Originally Posted by hawhill
I think you'd need two resistors between GND and TX (of your serial adapter), where in the middle you could grab a lower-voltage TX. Choose high enough resistor values, as a RS232 interface isn't really made for feeding current through ohmic resistors. But my e-tech skills are quite rusty.
There will be more difficulties, however, if your serial adapter doesn't like 1V8 levels on its RX pin (in that direction - coming from Kindle - lowering voltage doesn't make sense).
Rereading the thread, you do have some very good points. The 5v or 3.3v adapters may not work at a 1.8v level.
Rethinking , it may be better for the op to buy a correct 1.8v usb ttl serial adapter and be done with it.
The op may only use it once, but having it would give peace of mind. Also more electronics are going in the 1.8v direction.
I would recommend that you forget about any modifications and buy the correct cable/adapter. Then you can rest assured that no boggy men are hiding any where in your setup to fry or mess up anything in your hardware.
Here is a link in the proper direction. The TTL-232RG-VREG1V8-WE, sounds like exactly what you need.