ENCODER: A decoder is a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different. Binary Decoder has n inputs and 2n outputs also called as n-to-2n decoder. DECODER: Decoder is the combinational circuit which contains ‘n’ input lines to 2n output lines. The decoder is used for converting the binary code into the octal code. The IC74138 is the 3 x 8 decoder which contains three inputs and 8 outputs and also three enables out of them two are active low and one is active high. Decoders are used in the circuit where required to get more outputs than that of the inputs.
PROCEDURE 1. Connect as per the diagram given. 2. Apply Vcc to pin 16 and Gnd to pin 8 of the 74138. 3. Connect the inputs to pins 1,2,3. 4. Connect pin 4,5 to gnd and 6 to Vcc. 5. Similarly when E2 is HIGH all the outputs are high irrespective of the inputs. 6. When E3 is low all the inputs are high irrespective of E1 and E2 = high. 7. If E1 and E2 are low and E3 is high the inputs are low, the outputs 0o will be low with all the other outputs are low. 9. Change the inputs we get 1 and outputs as low and all the other outputs as high. 10. Note BCD output from the IC 74148 as the input to 74138. 11. Compare the truth table. 12. Note outputs of decoder truth table. RESULT: Thus the operation of 8 to 3 line Encoder and 3 to 8 Decoder using IC 74138, 74148 truth tables are verified.