You may want to check the pinout of the Ultra3000 serial port as it is not a standard serial connection. The RS232 and RS485 signals are both accessed through the same port, so a regular null modem cable might not have the right connections.
I can't say this for certain because I have always used the AB-supplied cable.. but I DO know that we have tried to use that cable for other applications where a null-modem cable is normally used, and it DID NOT work... leading me to believe that it is a special cable.
Hope this helps.