Ground Loop/where to hook5v-

Should the -5v logic supply go to the -dc side on drive
or to a gnd pin on db25?? It seems that if I do the latter
I end up with too much current on all signal lines.

I usualy wire the +5 to opto inputs on drivers,and to the + side of a
couple of solidstate relays. Then I run the -5 to ??

thanks
Carl