# How to Design a combinational circuit using a suitable decoder and OR gate

I need to construct a combinational circuit for the below truth table using a decoder and a OR gate. I'm familiar with the normal procedure of drawing combinational circuit diagrams but got confused when I saw "using a decoder" part.

According to the truth table it should be a 4X16 decoder. But How Can I mark the 16 outputs of the decoder with respective to the truth table's sum of production ??? I know how to use OR gate in this question, but implementing the outputs of the decoder is the problem.

• You just don't need to implement the decoder! It's a "building block" avalilable to you ready made. You don't have to syntetize it, just use it. – carloc Jun 17 '17 at 12:08
• Yes I know that it needs to be in Block diagrams. But I don't know how can I implement it – Code Bae Jun 17 '17 at 12:44
• Why do you need to implement it? – immibis Jan 4 '18 at 11:58

## 1 Answer

The decoder is a given. It is takes 4 inputs, and has 16 outputs. Outputs could be labeled 0 to 15. For any combination of inputs exactly one of the outputs will go high. Draw a block with 4 inputs on the left and 16 outputs on the right. That is given in the setup to the problem. For example, a 74LS154 IC is a decoder (but the outputs are active low).

The exercise is about combining the decoder and OR gates to get the output shown in the question.