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.

The encoder that shown in the fig indicates that only one input line can be equal to 1 at any time. The circuit has eight inputs and could have 2^8=256 possible input combinations. The most interesting thing is that above those 256 possible combination only eight combination has suitable meaning. The other input combinations are don’t-care conditions. The type of encoder available in IC form is called a priority encoder. 

 

Truth Table

Inputs  I0   I1   I2     I3   I4   I5   I6   I7

Outputs  Y2             Y1              Y0

1        0          0        0         0         0         0         0   0        1          0        0         0         0         0         0   0        0          1        0         0         0         0         0   0        0          0        1         0         0         0         0   0        0          0        0         1         0         0         0   0        0          0        0         0         1         0         0   0        0          0        0         0         0         1         0   0        0          0        0         0         0         0         1

0                0                 0    0                0                 1   0                1                 0   0                1                 1   1                0                 0   1                0                 1   1                1                 0   1                1                 1

 

Circuit Diagram

The outputs can be shown as

Y2=I4+I5+I6+I7

Y1=I2+I3+I6+I7

Y0=I1+I3+I5+I7