The low at E forward-biases the emitter-base junction of transistor Q 1 and shunts the resistor R 1 current away from transistor Q 2 so that Q 2 turns off, which turns transistor Q 4 off. When E = 0, the circuit goes into its Hi-Z state regardless of the state of logic input A. The circuit has two inputs : A is the normal logic input and E is an ENABLE input capable of producing the Hi-Z state. 47.8 (a) shows the circuit for a tristate INVERTER where the portion enclosed in a dotted rectangle has been added to a basic circuit. The tristate logic circuit operation is achieved by modifying the basic totem-pole circuit.

In practice the output terminal is not an exact open circuit, but has a high resistance of several MΩ or more relative to ground and V CC.

In other words, the output is an open or floating terminal that is neither a low nor a high. The Hi-Z state is a condition in which both transistors in the totem-pole arrangement are turned off so that the output terminal is a high impedance to ground and to V CC. The tristate logic circuit utilizes the high speed operation of the totem-pole arrangement while permitting outputs to be wiredANDed (connected together). Such circuits are called Tristate or three state logic. To overcome these difficulties, special circuits have been developed in which there is one more state of output, referred to as the third state or high impedance (Hi-Z) state, in addition to low and high states. This causes the problems of loading and operation speed.

Open collector outputs can be connected together with a common collector resistor connected externally.

Totem-pole outputs cannot be connected together due to very large current drain from the supply and consequent heating of ICs which may get damaged.

Totem pole output meaning