A decoder is a multiple input, multiple output logic circuit that changes codes i/ps into coded o/ps, where both the inputs and outputs are dissimilar for instance n-to-2n, and binary coded decimal decoders. Decoding is essential in applications like data multiplexing, memory address decoding, and 7 segment display. The best example of decoder circuit would be an AND-gate because when all its inputs are “High.”, the output of this gate is “High” which is called “active High output”. As an alternative to AND gate, the NAND gate is connected the output will be “Low” (0) only when all its inputs are “High”. Such o/p is called “active low output”.

2 to 4 Line Decoder 

In this type of decoders, decoders have two inputs namely A0, A1, and four outputs denoted by D0, D1, D2, and D3. As you can see in the following truth table – for every input combination, one o/p line is turned on.D0 =A1 A0, ( minterm m0) which corresponds to input 00 D1 =A1 A0, ( minterm m1) which corresponds to input 01 D2 =A1 A0, ( minterm m2) which corresponds to input 10 D3 =A1 A0, ( minterm m3) which corresponds to input 11.

The circuit is implemented using AND gates. In this circuit, the logic equation for D0 is A1/A0, and so on. Thus, each output of the decoder will be generated to the input combination.

Truth Table

Multiplexer is a combinational circuit that has maximum of 2n data inputs, ‘n’ selection lines and single output line. One of these data inputs will be connected to the output based on the values of selection lines.

Since there are ‘n’ selection lines, there will be 2n possible combinations of zeros and ones. So, each combination will select only one data input. Multiplexer is also called as Mux.

4x1 Multiplexer

4x1 Multiplexer has four data inputs I3, I2, I1 & I0, two selection lines s1 & s0 and one output Y. The block diagram of 4x1 Multiplexer is shown in the following figure.

One of these 4 inputs will be connected to the output based on the combination of inputs present at these two selection lines. Truth table of 4x1 Multiplexer is shown below.

TRUTH TABLE

Selection LinesOutputS1S0Y00I001I110I211I3

From Truth table, we can directly write the Boolean function for output, Y as

Y=S1′S0′I0+S1′S0I1+S1S0′I2+S1S0I3Y=S1′S0′I0+S1′S0I1+S1S0′I2+S1S0I3

We can implement this Boolean function using Inverters, AND gates & OR gate. The circuit diagram of 4x1 multiplexer is shown in the following figure.

CIRCUIT DAIGRAM

Binary to Gray code conversion

The Binary to Gray code converter is a logical circuit that is used to convert the binary code into its equivalent Gray code. By putting the MSB of 1 below the axis and the MSB of 1 above the axis and reflecting the (n-1) bit code about an axis after 2^n-1 rows, we can obtain the n-bit gray code.

In 4-bit gray code, the 3-bit code is reflected against the axis drawn after the 2^4-1-1^th =8^th row.

Just like Binary to Gray code conversion; it is also a very simple process. There are the following steps used to convert the Gray code into binary.

• Just like binary to gray, in gray to binary, the 1^st bit of the binary number is similar to the MSB of the Gray code.
• The 2^nd bit of the binary number is the same as the 1^st bit of the binary number when the 2^nd bit of the Gray code is 0; otherwise, the 2^nd bit is altered bit of the 1^st bit of binary number. It means if the 1^st bit of the binary is 1, then the 2^nd bit is 0, and if it is 0, then the 2^nd bit be 1.
• The 2^nd step continues for all the bits of the binary number.
  

8x1 Multiplexer

In the 8 to 1 multiplexer, there are total eight inputs, i.e., A₀, A₁, A₂, A₃, A₄, A₅, A₆, and A₇, 3 selection lines, i.e., S₀, S₁and S₂ and single output, i.e., Y. On the basis of the combination of inputs that are present at the selection lines S⁰, S^1, and S₂, one of these 8 inputs are connected to the output.

Let the 8x1 Multiplexer has eight data inputs I7 to I0, three selection lines s2, s1 & s0 and one output Y. The Truth table of 8x1 Multiplexer is shown below.

 BLOCK DIAGRAM