![]() The same is true for U2, with the result being that if R and S are both held high, Q and notQ must remain forced low - and are therefore stable - until either R, or S, or both change state. Note that the R input has been replaced with the complement (inversion) of the old S input, and the S input has been renamed to D. Referring to 'B' and perusing the NORs truth table, we find that if R is high (a logic 1) then, regardless of the state of U1-2, Q must be low. Such a circuit is called a D latch, and its internal logic looks like this: Since the enable input on a gated S-R latch provides a way to latch the Q and not-Q outputs without regard to the status of S or R, we can eliminate one of those inputs to create a multivibrator latch circuit with no "illegal" input states.
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