The Gated S-R Latch

The gated S-R latch

It is sometimes useful in logic circuits to have a multivibrator which changes state only when certain conditions are met, regardless of its S and R input states. The conditional input is called the enable,  and is symbolized by the letter E. Study the following example to see how this works:

When the E=0, the outputs of the two AND gates are forced to 0, regardless of the states of either S or R. Consequently, the circuit behaves as though S and R were both 0, latching the Q and not-Q outputs in their last states. Only when the enable input is activated (1) will the latch respond to the S and R inputs. Note the identical function in ladder logic:

practical application of this might be the same motor control circuit (with two normally-open pushbutton switches for start and stop), except with the addition of a master lockout input (E) that disables both pushbuttons from having control over the motor when it´s low (0).

Once again, these multivibrator circuits are available as prepackaged semiconductor devices, and are symbolized as such:

It is also common to see the enable input designated by the letters "EN" instead of just "E."