It seems to me that I remember something about the A500 interface being inverted from that of the A1000. That may make sense since the port on the A500 is on the left and on the right on the A1000. Perhaps the drawings you are looking at are for the A1000?
I beleive that I've seen posts on here about this interface in the past. Maybe try searching for A500IDE?
Just a few thoughts...
-AmigaEd
