Communicating with external accessories in iPhone

I think I could discuss this topic as it's open for everyone to access the document for it at apple develop center.  To search from google about this to see who is also interesting in, I found quite much people are looking for the solution of that communicating with external accessories in iPhone.




From searched article and question for this topic, they deem that iPhone could output data through UART by using the External Accessory Framework (ExternalAccessory,framework).  But actually, iPhone app can't select what interface it use for communication.  The interface could be USB, UART, or Bluetooth.  This depends on accessory.    While accessory connected to iPhone, Accessory registered what protocol it used.  At the time, the interface for accessory to communicate with iPhone is decided.  Then, app on iPhone declares what protocol it supports.  If app found the protocol that accessory registered and then gets the interface could used for outputing the data.  So If you would like to output data from iPhone through UART, you should have a accessory connected to iPhone and the accessory should use UART as its interface for data communication.    This means that accessory and app are paired.  It's impossible for you to write a app only in iPhone to output data directly through UART or specific interface.

Here the accessory should implement iAP for passing the identification and authentification process of iPhone/iPod, then app in iPhone/iPod could find the accessory and communicate with the connected accessory.