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Why do type A and B USB cables have 4 contacts and most mini and micro have 5?

Do the cables actually have 5 wires for mini and micro?

All the mini cables I have have 4 terminals on the A end and 5 on the B (mini) end, so is the 5th just a null connection or does it connect to one of the other 4 connections?

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2 Answers 2

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That extra pin is used for ID (for USB OTG). It's normally shorted to GND via resistor, or left not connected.

http://en.wikipedia.org/wiki/USB_On-The-Go#OTG_mini_plugs

OTG mini plugs The USB On-The-Go standard introduces a new plug receptacle called mini-AB. It can accept either a mini-A plug or a mini-B plug. Mini-A Adapters allow connection to standard-A USB cables, coming from peripherals. The standard OTG cable has a mini-A plug on one side and a mini-B plug on the other (it cannot have two plugs of the same type). The device that has a mini-A plugged in becomes an OTG A-device, and the one that has mini-B plugged becomes a B-device (see above). The type of the plug inserted is detected by the state of the pin ID (the mini-A plug has the ID pin grounded while the ID in the mini-B plug is floating). (There are also pure Mini-A plugs, used where a compact host port is needed but OTG is not supported.)

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Thanks, I still scratch my head because one end (A) of the cable has 4 contacts and the mini end (B) has 5, is there any schematic of a mini usb cable? Since all standard USB female sockets on PCs are 4 pin, why the fifth on one end if cannot be used. –  Moab Apr 16 '11 at 0:50
    
@MoabFollowing the links others provided would have lead you here maxim-ic.com/app-notes/index.mvp/id/1822 where there is a full explanation with drawings. –  BrianA Apr 16 '11 at 5:29
    
Thanks, I did and it does not have a diagram with 4 on the A end and 5 on the B (mini) end. What does this 5th connections do on the mini end, is it null? or somehow connected to the other 4 wires? –  Moab Apr 16 '11 at 14:36
    
@Moab The 5th pin is not connected to the other side. It's either shorted to GND (via a resistor) or left not connected. It signals the OTG host whether it should be in host / device mode. –  fseto Apr 18 '11 at 3:19
    
Thanks for the answer, appreciate your patience. –  Moab Apr 18 '11 at 3:29
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It's how the cable knows which is the host and which is the device.

From Universal Serial Bus - Wikipedia

ID  none    permits distinction of A plug from B plug
                * A plug: connected to Signal Ground
                * B plug: not connected
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