iPhone: how to communicate with external devices?

Hey, I would like to understand how iOS can communicate with external devices (example: http://www.icelsius.com). I know I could use OSC but if I understand this you need a network so.. d开发者_StackOverflow社区evices communicate over wifi. I don't know where to start. I would like to make a voltmeter, you would plug a cable into the iPhone and I would read signals with the iOS. thx...

You would have to apply to Apple's MFi program. If and when they accept you, they will provide you with documentation on how to communicate over the dock connector.