Connecting external devices to iPhone

Recently I heard about some opportunities involving creating apps running on iPhone, meant to talk to specialized hardware devices, so that got me thinking about ways in which the iPhone can connect and talk to external devices.

First category is wired connections. Here the choices are simple, since there's only a single connection port, the docking port (also used to charge the device). In order to be able to communicate through that port to the external device you'd need to be enrolled in Apple's MFi program. However there's another option, that doesn't require participation in this program, and uses the audio jack available on iPhone. That jack outlet provides connectivity to both headsets and microphone, so that gives you access to an input/output channel. Your external device should be capable to generate the required voltage that applied on the microphone pin would be detected in the iOS app as a sound, and should be able to understand the audio output received on the headset pins, as generated by the app. This is nothing new, as some people have already implemented it as either dedicated solutions like Square, or more general-purpose solutions like HiJack. For low data rates you may be able to get by with a simple detection technique, whereas for higher data rates you may need to implement a full-blown modem.

The other category is wireless connections. Here your best bet may be using the network connectivity (either WiFi or 3G) to connect to a server over TCP/IP or higher protocols. That's probably the approach that involves the least effort. Another appealing avenue is to use Bluetooth, however the public SDK only allows you to talk to another iOS on Mac device, via Game Center, or Bonjour. If you wanted to talk to your specialized external device over bluetooth, you would also need to be enrolled in the MFi program, and use the External Accessory Framework to talk to the device.

Another wireless channel, albeit one-way, is to use the camera to interpret either a pattern like QR code or barcode, or a pulsating light. Another one-way channel in the opposite direction is the screen of the iPhone. You could have your app display a sequence of patterns that could be interpreted by an external devices. Some of you greybeards may remember a version of Windows transferring data to a Timex watch in that manner.

The accelerometer falls in a category of its own, since it's not quite wired, neither wireless. It's obviously an input channel into the app, whereby the app can detect movement, if the iPhone is physically attached to an external device. That could be used for a monitoring/tracking the movement of physical goods, either intentional or due to theft or mishandling.

That pretty much sums it up. If you can imagine other ways of communication between the iPhone and external devices, please comment.