I've worked with all manner of computers for over 40 years and have not once seen a production circuit that claimed ot be RS232 compatible that didn't use a negative voltage. I've seen many ways of driving the RS232 signals, but never one that claimed to meet RS-(that means recommended standard) 232 without a negative level for either mark or space. Some invert the signal levels. Some change the handshakes, but all of them recognize that a '1' must be either > 3 volts or < 3 volts, with the range between those endpoints of allowable levels being a deadband, meaning it is not a legal static level.

I'd recommend everyone look for a copy of the EIA standard and read it. Pay attention to what it says, and, even more important, what it doesn't say.

RE