I know this is 6 years old thread but I had the same problem and this page always pop-uped, so if any one will be reading this.
The solution for me:
I was using USB - TTL adapter (logic level 0V to 3.3V/5V) and its no good (Arduino, USB-Serial etc...)
then when I switched to RS-232 (-3~-25V to +3~+25V), Bam! Works:)
I used the native COM1 port of my computer for RS-232
If you need to connect it to a micro-controller you can use MAX232 / MAX3232
you can read about the difference here: [Link: sparkfun.com]