By comparing Mackrakit's two codes in his above post to what I was trying to do, I noticed that he used the statement "HSERIN [X]" whereas I had been using "HSERIN [DEC X]". When I changed this statement in my code to match his the serial interface works! But I don't know why? I see in the PBP manual that DEC is a legitimate modifier for the HSERIN statement. How come it doesn't work in my code?