Tristate outputs

There is another important application in which circuit simplification can be achieved by paralleling gate outputs, namely when any one of several gates connected to a signaling line is to determine the logic state. This is then referred to as a bus system.

This end can also be attained using open-collector gates, as shown in Fig. 1.6, by placing all the outputs, apart from one, in the high-impedance H state. However, the main disadvantage of the low rate of rise can be avoided in this particular application by using gates with tristate output instead of gates with open-collector output. The tristate output is a genuine push-pull output with the additional property that it can be placed in a high-impedance state using a special control signal. This state is known as the Z state.

The basic circuit implementation is shown in Fig. 1.8. When the enable signal EN = 1, the circuit operates as a normal inverter: for x = 0, 21 = 0 and 21 = I, ie. T₁ is off and T₂ is on. For x = I, T₁ is turned on and T₂ is turned off. However, if the control variable EN = 0, we also have z₁ = z₂ = 0, and both output transistors are off. this is the high-impedance Z state.

Due to their favorable electrical characteristics, the variety of types available and their low price, low-power Schottky TTL circuits are the most commonly used family of logic circuits.

Fig. 1.8 - Inverter with Fig. 1.9 - Circuit symbol of an inverter

tristate output. with tristate output.

Date: 2015-01-12; view: 1056