<signal lines. Every board required it's own regulation, which <could take 20% or more of the board space, as well as being a <nightmare to keep cool. If you see early pictures of loaded <IMSAIs, the cover was always off. This was a necessity, the <heat was too much with the cover on. I had to use a 16" fan to <keep mine running with 64KB of 2102 based static RAM (not 21L02s <BTW, they cost more than the fan did). Some of the batter boxes had put some thought to air flw and this was not a problem. The Northstar Horizon was good, VECTOR MZ, COMPUPRO and there was an oufit that made mostly boxes all well cooled. In the mid to late 70s it was either boxed systems like TRS-80, Apple or designer systems like S100 or SS50. SS50 was 6800 based 50 pin bus and tended to be a very different thinking. Allison