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
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.
Table 18.1 Function table of 2-Input 4-Bit Multiplexer
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.
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.
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.
Table 18.3 Function Table of 16-bit Multiplex