THE S-R LATCH

Bi-stable multi-vibrator contains two stable states, as indicated by the prefix “bi” in its name. Typically, one state is referred as set and the other as reset. The simplest bi-stable device, therefore, is known as a set-reset, or S-R, latch. In order to create an S-R latch, wire two NOR gates in such a way that the output of one feeds back to the input of another  and vice versa, like this: The Q and not-Q outputs are supposed to be in opposite states. It is "supposed to" because making both the Sand R inputs equal to 1 results in both Q and not-Q being O. Due to this reason, having both S and R equal to 1 is called an invalid or illegal state for the 5-R multi-vibrator. Otherwise, making S=1 and R=0 "sets" the multi vibrator in such a way that Q=1 and not-Q=O. Conversely, making R=1 and s=0 "resets" the multi-vibrator in the opposite state. When S and R are both equal to 0, the multi-vibrator's outputs "latch" in their prior states. Note here that how the same multi-vibrator function can be implemented in ladder logic, with the identical results:

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