# Implement a function 3x8 Decoder with 4 variables

I'm required to implement a function

F = ~ABC~D + ~A~B~C~D + ~ABC + B~CD

using a 3x8 Decoder and any other additional combinational logic.

How can I implement this function if there are 4 variables (ABCD) in the expression but only 3 inputs in my 3x8 Decoder?

If I take only ABC as my 3 inputs on my decoder how can I include D to complete my function.

• For a start you can reduce the function to a more basic form. I can see that one part can be removed straight away. Then maybe draw a truth table to see if anything else drops out. "other additional combinational logic" gives you a lot of flexibility to combine terms also. – Andy aka Jan 11 '18 at 9:49
• I drew a truth table and K-map to reduce the expression but just got back the same expression. I think the function is already minimised? I don't know where to start really – zzzbai Jan 11 '18 at 11:10
• Reread the first comment and try again. – RoyC Jan 11 '18 at 13:55
• Actually, simplifying the function is a distraction. The ~A~B~C~D and B~CD terms make it impossible to use a single 3x8 decoder. – StainlessSteelRat Nov 26 '18 at 20:23