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Why do USB plugs only fit one way into USB ports?

Forgive my ignorance, but there are several types of plugs that are "omnidirectional" and do not have to be oriented a certain way to fit into the corresponding plug (pursuant to shape). In the case of USB, when you're trying to plug one in blind, it can be a bit annoying if you happen to be trying to do it upside-down.

I'm guessing this has to do with the pinouts, but then why doesn't the USB standard just negotiate for "pin 1" when something is plugged in, or use a functionally symmetrical pinout layout?

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MOST connectors in the world only allow one mechanical orientation.

Ones that are not orientation specific are usually "concentric" such as the familiar 2.5 / 3.5 / 6mm plugs on earphones and similar. Where these have more than 2 conductors the contacts for the conductors at the inside end of the socket ride over the conductors for the tip end as the plugs are inserted. Care must be taken to ensure that no problems are cause by these spurious short term connections.

AC power connectors in some systems can be polarity insensitive, but this can lead to safety concerns where there is some difference in attribute between the two contacts other than their ability to provide power. eg in many systems the mains power is ground referenced with one conductor essentially at ground potential. Reversing the twocontacts would still lead to a functioning power connection but may bypass protection and safety systems.

BUT the vast majority of plug and socket systems are orientation sensitive.
Consider the plugs for keyboards and mice (DB9, PS/2, now USB), any 3 pin power plug, trailer power connectors, telephone and network connectors (RJ10, RJ11, RJ45, ...), XLR/Cannon and similar audio connectors, video connectors for monitors ("IBM"/Apple/Other), SCART AV connectors, DMI, ...
People are well used to this.
Why should USB be any different?

BUT, full size USB has two power connectors and two signal connectors. Rhe signal connections could easily enough be interchanged.
But interchanging the two power connections involves routing +ve and -ve signals correctly.
This could be done with a diode bridge and two diodes but the voltage drop of about 1.2 Volts represents a loss of about 25% of the Voltage and an immediate 25% power loss. This could be addressed with mechanical automated switching - essentially relays, or with low voltage drop electronic switches (MOSFETs or other) but the cost and complexity is not justified in view of the ease of "just plugging it in correctly".

Im Mini and Micro USB systems with potentially more conductors this could have been addressed by redundant arrangements of contacts but that wastes potential resources (size or contacts) and still only results in two possible alignments, 180 degrees apart rotationally. You still could not insert it aligned long side vertical or at an angle.


Super Solution:

For the ultimate connector consider these two conductor wholly functionally symmetric hemaphroditic connectors.

  • Not only can these be orientated in two orientations rotationally but there is no "male" or "female" connector - both 'plug' and 'socket' are identical.

This scheme can be extended to more conductors using a coaxial arrangement. This is a General Radio GR874 connector. If you ever meet something using these you can be fairly sure you are in the presence of greatness :-).

enter image description here

Many many more of the same

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    \$\begingroup\$ @Russel - The easier way to solve the power connections problem is evidenced by the MagSafe connectors on Apple notebooks: Just Make it symmetrical by doubling up the connections: [D1] [+5] [GND] [+5] [D2] \$\endgroup\$ – Kevin Vermeer Feb 8 '12 at 16:43
  • \$\begingroup\$ You can't actually just swap the USB signal lines. The USB2.0 standard as far as I'm aware has no requirement for polarity inversion, and the enumeration sequences (as well as reset detection which uses a pull-up resistor on D+) would fail if you inverted the lines, not to mention all the data would get inverted. \$\endgroup\$ – Tom Carpenter Jul 19 '15 at 20:53
  • \$\begingroup\$ This is a great answer. Small note though - the GR874s also connect in only two 180-degree orientations relative to each other. So, they need to be turned up to a quarter turn each (in opposing directions) to be oriented correctly. \$\endgroup\$ – sscirrus Sep 17 '15 at 17:22
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Three reasons:

  1. Backwards Compatibility
    The USB standard was begun in 1994, and v. 1.0 became official in 1996. DB-25 (for "D-subminiature", no seriously) was the standard printer port. Everything since that date is still backwards compatible with the original specification. Making the connector omnidirectional would be an incompatibility, which is unacceptable for the standards organization which regulates USB.
  2. Cost
    As mentioned by mikeselectricstuff, this would add additional cost and/or size. Size is a design goal of USB (as evidenced by the trend from USB-B to USB-mini to USB-micro), and cost is always a design goal.
  3. Logo Placement
    No, really. The USB specification demands that all compliant cables put the USB trident: enter image description here
    on the top side of the connector. Here's a semi-official reference:

    The standard USB trident must be on the top of both plug overmolds as described in chapter 6 of the USB 2.0 specification.

