Abstract: An improved tristate enable circuit is described for activating a tristate enable gate to maintain the high impedance third state of a common bus tristate output device during "power down" and "power up" transitions of the common power supply V.sub.cc. The enable gate circuit element tends to turn off at a voltage level V.sub.cc2 generally greater than the voltage level V.sub.cc3 at which the tristate output device circuit elements turn off. As a result the high impedance state may be lost during "power down". A threshold activation circuit is coupled to the enable gate for activating the enable gate when the threshold activation circuit senses a higher common power supply voltage level V.sub.cc1. The threshold activation circuit operatively activates the enable gate over a voltage range from V.sub.cc1 to a lesser common power supply voltage level V.sub.cc4. Component values are selected for relating the voltage levels so that V.sub.cc1 >V.sub.cc2 and V.sub.cc3 >V.sub.cc4.