1994 Chevy S-10 no fire

I have replaced the park neutral switch, inigition coil, as well as the ignition module and still no fire plug wires and cap and rotor are fine what else could cause it not to fire I think the next step is to have a dealer test the computer it just seams odd because I pulled out of my drive way and it stalled when I went to put it in reverse and would not fire sence what could it be