Interesting problem -- I had one of those older USB to serial cables that had the old Prolific chip in it and it was blocked by the newest drivers that are in Microsoft, so I purchased a Plugable USB to serial converter which is supposed to have the later version of the Prolific chip in it.
I tried it using the RS92 software from Icom and it works just fine, but using it with chirp, it appears not to be communicating with the radio because if I disconnect the radio, I get the same information back as I do if the radio is connected -- all invalid. I don't suspect that the problem is in the cable, but it could be the issue and maybe I need to go out and buy a third cable with the acceptable chip in it to get chirp to work.
Does anyone know what the voltage of the signals are coming from an IC92AD? Are they "true" RS232 signals (ie +12 to -12 volts) or are they TTL voltages 0 to +5 volts? Is there a way to tell the Prolific chip that these are TTL voltages and not RS-232 voltages? Maybe that would help some people.
de WD6FIE Richard Ravich