[Elecraft] PSK PTT doesn't work with USB to serial converter

[casey at tomochka.com]

I have a working PSK31 setup with my K2 using a homebrew serial to K2
mic cable. That is, it works FB with my desktop computer that has an
actual serial (DB9) port. I'm trying to now use an XP netbook, which has
no serial port. Instead I'm using a USB to serial converter (Sabrent
brand). The driver seems to have installed correctly, but not
combination of settings of the device or Digipan results in PTT being
activated.

Anyone know how I can debug the device to see if it's setting RTS? 
Perhaps the signal level is too low?

I don't know for sure, but I think this converter uses a Prolific chip
and I have read that for CAT people have better luck with an FTDI based
converter. What is the difference between the chips that makes FTDI
better?

73,

Casey