The TTL logic currently accepted as standard calls for gate outputs no less than 2.4V for logic "true" ("1") and no greater than 0.4V for logic "false" ("0") at 25°C. Corresponding to these levels are the usually-specific logic input levels: a gate must respond to levels greater than 2.0V (or input "1") or to levels less than 0.8V (or input "0"). Read full article