Originally Posted by knc1
The photo shown on the page linked to showed one of those "general purpose" adapter boards, which from the silk screen markings looked to be a 5.0 / 3.3 volt board.
I requested the Make/Model of the board but am still waiting for that answer.
3.3 is not enough if it has an unbuffered LED lit by a logic zero on the kindle TxD. Both 3.3v and 5v TTL use the same logic zero/one switching zone, and the kindle cannot output below that with too high of an external pull-up resistor.
And 1.8v TTL uses lower voltage ranges for switching, so needs a logic zero on its RxD even closer to 0v. I made my k4 and k5 use "open collector" inputs with a series diode, so either side can pull it to GND, and both sides pull their end up to their preferred logic one. And I removed that pesky serial status LED on my 3.3v serial adapter that interfered with kindle TxD output.
It is easier to use a real 1.8v adapter, but they are hard to google because they come in a 1.8m standard length, which confuses the search results. Cheaper to hack a level converter (if needed) for a cheap 3.3v serial adapter using the "super simple" serial level shifter I published...
But only testing will tell if you need it, and there is that "unreliable GND" issue on the serial connectors...