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I mean, this is against the rules!

Most USB OTG devices claiming to be host capable (most Android smartphones and tablets) have a simple micro-B connector. Shouldn't they have a micro-AB connectors? Micro-B are supposed to be for devices unable to act as hosts.

It actually leads to a proliferation of USB OTG cables like this one:

enter image description here

Which are forbidden by the USB specs (micro-USB specs chapter 3.4: Compliant Cable Assemblies). We clearly see it is a micro-B, not micro-A plug (there are chamfers), and it is associated with a standard-A receptacle. I wonder how they are even allowed to print the USB logo on this, by the way.

So, why do manufacturers do this (both phone manufacturers and cable manufacturers)? How is it allowed by the USB consortium?

I'm asking this because I am actually designing a USB OTG device. I don't plan to make it certified by USB anyway (given the costs), but I'd like to know whether I should strictly follow the standards, or screw up with them like everyone else (the fact is - because of this - everybody is used to micro-B, not micro-A, and it's true the chamfers help prevent trying to put the connector the other way around).

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  • \$\begingroup\$ Purely mechanical reason: You can destroy an micro-AB port by forcing the cable in backwards - which is not possible with a micro-B port. \$\endgroup\$ – Turbo J Jun 24 '16 at 9:56
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    \$\begingroup\$ @TurboJ Is that really the reason? It would mean the USB consortium really screwed up, then. They define connectors that we don't really know in which way to insert, and that can break when inserted the wrong way? \$\endgroup\$ – dim Jun 24 '16 at 10:07
  • \$\begingroup\$ The micro-AB port on my Sony Mini Pro (5+ years old) did not break, but I often try to put the plug in the wrong way - which is rather annoying. You need a bit more force to be able to destroy the port. \$\endgroup\$ – Turbo J Jun 24 '16 at 10:16
  • \$\begingroup\$ When I look at USB 3.0 connectors I seriously doubt their capability of looking ahead. C'mon, its just an ugly hack added to the USB 2.0 connectors! (and then USB type C which is nice, but still another connector.. what a mess..) I'm less surprised by non B-AB type compliance now if compared to those other facts. \$\endgroup\$ – Wesley Lee Oct 27 '16 at 10:42
  • \$\begingroup\$ @WesleyLee I agree, but what bothers me is that people are not conforming to the specs. The mess could be acceptable if it's a weel-specified and agreed-upon mess. But I don't know what to choose, here. \$\endgroup\$ – dim Oct 27 '16 at 10:49
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The more I think about it, the more I'm convinced the reason is not technical. It likely is just to avoid the burden of certifying a device with dual-role OTG capability.

Apparently (see here), testing a device for OTG compliance costs almost twice as much as testing a simple high-speed device. There is also much more work in the design phase, and much more risk of failing the compliance, having to make design iterations, and going through testing again. Since the OTG capability isn't used by many consumers, manufacturers don't feel the need to advertise full OTG compliance.

Instead, they simply certify the phone/tablet/whatever/... for "USB device-only" usage, then use clever marketing wording (claiming "OTG-compatibility", for example), and call it a day.

Because they don't test for dual-role OTG compliance, they are not allowed to use micro-AB connector (or they would fail the simple device-only compliance). So they simply put a micro-B receptacle, and expect cable manufacturers to provide adapters such as the one shown above (although such adapters are officially forbidden by the USB spec - but cable manufacturers don't really care).

Also note that the logo on the cable above is not the USB logo that is subject to strict licensing. It is the "trident" logo, whose usage is much more relaxed. This logo, for example, does not imply that the item has passed the USB compliance tests.

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I believe this is because of the cost of the devices. USB Micro-AB have the same soldering footprint as Micro-B. Micro-B is much wider used hence the connector is much cheaper due to the economy of scale.

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    \$\begingroup\$ Yes, but if they used micro-AB instead of micro-B on all phones, wouldn't micro-AB be much more widely used and hence as cheap as micro-B? That looks like a chicken and egg problem. \$\endgroup\$ – dim Jun 25 '16 at 7:17
  • \$\begingroup\$ @dim Micro-B is also used where OTG is NOT needed, and that is a much wider market than places where OTG is needed. Still, economy of scale. \$\endgroup\$ – Maxthon Chan Jun 25 '16 at 8:11
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    \$\begingroup\$ Is it really "much cheaper"? I believe B and AB connectors are roughly the same price. \$\endgroup\$ – Dmitry Grigoryev Jun 29 '16 at 8:25
  • \$\begingroup\$ How close is roughly? 1 cent? for products with 1M production runs it is a substantial amount. \$\endgroup\$ – Lior Bilia Oct 27 '16 at 10:56

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