The following figure shows the outputs of the SR-latch for two different sets of input values:
You can find the output values by starting with the input signal that is 1 (do not start with the input signal that is equal to 0, since you don't know what value the other signal is). That signal has been highlighted in red in the figure above.
The fact is: Nor(1, 0) = 0 and Nor(1, 1) = 0 (see: click here), so Nor(1, x) = 0, regardless what value x is.
Use the fact that Nor(1, x) = 0 to determine the output that is 0, and use that value in the other input to compute the second output value. (It's a lot easier to show that in class than writing it down...)
The figure on the left shows the signal before we change S from 1 to 0. You can see in the left figure that:
In the right figure, we can again compute Q and Q-bar (with S = 0):
Conclusion: S = 0, R = 0 ---> Q = 1, Q-bar = 0
The figure on the left shows the signal before we change R from 1 to 0. You can see in the left figure that:
In the right figure, we can again compute Q and Q-bar (now with R = 0):
Conclusion: S = 0, R = 0 ---> Q = 0, Q-bar = 1
So for S = 0 and R = 0, is Q = 0 or is Q = 1 ???
Did we make a mistake here ?
No, this is a case where the output of a circuit does not only depends on its inputs, but also on the state:
You have just seen your first sequential circuit !
Note: we will not bother with the input S = 1 and R = 1, because we will not use that input at all.
The file contains 2 SR-latches. The top one shows the Q-bar output for completeness and the bottom one shows only the Q output, which is the output that really matters. The file contains some comments on what to do.