Video: Evaluating the Output of NOR Gates

The diagram shows a NOR gate. If input A is 0 and input B is 1, what will the output be?

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Video Transcript

The diagram shows a NOR gate. If input A is zero and input B is one, what will the output be?

Okay, so, in this diagram we can see that we’ve been given a NOR gate. And we’ve also been given the values of the inputs. We’ve been told that input A is zero and input B is one. Based on this, we need to work out what the output is. So, to answer this we need to recall the truth table for a NOR gate.

So, here is the truth table for a NOR gate. Now an easy way to remember this is to think of a NOR gate as a NOT OR gate. And the reason for this is the following. Firstly, let’s think about an OR gate. Now what an OR gate does is that it gives an output of one if either input A or input B is one. In other words, for an OR gate if both inputs are zero, then the output is zero as well. However, if one of the inputs or both of the inputs are one, then automatically the output becomes one in all of those cases as well. So, that’s what an OR gate does. When either input A or input B is one, the output is one.

But in this case, we’re looking at a NOR gate, in other words, and NOT OR gate. And so, here’s where the NOT part comes in. Now a NOT gate has only got a single input, so we’ll call it input A. And it takes this input if it’s a zero, and turns it into a one. And if the input is one, it turns it into zero. In other words, it flips, or inverts, the input.

So, we take this OR gate, and, in each case, we send the output from the OR gate as an input into the NOT gate. This way the zero becomes a one and the ones become zeros. And so, now we’ve got the outputs from a NOR gate. And that’s why it’s beneficial to think of a NOR gate as a NOT OR gate.

But anyways, so coming back to the question at hand then, we’ve been told that the value of input A is zero and the value of input B is one. So, looking at the second row of the truth table of the NOR gate, we find that the output must, therefore, be zero. And hence, we’ve found the answer to our question.

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