Just like network cables there are two types of db9 cables, if you are grabbing some that are laying around from the office or from your home PC setup. There are straight through types and crossover types. Make sure you have the right type, measure with a multi meter to verify the pinout on both ends of the cable. Megamanual should explain this, but I didn't go look. I went through about 20 cables here at work and only found 2 that were the correct style to work with megasquirt. I don't remember off hand which one is correct, check the megmanual.