Pin Signal Description
1 GND Ground (-), internally connected with Pin 2 on iPhone motherboard
2 GND Audio & Video ground (-), internally connected with Pin 1 on iPod motherboard
3 Right Line Out - R (+) (Audio output, right channel) . See also Apple iPhone headset connector pinout
4 Left Line Out - L(+) (Audio output, left channel)
5 Right In Line In - R (+)
6 Left In Line In - L (+)
11 GND If connected to GND the iPhone sends audio signals through pin 3-4, otherwise it uses onboard speaker.
12 Tx iPhone sending line, Serial TxD
13 Rx iPhone receiving line, Serial RxD
14 RSVD Reserved
15 GND Ground (-), internally connected with pin 16 on iPhone motherboard
16 GND USB GND (-), internally connected with pin 15 on iPhone motherboard
17 RSVD Reserved
18 3.3V 3.3V Power (+) ?
19,20 +12V Firewire Power 12 VDC (+)
21 Accessory Indicator/Serial enable Different resistances indicate accessory type: 1kOhm - docking station, beeps when connected 68kOhm - makes iPhone 3g send audio through line-out without any messages 500kOhm - related to serial communication / used to enable serial communications Used in Dension Ice Link Plus car interface 1MOhm - Belkin auto adaptor, iPod shuts down automatically when power disconnected Connecting pin 21 to ground with a 1MOhm resistor does stop the ipod when power (i.e. Firewire-12V) is cut. Looks to be that when this pin is grounded it closes a switch so that on loss of power the Ipod shuts off. Dock has the same Resistor.
22 TPA (-) FireWire Data TPA (-)
23 5 VDC (+) USB Power 5 VDC (+)
24 TPA (+) FireWire Data TPA (+)
25 Data (-) USB Data (-)
26 TPB (-) FireWire Data TPB (-)
27 Data (+) USB Data (+) Pins 25 and 27 may be used in different manner. To charge an iPhone 3G / iPod Touch 2nd gen 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. 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.
28 TPB (+) FireWire Data TPB (+)
29,30 GND FireWire Ground (-)