Experiment 4 - The Gated RS Latch (RS-E Latch)

An RS latch changes its result Q each time the inputs R or S are pressed. However, this is not always desirable. Sometimes it is useful to "switch off" the inputs. This is possible by expanding the RS latch.

Circuit Diagram Gated RS Latch.

Symbol Gated RS Latch in Circuit Diagrams.

For this purpose consider the circuit diagram shown. You recognize the RS latch (only this time NAND gates were used instead of NOR gates). Furthermore, the circuit is expanded by two more NAND gates, and thereby you get an additional input labeled E for "ENABLE"...

• As long as E = 1, the latch behaves like an ordinary RS latch. With R and S you switch between the two results back and forth. The flip-flop is "enabled" or "open".
• As soon as E = 0: The last result freezes, no matter what happens to the inputs R and S. The latch is "disabled" or "closed".

Circuit of a Gated RS Latch. (Enlarge)

Build the circuit shown in order to experience this. The ENABLE input is of vital importance for the further development of latches in its transformation to flip-flops. You come back to this shortly. But first back to the RS latch.