An encoder is a combinational circuit that converts binary information in the form of a 2N input lines into N output lines, which represent N bit code for the input. For simple encoders, it is assumed that only one input line is active at a time.

One limitation of this encoder is that only one input can be active at any given time. If more than one inputs are active, then the output is undefined. For example, if D6 and D3 are both active, then, our output would be 111 which is the output for D7. To overcome this, we use Priority Encoders.