USBFacts - Knowing Connectivity

 
USB Connectors

Different types of USB connectors from left to right
• 8-pin mystery plug
• Mini-B plug
• B-type plug
• A-type receptacle
• A-type plug

Pin configuration of the USB connectors Standard A/B, viewed from face of plug
There are several types of USB connectors, including some that have been added while the specification progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles. The data slots in the A - Plug are actually farther in the plug than the outside power slots, to prevent data errors by instead powering the device first, then transferring data. Additionally, some devices operate in different modes depending on whether the data slots are connected. For example, some battery-powered MP3 players switch into file transfer mode (and cannot play MP3 files) while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode indefinitely, without having to interrupt playback to recharge or replace batteries.

 USB-A
The Standard-A type of USB connectors takes on the appearance of flattened rectangles that plugs into downstream-port sockets on the USB host or a hub. This kind of connector is most frequently seen on cables that are permanently attached to a device, such as one on a cable that connects a keyboard or mouse to the computer.
 USB-B
Standard-B connectors—which have a square shape with beveled exterior corners—typically plug into upstream sockets on devices that use a removable cable, e.g. between a hub and a printer. This two-connector scheme prevents a user from accidentally creating a loop.
 Mini, Micro
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Please improve this article if you can. (December 2008)

Various non-standard connectors; the now-deprecated[citation needed] (but official) Mini-A and Mini-B; and the current standard Micro-A, and Micro-B connectors ... all of these are used for smaller devices such as PDAs, mobile phones or digital cameras. The Standard-A plug is approximately 4 by 12 mm, the Standard-B approximately 7 by 8 mm, and the Mini-A and Mini-B plugs approximately 2 by 7 mm.
 

Micro-USB Receptacles
The Micro-USB connector was announced by the USB-IF on January 4, 2007 and the Mini-USB connectors were withdrawn.[citation needed] In late 2008, most small USB connectors and cables in use are still the Mini style, but some devices use the new Micro connectors.
The Micro-USB connections are intended to replace the Mini-USB plugs used in new devices including smartphones and Personal digital assistants. This Micro-USB plug is rated for 10,000 connect-disconnect cycles. It is about half the height of the mini-USB connector, but features a similar width. In the Universal Serial Bus Micro-USB Cables and Connectors Specification, details have been laid down for Micro-A plugs, Micro-AB receptacles, and Micro-B plugs and receptacles, along with a Standard-A receptacle to Micro-A plug adapter. The carrier led group OMTP have recently endorsed micro-USB as the standard connector for data and power on mobile devices. These include various types of battery chargers, allowing Micro-USB to be the single external cable link needed by some devices.

USB OTG Sockets: Mini-AB, Micro-AB

Except for special standard-to-Mini-A and standard-to-Micro-A adapters, USB cables always have an A-connector and a B-connector, on opposite ends. A-connectors can always connect to A-sockets; B-connectors can always connect B-sockets. These sockets all come in standard, mini, and micro versions.
For USB On-The-Go (or 'OTG') support another socket type is defined: the AB, in both mini and micro versions. It can accept both A and B connector, through careful mechanical design. OTG software detects the difference by use of the ID pin, which is grounded in A-connectors and is otherwise floating. When an A-connector is connected to an AB socket, the socket supplies VBUS power to the cable and starts in the host role. When a B-connector is used, the socket consumes VBUS power and starts in the peripheral or device role. OTG allows those two roles to be switched by software, as needed for the task at hand.