There's something that's been puzzling me for a while now. If you look at an Apple iPod/iPhone cable, there are 6 visible and distinct contacts. Yet, it is a USB connection, which (for at least 2.0; Don't know much about 3.0) only has 4 wires. What's the point of having two extra contacts?
Hang on, what cable are you looking at? As far as I recall, the Apple Dock connector has 30 pins. which consist of not just USB pins, but also firewire pins, video pins, line-in and line-out pins, Tx/Rx pins, respective ground pins, etc. I'm not entirely sure what Apple cable only has six pins.
I'm looking at the cable that came with my iPod classic a few years ago. It has 30 little slots on the end that connects to the iPod, but only 6 of them appear to have any sort of contacts in them. What's confusing me is why there would be 6 instead of 4 since USB only uses 4 wires, and therefore would only need 4 contacts.
I think some of them are to identify the device and authenticate it. So, for example, the iPod knows that it's a USB cable and not one of those Volume/Track/Pause control pad things (Although on the iPod Classic I think there's another slot for that thing...). Or an FM transmitter, a keyboard, etc.
And have you ever plugged your iPod into, say, an FM Transmitter, and got that message "Charging is not supported with this accessory"? I have a feeling Apple charges accessory companies to have their Vendor ID recognized by the device, so that their charger will be supported by the iPod. The cheaper ones probably just fake it, or clone a bigger company's ID.
But maybe not. It could just be identifying the nature and capabilities of the accessory.
That's my theory anyway.