Question: 3 to 8 Decoder and truth table of 3 to 8 decoder.
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3 to 8 Decoder.

• A 3 to 8 decoder has three inputs (A,B,C) and eight outputs (DO to D7).

• Based on the 3 inputs one of the eight outputs is selected.

• The truth table for 3 to 8 decoder is shown in table (1).

• From the truth table, it is seen that only one of eight outputs (DO to D7) is selected based on three select inputs.

• From the truth table, the logic expressions for outputs can be written as follows:

Truth table of 3 to 8 decoder.

| Inputs | Outputs | |----|----|

A B C D0 D1 D2 D3 D4 D5 D6 D7
0 0 0 1 0 0 0 0 0 0 0
0 0 1 0 1 0 0 0 0 0 0
0 1 0 0 0 1 0 0 0 0 0
0 1 1 0 0 0 1 0 0 0 0
1 0 0 0 0 0 0 1 0 0 0
1 0 1 0 0 0 0 0 1 0 0
1 1 0 0 0 0 0 0 0 1 0
1 1 1 0 0 0 0 0 0 0 1 • Using the above expressions, the circuit of a 3 to 8 decoder can be implemented using three NOT gates and eight 3-input AND gates as shown in fig (1).

• The three inputs A, B and C are decoded into eight outputs, each output representing one of the midterms of the 3-input variables.

• The three inverters provide the complement of the inputs and each one of the wight AND gates generates one of the midterms.

• This decoder can be used for decoding any 3-bit code to provide eight outputs, corresponding to eight different combinations of the input code.

• This is also called a 1 of 8 decoder, since only one of eight output lines is HIGH for a particular input combination. Fig (1): Logic diagram of 3 to 8 decoder.

• It is also called a binary-to-octal decoder, since the inputs represent 3-bit binary numbers and the outputs represent the eight digits in the octal number system. Fig (2): 3-to-8 decoder in IC form.

renu • 125 views
 written 10 weeks ago by RB ♦♦ 100