Quadruple 2 to 1 line multiplexer
As in decoders, multiplexers may have an enabled input to control the operation of the unit. When the enable input is in the inactive state, the outputs are disabled, and when it is in the active state, the circuit functions as a normal multiplexer. The enable input is useful for expanding two or more multiplexers to a multiplexer with a larger number of inputs.
In some cases, two or more multiplexers are enclosed within a single integrated circuit package. The selection and the enable inputs in multiple-unit construction are usually common to all multiplexers. As an illustration, the block diagram of a quadruple 2-to-1-line multiplexer is shown below:
The circuit has four multiplexers, each capable of selecting one of two input lines. Output Y0 can be selected to come from either input Ao or Bo. Similarly, output Y; may have the value of A1 or B1, and so on. One input selection line S selects one of the lines in each of the four multiplexers. The enable input E must be active for normal operation.
Although the circuit contains four multiplexers, we can also think of it as a circuit that selects one of two 4-bit data lines. As shown in the function table, the unit is enabled when E = 1. Then, if S = 0, the four A inputs have a path to the four outputs. On the other hand, if S = 1, the four B inputs are applied to the outputs. The outputs have all O’s when E = 0, regardless of the values of S.