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What is the extra, 5th, pin on micro usb 2.0 adapters for?

Here is an image with the different connectos. Most of them have 5 pins, but the A-type host only has four.

USB Connectors

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3 Answers

up vote 12 down vote accepted

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 .

OTG ID

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And since a Type-A connector is nearly always host it doesn't need to have a 5th pin? – Keegan McCarthy Jul 10 '12 at 22:42
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If you mean a standard 4-pin Type-A plug, this would be used for a permanent host - you wouldn't use this in an OTG device. The ID pin is only used if the device can change between host and slave. For standard USB the ID pin is just left disconnected in the device. – Oli Glaser Jul 10 '12 at 23:28
Yeah, that answers my question. Thanks! – Keegan McCarthy Jul 11 '12 at 3:27
up vote 1 down vote

It's for host:client negotiation.

Permits distinction of host connection from slave connection

host: connected to Signal ground

slave: not connected

source

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I'm a real newbie when it comes to serial connection. I've only just started using RS232... Can you please explain a little more? – Keegan McCarthy Jul 10 '12 at 22:35
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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.

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Ok, so what info does the ID signal carry? When does it use the ID pin? – Keegan McCarthy Jul 10 '12 at 22:37

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