    You'll have to buy the USB spec if you want it straight from the horse's mouth. (Incidentally, I consider that a very appropriate idiom to apply to standards organizations that don't release their standards for free!) If the connector were reversible, the branding might not be visible, or the branding would have to be on both sides, which would make cable manufacturers unhappy.

Incidentally, the last point, while it may seem rather picky of the USB standards organization to demand it, is useful. As described in this Lifehacker article, you can determine which way to connect the USB cable by looking for the trident and orienting this "upwards". Unless, of course, it's dark and you can't see the trident...in which case I recommend that you turn the lights on and move to a position where you can see what you're doing.

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    \$\begingroup\$ All that answer says is "they never have been". You asked WHY they are not - this says that they weren't then so they still aren't now. That's the same reason most people drive to work on the same side of the road each day, but not why a given side of the road is used in absolute terms. With a full size USB A connector you can determine the orientation in pitch blackness using a fingertip / fingernail. Harder for Mini / Micro bus still doable. \$\endgroup\$ – Russell McMahon Feb 7 '12 at 23:32
  • \$\begingroup\$ Just as a side note, I had an Acer computer that had the front USB connectors reversed - just one of their lacks :) \$\endgroup\$ – clabacchio Feb 8 '12 at 8:32
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    \$\begingroup\$ You still can find the right orientation of the plug in the dark, since the logo trident is embossed and you can feel up on which side of the plug it is... \$\endgroup\$ – Count Zero Feb 8 '12 at 9:21
  • \$\begingroup\$ Why would manufacturers be unhappy to emboss the logo on both sides? It seems a trivial operation. \$\endgroup\$ – sharptooth Feb 8 '12 at 12:43
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    \$\begingroup\$ @RussellMcMahon - Sorry, I didn't clarify well enough on point #1 - They've never been, because when the alternative was DB-9, USB was already far better. The other reasons (mostly cost) would dictate that it wasn't omnidirectional in '96. It may seem obvious now that USB could be smaller, faster, and have an omnidirectional plug, but when it was invented these features weren't feasible. \$\endgroup\$ – Kevin Vermeer Feb 8 '12 at 16:39
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There is also a reason that has not been mentioned, and it's related to the concept of poka-yoke (thanks to my friend that studies industrial engineering).

The principle is that well-designed connectors shouldn't leave room for ambiguity, especially when potential failures or safety risks are involved. Paraphrasing Murphy's law,

If there is any way to do it wrong, he (someone) will.

like the old floppy disk, that enters in the hole only in one direction, and also SD cards, good design implies also that the final user has nearly no chance to connect it improperly, and ideally shouldn't have any doubt.

This is not the reason for not making it symmetrically connected, but since it has a orientation, it's made in a way that you cannot connect it in the wrong way.

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  • \$\begingroup\$ Floppy disks will quite happily go in upside down, and there was even a whole tradition of doing so amongst owners of cheaper home computers with single sided drives. Or did you mean the rigid plastic encased 3.5" last gasp of a much older technology? \$\endgroup\$ – Chris Stratton Apr 23 '13 at 14:04
  • \$\begingroup\$ @ChrisStratton I meant indeed the rigid, 3.5" one, the others are just a few years too old for my experience :) \$\endgroup\$ – clabacchio Apr 23 '13 at 16:17
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I understand the other answers here, and also I agree that this is a significant inconvenience. I've drawn up an omnidirectional 4-pin connector which only uses 2x contacts to achieve all 4 possible orientations.

