Page 76 - Computer science 868 Class 12
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The truth table is given below:

                                    Octal Number         B 2              B 1             B 0
                                          0               0               0               0
                                          1               0               0               1
                                          2               0               1               0
                                          3               0               1               1
                                          4               1               0               0
                                          5               1               0               1
                                          6               1               1               0
                                          7               1               1               1

              Here B , B  and B  represent the OR gates where the binary outputs are obtained. If the Octal numbers are represented
                             2
                    0
                       1
              by switches I0 to I7 then from the truth table we can conclude that
              B  =  I  + I  + I  + I 7
                   1
                       3
               0
                          5
              B  =  I  + I  + I  + I 7
                          6
                       3
                   2
               1
              B  =  I  + I  + I  + I 7
               2
                          6
                   4
                       5
              The logic circuit diagram of an Octal to Binary Encoder is as follows:
                                         0
                                         1
                                         2
                                         3
                                         4
                                         5
                                         6
                                         7








                                                       B 2              B 1            B 0
              Working Principle of Octal to Binary Encoder: Say the switch of Octal number 2 is closed. 2 is connected only with OR
              gate B . So a trigger applied makes B  = 1 while B  and B  will retain their default value of 0. Thus B = 0  B  = 1 and
                                                1
                    1
                                                                 2
                                                                                                        2
                                                                                                               1
                                                           0
              B  = 0  or 0 1 0 which is the binary equivalent of 2
               0
              Similarly on pressing switch 5 which is connected with B  and B , we get B  = 1  B  = 0 and B  = 1  or 1 0 1  which is the
                                                                                2
                                                                      0
                                                                                               0
                                                                                      1
                                                                2
              binary equivalent of 5
              2.4.2 Decimal to Binary Encoder
              We know the base of a decimal  number is 10 and the digits are from 0 to 9. We need 4 bits to represent the binary
              value. So a binary encoder has 10 input lines and 4 output lines.




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