Hi I couldn't remember exactly what it was so I looked it up. I was both right and wrong. First, it wasn't negative levels, it was positive levels. The 8080 requires 3.3 volt highs on their inputs. Most TTL only require 2.5 volts. Intel made a number of parts compatable with these higher input threshold levels. The statement that 8080's are TTL compatable is only mostly true. They could drive TTL but to receive from TTL often required pull-up resistors or drivers that had higher outputs. So, it was drive level and not input voltage that was significant. Dwight