Multivibrators

Read this chapter, which discusses how logic gates are connected to store bits (0s and 1s). Combinational circuits, described in the previous section, do not have memory. Using logic gates, latches and flip flops are designed for storing bits. Groups of flip flops are used to build registers which hold strings of bits. For each storage device in Chapter 10, focus on the overview at the beginning of the section and the review of the device's characteristics at the end of its section. While you do not absolutely need to know the details of how latches and flip flops work, you might find the material of interest. We strongly recommend that you read the details of the design of each storage device, because it will give you a stronger background.

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:


A practical application of this might be the same motor control circuit (with two normally-open push button switches for start and stop), except with the addition of a master lockout input (E) that disables both push buttons from having control over the motor when its 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.”

REVIEW:

  • The enable input on a multivibrator must be activated for either S or R inputs to have any effect on the output state.
  • This enable input is sometimes labeled “E”, and other times as “EN”.