Experiment 15 - RS Latch

The concept of feedback is not only a mind game. It forms the basis for an entire class of circuits, latches and ultimately flip-flop.

Consider the circuit shown, built with NOR gates. This is a classic latch circuit. Because of its functionality it is called RS latch, where R stands for "RESET" and S for "SET".

R S Q Q' 0 0 stored 0 1 1 0 1 0 0 1 1 1 not defined Truth Table RS Latch.

Circuit Diagram RS Latch.

Due to the feedback an unusual truth table is produces...

• If R and S are not equal: Q follows S and Q' follows R.
• If R and S are equal to 0, the circuit stores the last results of Q and Q'.

Circuit of a RS Latch. (Enlarge)

By setting up the circuit, this will become more evident to you. When connecting the battery, either Q or Q' will light up. This is determined by chance. Suppose Q is lit, you can use the push button switch R to switch the lit LED's: Q' lits and Q extinguishes. With S you switch it all back again. If you release both push button, the circuit stores which push button you pressed last... Either R (Q' =1, Q =0) or S (Q =1, Q' =0). Try this for some time.

To make it even clearer how latches store information, you expand the RS latch in the last experiment.