I have an FTDI-based USB to serial adapter, and I was able to get it working under XP but with limited speeds. I use it to program a microprocessor (like others in the forum, I do embedded development), and it takes significantly longer (than on my Dell) to link up with the processor and then send the program to it. I was able to install FTDI's driver and the adapter now shows up as /dev/tty.usbserial-DEVSN (where DEVSN is it's serial number). I know that Parallels Desktop can tie a virtual COM port to a file or a socket, but neither of those is a complete solution. Both are limited in that changes to the port speed are ignored, and the file is a write-only device. If you always used a port at a fixed speed without needing to access DTR/CTS/RTS then you could write a small command-line app under Mac OSX to open the tty at that speed, and then provide a socket interface to it. Unfortunately, my microprocessor programming interface changes speed and uses the control lines. I'm hoping that if the fine Parallels developers haven't already considered this solution, that they'll explore it now.
If you have a USB serial adapter (I have a Keyspan), you can try this: http://eudyptes.com/SerialClient.php Thanks to Eudyptes for a great tool to make Parallels talk to serial devices! Now if someone could tell me a trick to get it to work with my Apple USB modem so I could use it to dial from a Door Security app in Windows I'd be set!
How about using an RFC2217 server/client? I'm now considering trying to use an RFC2217 (protocol for sharing a serial port over a TCP connection) server running on the Mac to share the /dev/tty created by the USB adapter, and an RFC2217 client running under Windows. If I get something working, I'll report back. Right now, I'm using an USB adapter with the driver in Windows controlling it. Unfortunately, I'm just not seeing the performance I need. I'm really hoping that the Parallels developers can extend the serial port support to tie a COM port to a /dev/tty device, with full support for hardware handshaking lines and baud rate changes.
