I connect to them all the time, but I use a PCMCIA serial card. All USB to serial adapters are implemented via software on the USB side and therefore conflicts can and do arise. PCMCIA cards are a physical hardware port to the computer and not subject to a USB software driver. I use the Quatech SSPR-100 card.

You say you can connect to a B&K keypad from your machine. There are two differences here - the B&K software for the CT and the serial to ethernet adapter. Check the second carefully, I have found broken or incorrect wiring inside these adapters.