Physical appearance
The size difference is large and quite obvious! Someone mentioned that microUSB is mechanically flawed. Unless he meant something very specific, evidence suggests otherwise.
Durability
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug,
compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.
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USB On-The-Go
USB On-The-Go introduces the concept that a device can perform both the master and slave roles, and so subtly changes the terminology. With OTG, a
device can be either a
host when acting as the link master, or a
peripheral when acting as the link slave. The choice of whether to be host or peripheral is handled entirely by which end of the cable the device is plugged into. The device connected to the "A" end of the cable at start-up, known as the "A-device", acts as the default host, while the "B" end acts as the default peripheral, known as the "B-device".
OTG Micro Plugs
With the introduction of the USB Micro Plug, a new plug receptacle called Micro-AB was also introduced. It can accept either a Micro-A plug or a Micro-B plug. Micro-A Adapters allow for connection to Standard-A plug type USB cables, as used on standard USB 2.0 Devices. An OTG product must have a single Micro-AB receptacle and no other USB receptacles.
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USB 3.0 introduced a backwards compatible, SuperSpeed extension of the Micro-AB receptacle and Micro-A and Micro-B plugs. These contain all of the pins in the USB 2.0 Micro and use the ID pin to identify the A-device and B-device roles. Additionally they contain the additional SuperSpeed pins.
OTG micro Cables
When attached to a PC an OTG device requires a cable which has a USB Standard-A plug on one end and a Micro-B plug on the other end. In order to attach a peripheral to an OTG device the peripheral either needs to have a cable ending in a Micro-A plug which is inserted into the OTG device's Micro-AB receptacle or the OTG device itself needs an adapter cable [
O: I call it a cable cable, but whatevs] which has a Micro-A plug on one end and a Standard-A receptacle on the other. The adapter cable enables any standard USB peripheral to be attached to an OTG device. In order to attach two OTG devices together requires either a cable with a Micro-B plug at one end and a Micro-A plug at the other or can be achieved using a combination of the PC cable and adapter cable.