If the DX has the same series 1K resistor as the K4/K5, and if the serial adapter has an LED powered sinking current through the kindle TxD, you may need to remove that LED from your adapter and replace it with a 22K resistor like I did. The kindle was not able to output a low enough voltage on the external side of the 1K resistor with the current load of that LED to appear as a logic zero.
In other words, you need some level-shifting circuitry, which you can purchase, or you can make one with resistors and diodes as shown in the level shifter circuit I posted to a thread (look in the tools index).
Some serial adapters work with no level shifter, but others need part or all of it. It helps to have an oscilloscope so you can see peak voltage levels you are sending or receiving serial data.
You can harvest diodes and other components from the circuit board in a dead compact fluorescent lamp.
Also, the serial GND does not seem reliable in some kindles. It helps to attach to a known ground like a metal shield in the kindle, or via the USB cable shell by connecting the USB cable to the same computer as your serial adapter.
Last edited by geekmaster; 09-20-2012 at 02:25 AM.