Truth table for 2 inputs selector are: A0 A1 D0 D1 D2 D3 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 1 0 1 1 0 0 0 1

For each combination of A0 and A1 only one of the options of Di will get the value of 1. In general 2   input bits decoder has   22 = 4  output lines. 3   input bits decoder has   23 = 8  output lines. n   input bits decoder has   2n  output lines.

Truth table for 3 inputs decoder: A B C EN s0 s1 s2 s3 s4 s5 s6 s7 X X X 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 1 0 1 0 0 1 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 1 0 1 1 0 0 0 0 0 1 0 0 1 1 0 1 0 0 0 0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0 1

The 3 inputs decoder has 8 outputs possibilities, it means that for any input combination of A, B and C only one different output exists (s0 to s7). Generally, decoders are provided with enable input (EN) so as to activate the decoded output based on the inputs. The decoder can be widely used in many fields, for example 4 inputs can be used to represent the decimal digits zero to nine.

Truth table: A s0 s1 0 0 1 1 1 0

This kind of circuits can be used for turning on and off processes for example to activate a pump to fill a container, if it has a full gage indicator than it turns off and another process can be activated.

Truth table: A EN s0 s1 X 0 0 0 0 1 1 0 1 1 1 0

This circuit is the same as the previous one but we added an input EN that turns all the outputs to zero no matter what the input is. this can represent an emergency stop for all processes.