Experiment 5 - The D Latch

Do you remember the RS latch from the third experiment? The result of the RS latch is not defined when the inputs R and S jump simultaneously to 1. With a simple trick you make that impossible. Yet this has consequences.

With a simple NOT gate you prevent that R and S switch simultaneously to 1. The circuit diagram shows you how. Now the former input signal R is always the opposite of the former input S and you are left with a single input, which is renamed to D (for "DATA").

Circuit diagram D Latch.

Symbol D Latch in Circuit Diagrams.

Remember the truth table for the RS latch from experiment 3? With the D latch you prevent not only the undefined state (R=1, S=1), but also the condition in which the last result is stored (R=0, S=0).

Circuit of a D Latch. (Enlarge)

In fact, the D latch has lost its memory. Q always follows D! The D latch is strictly speaking not a latch! Convince yourself with the circuit shown. In the next experiment, you give the circuit its memory back.