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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.**