The unit does indeed require a null modem when controlling it from an RTI processor. The settings are Baud 9600, Data bits 8, Stop bits 1, Parity None. Also make sure you have the proper port selected as well. When entering the string onto the button make sure you press the Hex button first before putting in a string. An example command for Main Zone off would be 02 04 A0 60 00 00 FC. Hopes this helps.