Logic Gates, Level III

Welcome back for another round with logic gates. If you haven’t already read Logic Gates, Level I or Logic Gates Level II, please do so in order to make sure you are fully caught up. Moving along…

We will start this lesson with the “buffer” gate. This gate is also known as a digital buffer or voltage buffer because of its function. The buffer provides a way to pass along an output that is the same value as the input. For instance, passing in a 1 will give you a 1, and a 0 will give you a 0. Here is the truth table just for completion:

Input A: 1 | Output Q: 1
Input A: 0 | Output Q: 0

The buffer gate is represented as such:

A “buffer” gate in “Distinctive Shape” symbolic illustration

Why use a buffer gate if there is no change in the output? Simply put, the purpose of the buffer gate is to increase the propagation delay in a circuit, which is just to say that it makes the signal take longer to reach its destination.

A buffer gate can also restore the strength of a weak output. This is because of the design of the buffer gate, which is usually two inverters back-to-back. This takes a weak input, inverts it twice, and with the inversion the input strength is restored in the strong output.

There is a version of the buffer gate known as the Tri-State Digital Buffer, which can take input as a control input alongside the data input. In this case, the control acts as gatekeeper.

When the control input is 1, then the data input is passed on as usual. However, if the control input is 0, then nothing is passed on, and this state is called the “Hi-Z” state. This is effectively the same as no output whatsoever.

Here is a truth table for the Tri-State Digital Buffer:

Data Input: 1 | Control Input: 0 | Output Q: Hi-Z
Data Input: 0 | Control Input: 0 | Output Q: Hi-Z
Data Input: 1 | Control Input: 1 | Output Q: 1
Data Input: 0 | Control Input: 1 | Output Q: 0

Notice that a 1 is only output if both the control and the data input are 1 as well.

That is about all for Level III, which gives a detailed description of the buffer gate and its purpose. More info coming in the next Level, so keep an eye out.

Good luck!

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