Accessory Indicator/Serial enable
Different resistances indicate accessory type:
1kOhm - iPod docking station, beeps when connected
10kOhm - Takes some iPods into photo import mode
6.8 kΩ - Serial port mode. Pin 11-13 are TTL level. Requires MAX232 chip to convert to RS232 levels.
USB Data (+)
Pins 25 and 27 may be used in different manner. To force the iPod 5G to charge in any case, when USB Power 5 VDC (pin 23) is fed, 25 must be connected to 5V through a 10kOhm resistor, and 27 must be connected to the Ground (for example: pin 1) with a 10kOhm resistor.
iPod 5G can also be forced to charge by attaching the data + and the data - pins to the 5v via a 10k Ohm resistor ( BOTH PINS) and connecting pin 16 to the 5v (ground). (Confirmed working with iPod 5G 20GB). This provides 500mA of current for charging. For quicker charing, up to 1A, see below.
To charge an iPhone, 3G, 3GS, 4 / iPod Touch, 2nd gen, 3rd, 4th or Ipod Classic (6th Gen). usb data- (25) should be at 2.8v, usb data+(27) should be at 2.0v.
This can be done with a few simple resistors: 33k to +5v (23) and 22k to gnd(16) to obtain 2v and 33k to +5v and 47k to gnd to obtain 2.8v. This is a notification to the iphone that it is connected to the external charger and may drain amps from the usb.
To charge iPod Nano pins 25 and 27 should be tied together and then connected to a 10K ohm resistor, and the other side of this resistors then needs to be connected to 5v power.
It's also possible to charge the iPod's or iPhone's battery to make use the of internal +3.3v output (18) terminal to connect the USB Data + (27) thru a 47k ohms resistor and the USB Data- (25) thru a 47k resistor to the USB Power source +5v (23). This way the USB function is still useable for normal operations and makes it easier the fit in a plug. The resistors are not to critical 2x 150k's still work.
Added correction: iPod 2.1A charger advertises 2.8V on D+ and 2.0V on D-. Tying either wire to 5V could damage the target - use resistors tied to 5.1V and ground to be safe.