2-INPUT 4-BIT MULTIPLEXER

The MSI, 74X157 is a 2-input, 4-bit Multiplexer. This multiplexer has two sets of 4-bit inputs. It also has 4-bit outputs. The single select input line allows the first set of four inputs or the second set of 4-inputs to be connected to the output. Thus four-bits of data from two sources are routed to the output. The function table and the circuit of the multiplexer are shown. table 18.1, figure 18.1

digital logic design  2 INPUT 4 BIT MULTIPLEXER

The multiplexer has two sets of 4-bit active-high inputs 1A, 2A, 3A, 4A and 1B, 2B, 3B, 4B respectively. The multiplexer has 4-bit active-high outputs 1Y, 2Y, 3Y 4Y. The single select input allows either the 4-bit input A or the 4-bit input B to be connected to the 4-bit output Y. The G active-low pin enables or disables the Multiplexer.

Inputs Outputs
G S 1Y 2Y 3Y 4Y
1 X 0 0 0 0
0 0 1A 2A 3A 4A
0 1 1B 2B 3B 4B

Table 18.1 Function table of 2-Input 4-Bit Multiplexer

digital logic design  2 INPUT 4 BIT MULTIPLEXER

Expanding Multiplexers

Multiplexers have to be connected together to form larger multiplexer to fulfil specific application requirements.

1. 8-Input Multiplexer

A single dual, 4-input multiplexer 74X153 can be connected to form an 8-input multiplexer. The circuit diagram and the function table are shown in fig. 18.2 and table 18.2 respectively. The two active-low enable inputs of the two 4-input multiplexers are connected together using a NOT gate to form the C input of the 8-input multiplexer. When C is set to 0, the first multiplexer is selected allowing its inputs 1C0, 1C1, 1C2 and 1C3 to be selected through select inputs A and B. When C is set to 1, the second multiplexer is selected allowing its inputs and outputs to be used. The two outputs are connected through an OR gate.

digital logic design  2 INPUT 4 BIT MULTIPLEXER

Input Output
C B A F
0 0 0 1C0
0 0 1 1C1
0 1 0 1C2
0 1 1 1C3
1 0 0 2C0
1 0 1 2C1
1 1 0 2C2
1 1 1 2C3

Table 18.2 Function Table of a 8-to-1 Multiplexer

2. 16-Input Multiplexer

Two 74XX153 Dual, 4-input multiplexer can be connected to form a 16-input multiplexer. The circuit diagram and the function table of the 16 input multiplexer are shown in Figure 18.3 and table 18.3 respectively.

The select inputs A and B of the two dual, 4-input multiplexers are connected together which allows selection of any one input out of the four set of 4-bit inputs. The four active-low multiplexer enable inputs which allow selection of any one of the four multiplexers are connected to the active-low outputs of a 2-to-4 decoder. The decoder inputs C and D enable one out of the four multiplexers. The four outputs are connected together through a 4-input OR gate. The G enable input of the decoder when set to 1 disables the decoder and the multiplexers.

digital logic design  2 INPUT 4 BIT MULTIPLEXER

3. 2-Input, 8-bit Multiplexer

Two 2-input, 4-bit multiplexers 74X157 can be connected to implement a 2-input, 8-bit multiplexer. The circuit diagram is shown in figure 18.4. The select S inputs of the two multiplexers are connected together so that the 4-bit inputs A of both the multiplexers are selected simultaneously when S is set to logic low. Similarly, by setting the S input to logic-high the B inputs of both the multiplexers are selected. The active-low enable inputs G of both the multiplexers are also connected together so that both the multiplexers are enabled and disabled simultaneously by setting the G input to 0 or 1 respectively.

Inputs Output
G D C B A F
1 x x x x 0
0 0 0 0 0 1C0 (M1)
0 0 0 0 1 1C1 (M1)
0 0 0 1 0 1C2 (M1)
0 0 0 1 1 1C3 (M1)
0 0 1 0 0 2C0 (M1)
0 0 1 0 1 2C1 (M1)
0 0 1 1 0 2C2 (M1)
0 0 1 1 1 2C3 (M1)
0 1 0 0 0 1C0 (M2)
0 1 0 0 1 1C1 (M2)
0 1 0 1 0 1C2 (M2)
0 1 0 1 1 1C3 (M2)
0 1 1 0 0 2C0 (M2)
0 1 1 0 1 2C1 (M2)
0 1 1 1 0 2C2 (M2)
0 1 1 1 1 2C3 (M2)

Table 18.3 Function Table of 16-bit Multiplex

digital logic design  2 INPUT 4 BIT MULTIPLEXER

VN:F [1.9.14_1148]
Rating: 10.0/10 (1 vote cast)
VN:F [1.9.14_1148]
Rating: 0 (from 0 votes)
2-INPUT 4-BIT MULTIPLEXER , 10.0 out of 10 based on 1 rating