What is the extra, 5th, pin on micro usb 2.0 adapters for?
Here is an image with the different connectors. Most of them have 5 pins, but the A-type host only has four.
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It's for On-The-Go, to select which device is the host or slave:
The OTG cable has a micro-A plug on one side, and a micro-B plug on the other (it cannot have two plugs of the same type). OTG adds a fifth pin to the standard USB connector, called the ID-pin; the micro-A plug has the ID pin grounded, while the ID in the micro-B plug is floating. The device that has a micro-A plugged in becomes an OTG A-device, and the one that has micro-B plugged becomes a B-device. The type of the plug inserted is detected by the state of the pin ID .
To complete Oli Glaser's answer, 5 pins USB respects the On-The-Go standard (OTG). The additional pin added to the conventional USB port is the ID pin added to the 4th electrical pin, and allow to recognize the device. Here is the resulting electrical setup of the pins:
As compared to other 4-pins USB devices, where there is no ID pin, the advantage is to be able to distinguish the host device from slave devices.
It's for host:client negotiation.
Permits distinction of host connection from slave connection
host: connected to Signal ground
slave: not connected
There is no info carried by the 4th pin. When connected to the ground (5th pin) it serves to notify the host that it is connected to a dumb-client device instead of a smart-client device. This is confusing at best because some client devices only act as dumb-clients and some client devices can be either smart-client, another peer-host, or a pass-through repeater. You will probably only ever see OTG actually used in case of a keyboard being plugged into the micro or mini connector on a tablet computer. Other client devices usually have enough inherent software capability to notify the host that they are a client using the normal 4-wire USB connection.
As shown here the original type A and B connectors use four connections, D+ and D-, which are differential data signals, along with ground and +5v. The newer mini and micro connections add an ID signal.
Before USB OTG was popular, the 5th pin was an auxiliary pin to allow the USB port on portable devices to be used for other purposes via passive components/circuits. A resistor array in the cable would indicate the function of the cable to circuitry in the device. Sometimes this could be a composite TV out, but it was mostly used for audio. Manufactures HTC and Motorolla did this audio out on many phones, with different pinout schemes.