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Chapter 13: Problem 25

What is the difference between open-collector and tri-state output?

Short Answer

Expert verified
The key difference between open-collector and tri-state outputs is their active states. An open-collector can only drive a logic Low and relies on external means to achieve a logic High. Meanwhile, a tri-state can actively drive both logic High and Low in addition to its Hi-Z state. Thus, open-collector outputs are used for wired-AND functions, while tri-state outputs are used in data buses.

Step by step solution

01

Open-collector Output Explanation

An open-collector output in digital logic is a type of output in which the output device, such as a transistor, is either in the OFF state or it provides a path to ground. When it is in the OFF state, the output is in a 'floating' state, and not driving any particular logic level. An external pull-up resistor or voltage source is typically used to drive the output high when the transistor is not conducting. Open-collector outputs can be used to implement a wired-AND function.
02

Tri-state Output Explanation

A tri-state output in digital electronics is an output that can have three states - logic High, logic Low, and high impedance (Hi-Z). When in the Hi-Z state, the output effectively becomes disconnected from the rest of the circuit, as if an open switch was placed in the line. Tri-state outputs are commonly used to allow multiple devices to share a common data bus without driving conflicting signals onto the line.
03

Comparing Open-collector and Tri-state Outputs

While both open-collector and tri-state outputs can disconnect themselves from the output line (Hi-Z), the key difference lies in their active states. In an open-collector setup, it can only drive a logic Low, and relies on external means to achieve a logic High. Tri-state, meanwhile, can actively drive both logic High or Low, in addition to its Hi-Z state. Therefore, open-collector outputs are often used for wired-AND functions, while tri-state outputs are often used in data buses to prevent signal contention.

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