If you are using S5 for Windows you can use just about any USB to RS232 converter, the Keyspan is very good, we also have our USB to RS232 that works very well, the Keyspan has good troubleshooting software that helps
If you are using Step5 then it will not work, we will be coming out with our new version next week this will be a USB to TTY converter and should work with both
Regards!