Are you using a usb to serial adapter as I do ? Than you probably use one from the Profilic company which are most common. I tried to connect my Elecraft K3 with such a usb to serial adapter and had trouble with the CAT connection using OMNI RIG. I have in my shack two from the company and only one is working fine – the other one hangs up and stops responding. Searching the INTERNET I found out that there a many different chips sets used by Profilic. To find out you can use a tool by them called „PL2303 CheckChipVersion tool program„ (checkChipVersion_v1006). Now to the point I found out: the one working has a chip labeled „PL-2303 XA/HXA“ and the one not working has a chip labeled „PL-2303 RA“ which is the low cost version. So if you buy one pay attention you have the „XA/HXA chip“ inside. Also and only for them you might get drivers for windows 8 + Windows 10.
Update January 2018:
For Windows 10 if you have already installed a driver and it is not working first uninstall it, plug off the adapter, install THIS DRIVER [ download driver] and plug in again the adapter – that should work with Win10 for 100%.