Encoder Circuit

An encoder is a digital function that produces a reverse operation from that of a decorder. The main difference between a decorder and an encoder is that a decorder has n number of input and 2^n number of output lines or channels. On the other hand an encoder has 2^n number of inputs lines and n number of outputs lines or channels. As example we can say about octal to binary encoder which consists of eight inputs and three outputs lines or channels. It is simply constructed with OR gate whose inputs can be determined from the truth table. The low order output Y0 is 1 if the input octal digit is odd. Output Y1 is 1 for octal digits 2, 3, 6 and 7. Output  is 1 for octal digits 2, 3, 6 and 7. Output Y2 is a 1 for octal digits 4, 5, 6 or 7. It is seen that input I0 is not connected to any OR gate, the binary output must be all 0’s in this case. All the 0’s output is also obtained when all inputs are all 0’s.

Decorder

In digital communication system there is a common name (decorder and encoder) to transfer data to desire position. In digital electronics we use decorder and encoder for this purpose. To simply speaking we use encoder to collect data of from a large number of channels or lines and transmitted as a few number of channels or lines. On the other hand we use decorder to collect the data that is transmitted by the encoder by a few number of channel or line and then distributed it to greater number of channels or lines (as required by the authority).

Discrete quantities of information are represented in digital system with binary codes.A binary code of n bits is capable of representing up to 2^n distinct elements of the coded information.A decorder is a combinatinal circuit that converts binary information from n inputs lines or channels to a maximum of 2^n unique lines.If the n-bit decoded information has unused or don’t-care combinations, the decorder output will have less than 2^n outputs.

Now we will try to construct a 3 to 8 line decorder. As mentioned earlier that, if a decorder has 3 input lines, it can handle data up to 2^3 or 8 output lines. Let we represent inputs as A0, A1 and A2 and outputs as D0, D1, D2, D3, D4, D5, D6 and D7.

Truth Table

Inputs    A0   A1     A2      Outputs 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