Like most devices designed and built by Apple, the iPhone uses a proprietary 30-pin connector for charging, audio and video transfer, synchronization and accessory connectivity. This connector has become known as the "Mac connector." To build a USB cable that both charges and allows data transfer between an iPhone and a computer, the Mac connector requires more than just the four wires connecting from the USB plug.
The USB plug itself is a standard connector found on most every computer and laptop. It's the primary input and output port for communicating with devices such as the iPhone, allowing the handset to synchronize with the computer and its software. The USB standard also includes a regulated 5-volt output, so devices can be charged or powered while connected to the computer. The iPhone requires any standard, four-pin USB plug for the computer end of the cable.
This proprietary 30-pin connector was designed by Apple, and is used on almost all of its devices for power and data transfer. Although the USB cable only has four pins, the Mac Connector requires additional connections between its pins to allow the iPhone to communicate with an external device, and to receive a charging voltage. See References for a full description of each pin's function on the iPhone connector. The essential data and power system remains the same as the USB standard, however, with positive voltage, negative voltage reference, and positive and negative data lines connected between the two plugs.
USB to Mac Connections
To transfer data between the iPhone and a computer using a USB cable, the iPhone requires the USB's positive data line connecting from pin three of the USB plug to pin 27 on the Mac connector. The negative data line is connected from pin two on the USB plug to pin 25 on the Mac connector. The positive 5 volt power output from the USB is achieved by connecting pin one on the USB plug to pin 23 on the Mac connector. The negative power output requires a wire from pin four on the USB plug to pin 16 on the Mac connector.
In order for the iPhone to accept a charging voltage from the USB, resistors must be connected to the USB data lines to provide a specific voltage signal. The positive data line at pin 27 must carry a 2 volt signal, which is achieved by connecting a 33K ohm resistor between pins 27 and 23 on the Mac connector, and a 22K ohm resistor between pins 27 and 16. The negative data line must be at 2.8 volts, achieved by connecting a 33K ohm resistor between pins 25 and 23, and a 47K ohm resistor between pins 25 and 16. The cable will now charge and synchronize the iPhone.
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