USB to TTL converter: why doesn't TX need a voltage selector?
Why doesn't TX need a voltage selector?


Because it's a cheap, low-quality adapter that only supports one output level.

The Prolific PL2303, around which this adapter is based, can't output 5V signals. By default, it uses a 3.3V voltage standard, and will output at least 2.4V for high outputs. This will be high enough for most 5V TTL devices, but some devices may have trouble with it.