optic tables). Geometry means shorts are rare, but they do happen, especially if the tables are earthed.

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    \$\begingroup\$ The iPod Shuffles (except first generation) also use that kind of connector; I've used the MP3 player you're thinking of or at least a similar device. The data rate feels USB 1.0-ish. \$\endgroup\$ – mu is too short Oct 24 at 5:01
  • \$\begingroup\$ @mu that seems reasonable, and not too bad if you don't make a habit of it. I've never used mine - I got it on a whim for next to nothing, and by the time I came to use it the battery wouldn't charge \$\endgroup\$ – Chris H Oct 24 at 6:21
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It's not an effective use of space.

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