*3.2.1 Working*

*Multiplexing*

**22**

**Figure 17**.

**3.1 Four-in-one multiplexer**

*Logic diagram of eight-in-one multiplexer.*

**3.2 Eight-in-one multiplexer**

**Figure 15**.

**Figure 18.**

The logic symbol and circuit for a four-input multiplexer are shown in

Here D0, D1, D2, and D3 are data input lines. S0 and S1 are data selector or logic switches. When S0 = S1 = 0, then the two inputs of the first AND gate become actively high because the selectors S0 and S1 are inverted using NOT gate and given to this gate. Thus the data from D0 line is outputted through this AND gate. At that time the other gates are in 0 output position. Similarly when S0 = 1 and S1 = 0, then the two inputs of the AND gate 2 become actively high; thus, data from D1 is transmitted through gate 2 as output, and all other gates are in 0 output position. In this manner D2 and D3 are inputted to consecutive switch positions. Here an OR gate is used to combine these four output lines as a single output (**Figure 16**).

The logic symbol and data selector of eight-in-one multiplexer is shown in

If input is 2, then one data selector switch is needed; if input is 4, then two selector switches are needed; if input is 8, then three selector switches are needed; if

input is 16, then four selector switches are needed; and so on.

When the three selector switches are actively low, then the three inputs of the first AND gate become actively high because the selector outputs are NOTed and given to the first AND gate. Thus the data from D0 line is outputted through the first AND gate, and all other AND gates are in 0 output position. When S0 = 1, S1 = 0, and S2 = 0, then the three input terminals of the second AND gate become actively high, and D1 is outputted through this gate. At that time all other gates are in 0 output position. In that manner D2, D3, D4, D5, D6, and D7 data are outputted in the next consecutive switch positions.

Important TTL multiplexer IC's are as follows:

74,157—2 input data selector/MUX.

74,151—8 input data selector/MUX.

74,150—16 input data selector/MUX.
