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I am a novice when it comes to electrical engineering and I have a very basic question concerning the operation of RS latches:

I am currently working my way through Roth/Kinney's Fundamentals of Logic Design and I have some problems understanding the part describing the operation of RS latches. The book gives this table for the states of RS latches where Q^+ = R'S + R'Q and P = S'Q' and the stable states of the latch are circled:

enter image description here

Now, how comes that for SR = 01, Q = 1 and SR = 10, Q = 0 the latch does not end up in a stable states. I thought that that as long as SR != 11 the latch always ends up in a stable state.

Also, for SR = 11, Q = 0, how can the latch end up in a stable state. I thought that setting SR to 11 is forbidden and the latch state is undefined for this input combination.

Can anybody clarify that for me?

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  • \$\begingroup\$ 'stable' means 'is going to stay in the same state'. For Q=1, SR = 01 (i.e. Q=1, S=0, R=1) (R)eset is high. Next state is going to be 0. So the state will change. Q=0, SR=01 is stable because R doesn't do anything if Q is already 0. We can also see from SR=11, Q=0 that R overrides S, i.e. if you try to both set and reset this latch you get 0 as your next state and that it stays there rather than oscillates \$\endgroup\$ – Will Apr 2 '14 at 15:13
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If SR is 11 then the outputs of the cross-coupled NOR gates is 00 (defined).

(from the book) enter image description here

Assume typos in this book..

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The particular latch circuit you describe is not the typical SR latch. Notice that there is no feedback from Q` to Q. I think this latch gives precedence to R over S, so that a transition from SR=11 to SR=00 will leave the latch in the reset state.

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  • \$\begingroup\$ I'll go one further, and say that the circuit described IS NOT an SR latch. \$\endgroup\$ – Scott Seidman Apr 1 '14 at 15:30

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