I know that each side is on a different voltage (2 and 3?) but is their anything reason why USB has a top and bottom? It seems like it would be pretty simple to design a system that could go in anyway. Why was this not done?
It would require 7 contacts instead of 4. Since price and size are always issues, and the ability to insert the connector both directions is pretty pointless, practicality won out.
The only sort of plug with more than two contacts that can physically go in any which way would be round like a headphone plug.
Oddly enough, such plugs exist with 4 contacts.
Designing a multi-contact rectangular plug that can be plugged in multiple ways would be a design headache and would be needlessly complex and expensive.
That’s not true - any plug which is symmetrical can be made non-polarized. Just put ground in the center and duplicate the other signals outward from that pin on either side. it’s just not worth doing.
I understand this, but why couldn’t they shape the connector differently so it would be obvious at a casual glance which way it should go in? Put a notch in it, or something?
Almost (and yes, I’m using that weasel word on purpose) every type “A” USB connector (the type I assume you’re on about) I’ve ever seen has a seam where the edges of the metal forming the outer sleeve of the connector join. This is almost (again) always points “down,” i.e., the same direction as the “root” end of the USB symbol (toward the left in the linked illustration).
There are some very thin USB sticks that lack the outer metal casing. These can be inserted both ways but since there are only contacts on one side if inserted the other way around they will not work.
It would be trivial to add contacts on the other side too, so the stick would work either way. Maybe somebody has already though of it and there is such a usb stick on the market.
Probably they didn’t think of it at the time they designed the socket. Maybe they thought that most USB devices would be plugged once and left plugged for a long time. Besides, back in 1995 when the USB standard was introduced we didn’t have all these fancy devices like card readers, dongles, sticks etc.
Yes, but some computers have the sockets upside-down 
A cable could be made with four contacts like USB, which would have identical connectors at both ends and on the devices that it is being plugged into, therefore being both easily daisy-chained and without having to deal with male-female ends. Picture a plug which has both normal stereo headphone jack (two contacts) and a receptacle for such a jack (two contacts.) You’d be able to connect them in a chain forever, plus you’d never need an awkward male-male or female-female adapter. I’m still waiting for someone to start using these; they could be easily assembled from already-existing parts and just be so much more convenient.
I would be leery of such drives. My understanding is that the outer shell of the contact area serves as the electrical ground, ensuring that the drive is grounded before any power or data starts flowing to it. This was, again as I understand it, a primary design criteria for the USB standard, to avoid the weaknesses of connectors like RCA plugs where the ground is actually the last thing to make contact.
IANA Electrical Engineer.
Wait, what? What does the the “root end of the USB symbol” have to do with the direction in which you insert the connector? I mean, the USB symbol is generally (yeah, Imma use those weasel words, too, we’ve already gone over the fact that computer manufacturers are conspiring against us ;)) on the top of the connector, so the entire symbol serves as a handy clue for the vertical alignment, as do the contacts inside the connector, which are generally on the bottom, and the seam which you’ve already mentioned.
However, and maybe I’m just dense this morning, using the USB symbol as a horizontal guide doesn’t really make any sense to me, as it should be fairly obvious which side, horizontally, the connector should be inserted. :dubious:
I’m referring to the symbol next to the jacks on the computer, not on the flash drive (or whatever).
Good quality USB cables have a metal braid beneath the insulator, like in coaxial cables. The outer shells at each side of the cable are connected through this braid.
But in most USB cables I’ve seen the braid is missing and the outer shells aren’t connected anywhere. So I guess it isn’t that important.
Ohhhh, gotcha. That definitely makes sense for computers with vertically positioned USB ports.
I guess that hadn’t occurred to me because all of my recent computers (notebooks, the lot of em) have had the USB ports positioned horizontally, which makes the symbol on the port itself less useful.
You could physically make such a cable, but by definition it wouldn’t be a USB cable, nor work with USB devices.
USB was designed on purpose so you can’t (easily) daisy-chain them, because that wouldn’t work (for timing and electrical reasons), and so you can’t (easily) plug two peripherals or two hosts into each other, which also won’t work for even more fundamental reasons.
Which end did you check? The shield in a USB cable is only supposed to be connected at the host (i.e. type A) end.
Whenever I try to plug in a USB cable I wonder about the design.
It’s not like they were designed in the stone age or anything – we aren’t talking about Univac mercury delay line memory.
You would think that part of the criteria for choosing a good connecter would be how goofproof it is; can you plug it in while reaching behind a machine without bending pins or putting it in backwards.
USB is close—there are no pins to bend, but it really takes some fiddling to get the connector in the right orientation. But even when you think you have it right, it’s hard to tell.
A-Ha! but there is such a thing!.
Most people when they think about USB plugs they think about the USB-A plug, but there is also a USB-B plug that is square with chamfered edges at the top. You can see one at the bottom of this page.
Usually the USB-B connector are found in peripherals, printers, external DVDs, etc. For example, the microcontroller programming board I have has a socket for a USB-B plug in it.
USB connectors are designed so that the two outer pins which carry the power are longer and make connection before the two inside data pins which are shorter.