Take a look:

omnidirectional cable

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  • \$\begingroup\$ Very interesting. Going with a rectangular connector (like USB) modeled after this design might be even more generally useful, since some devices (phones) really need something that's as thin as possible along one axis. And I don't think a rectangular connector is really much harder to plug in "blind" vs. a square connector, assuming that it can be plugged in either way around. \$\endgroup\$ – joelpt Feb 12 '14 at 18:54
  • \$\begingroup\$ Join the USB Forum and push for them to make a symmetric connector, or start your own forum and get buy-in from a big vendor or two... \$\endgroup\$ – Dagelf Nov 5 '15 at 10:01
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    \$\begingroup\$ @Dagelf: Your comment comes a bit late for that, since they already did that back in August 2014 \$\endgroup\$ – Ben Voigt Mar 30 '16 at 6:20
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Because it would add unnecessary cost. The vast majority of connectors are directional, and providing an omnidirectional one would probably cause confusion as people would think it needed to be plugged in a particular way round.

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    \$\begingroup\$ I'm not so sure about people being confused about that. See the AC power plug in many places. But I've seen people get confused about being unable to insert a USB plug the wrong way. \$\endgroup\$ – Count Zero Feb 8 '12 at 9:23
  • \$\begingroup\$ +1 for cost. The computer and peripheral makers who drove the design of the USB connector care about every nickel of the manufacturing cost of their product, even if it's a $1000 PC. Even more so if it's a $5 memory stick. \$\endgroup\$ – The Photon Feb 9 '13 at 6:13
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Since August 2014, USB has a new "Type C" connector that is fully reversible, in addition to having higher power limits, pins for video signals (no more need for "MHL" ports on cell phones that are only mostly compatible with micro-USB) and SuperSpeed (up to 10Gbps) USB transfer rates.

http://www.usb.org/press/USB_Type-C_Specification_Announcement_Final.pdf

The legacy USB pins have parallel contacts in symmetric locations on both faces of the connector, to allow converter cables for old devices that can't perform orientation detection. The new pins also are placed in symmetric locations, but rather than being shorted to their reflections, the polarity is auto-detected (probably with the help of the orientation-invariant legacy pins).

These connectors are becoming popular pretty rapidly, so connectors fitting only one way should soon be just a memory.

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USB client devices rely on power supply from USB hosts.

Quote from Wikipedia:

The standard connectors were deliberately intended to enforce the directed topology of a USB network: type A connectors on host devices that supply power and type B connectors on target devices that receive power. This prevents users from accidentally connecting two USB power supplies to each other, which could lead to dangerously high currents, circuit failures, or even fire. USB does not support cyclical networks and the standard connectors from incompatible USB devices are themselves incompatible. Wikipedia

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  • \$\begingroup\$ But this explains the reason for unidirectional (female-male) connection, not the asimmetry \$\endgroup\$ – clabacchio Feb 11 '12 at 9:22
  • \$\begingroup\$ The asymmetry makes the directionality more obvious. You will not attempt to connect an "A type" plug to a "B type" receptacle. \$\endgroup\$ – msms Feb 11 '12 at 10:21
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    \$\begingroup\$ It's still another topic: what the OP asks is why you can insert the usb plug reversed, but in the right receptacle...and this doesn't depend on the type or in being male or female. \$\endgroup\$ – clabacchio Feb 11 '12 at 17:49
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I can't see why not either.

I have a USB flash drive that has no metal housing; it's just a plastic rectangle which goes in the host slot. It's less robust than with, but obviously cheaper to make. It fits in USB host slots both ways - but with only with one set of pins, does have a required orientation.

So if I were to put a set of contacts on the other side of the device, and crossover to the existing ones, it wouldn't matter which orientation I plugged it in. Then put a trident on /both/ sides of the thing. Same thing would work for cables, not just memory sticks.

Downsides I can see are: - Decreased robustness of cable & connection - but cables are replaceable and disposable anyway - Increased manufacturing cost for contacts + crossover - though this would be somewhat (perhaps entirely) offset by not having the metal housing. Plus, I'd be willing to pay $5 more for a cool bidirectional cable anyway

It would just work, and many lives would be - in a very small way, but very frequently - improved!

edit: Ah. Sandisk have a patent on it. http://www.google.com/patents/US7666035?printsec=drawing#v=onepage&q&f=false

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  • \$\begingroup\$ Actually, if you were to put contacts on the other side of that device it would simply short out against the shield of the receptacle. Still, the connector could have been designed like firewire (which uses pads on both sides of the tongue) in the first place. \$\endgroup\$ – Yann Vernier Mar 20 '13 at 18:55

protected by Dave Tweed Jul 20 '15 at 1:03

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