I've done some measurements on the RS232 adapter I used to unlock my drive. The logic 1 level on the HDD TXD pin is 2.6V. That is OK for both 3.3V and 5V RS232 adapter parts. But the margin is lower on the 5V parts - max level needed to give logic 1 is 2.4V I checked my adapter with power supplies set to 3.3V and 5V. It works fine at both settings. 3.3V would be my choice, but I don't think you would have a problem with the 5V parts. You mentioned that you were trying to use a serial cable from a mobile phone. I have one similar cable which does not work in this application. Some of these cables used other pins on the RS232 interface to supply power to the IC, and would not work with a simple hyperterminal setup.