From #kindledev@freenode IRC channel:
yifanlu: Anyone who used the serial console on the k4 or touch give me some advice? Where's the location of TxD, RxD, GND. What chipset to use for my ttl adapter. etc Wed Jan 25 2012 - 3:57
Short answer: The K4NT unpopulated serial port connector labelled P2 is about 1 inch up from the lower-left corner of the circuit board. The solder pads from top to bottom are: GND, RxD, TxD.
The answer to your question was previously posted here:
The relevant part of that post is duplicated here so that it can be found more easily:
The serial port is an unpopulated printed connector outline at the board edge, near the bottom. The GND pad at the top looks different from the others. RxD is the center pad. TxD is the bottom pad. It looks like that connector for the Touch would fit here. The pin spacing was identical to the wire spacing on an old 4-pin IDE cable. I peeled off a strip of 3 wires, and without separating them, I stripped and solder-tinned the ends, then clipped them to about 1 mm of wire showing. I added a little extra solder to the serial pads on the board, then touched the 3 wires to the pads and touched them with my hot soldering iron to bond the wires to the pads. I also used my 2-resistor and 3-diode level shifter circuit to make my usb serial adapter work with my k4nt. It allowed me to see the error message about serial number = "1" and board ID = "1" and u-boot "bailed!" because it cannot init ram on an unknown board type.
The level shifter circuit that I posted previously will let the kindle work with any usb ttl serial adapter (5v, 3.3v). You may need to remove the RxD LED from your adapter like I did, because it supplies too much current to allow the kindle TxD to pull it down to a logic zero.
You can also use usb serial cables from cell phones, with level shifting.
Some 3.3v or 5v adapters may work fine with no level shifter.
The correct way to go is to buy a real 1.8v usb serial adapter, but from places like mouser or digikey they are about $25 plus shipping. Good luck Googling "1.8v usb serial" though, because most Google hits are for 1.8m (6 ft) cables rather than 1.8v.
I got my 3.3v usb serial adapters from an ebay store for about $3 buy-it-now and free shipping, but I had to wait for them to arrive from Singapore (about 2 weeks), and I had to modify them by removing the RxD LED that exceeded with the kindle TxD current sinking ability.