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Experiment 3 - The RS Latch

Due to the feedback, the circuit from the last experiment obtains an unusual truth table...

  • R and S are not equal: Q follows S and Q' follows R.
  • R and S are equal 0: the last result of Q and Q' is stored.
R S Q Q'
0 0 stored
0 1 1 0
1 0 0 1
1 1 undefined

Truth Table RS Latch.

Symbol RS Latch in Circuit Diagrams.

This is a classic latch circuit. Because of its functionality it is called RS latch, where R stands for "RESET" and S for "SET".

Circuit diagram RS latch with NOR gates.

Circuit RS latch with NOR gates. (Enlarge)


A RS latch can also be constructed using NAND gates. This results in some differences, which are discussed in the section "Know more..."

For practical reasons, in the following experiments, the RS latch is sometimes contructed from NOR gates and sometimes from NAND gates. Therefore, pay close attention to which gates are used in the circuit.

This basic RS latch has one big drawback: If R and S are going to 1 simultaneously, the result is undefined.

In the next experiments, you will expand the RS latch.