MUX 8 to 1 Problem

[melvinphil91]

Hello to all!

I have recently just started using MultiSIM for my electrical engineering course and I could use some assistance. 

My task is to create a 4 bit ALU which is capable of performing 5 functions [ ADD, SUB, OR, NOT, AND ].

After a couple of days playing around the problem I have been able to come up with a reasonable solution that involves using 4x 8 to 1 MUX's which based on the control lines [ AND = 000, OR = 001, NOT = 010, ADD = 011, SUB = 100 ] should spit out the correct result. 

I have slowly been adding the opcodes as I go along and complete each subsystem. However I have run into a problem where the opcode 001 [output D1] is not "triggering" for some reason I have yet to figure out. I tried isolating the particular system and tried to reproduce the problem, which I have done successfully. 

It seems even though G = logic 1 (input logic 0 since NOT gate is present ) and ABC = 001 it doesn't give any output 😕 however for the other two opcodes that I have implemented [AND and NOT ] it works perfectly. 

I have attached the isolated circuit for OR and I hope someone can help me figure out why this wont give me an output.

Kind regards

Melvin

[melvinphil91]

Update : Figured out my own problem lol.

Turns out I wired up C0-C3 wrong, the way I wired it, it would be read backwards, i.e 001 would be read as 100 and give the wrong output!

Using the multisim I posted above if you input 100 on control lines it will be read as 001 and give the intended output! 

Thanks anyway!