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f-alpha.net » Electronics » Digital Electronics » Boolean Logic » Go on... » Experiment 15 - The Latch

Experiment 15 - RS Latch

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.

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