Electrical – 4ِ-bit adder in Multisim


I want to design a 3-bit by 3-bit number adder circuit. The result must be in BCD. And I want to show the result by using 7-segment displays.

I built this circuit. I'm not sure from it, and when I run the circuit it doesn't show anything.

Where is my fault?!

enter image description here

Best Answer

You actually got a lot of it right. But I don't use Multisim and can't help there, much. I think I see a wiring mess around your three gates -- you should look closely there. There might be "crossed wires" in there somewhere.

Also, you do NOT need a three-input OR gate. Just a two-input one. The logic should be something like:


simulate this circuit – Schematic created using CircuitLab

I'm not showing the 7447's there. But you should get the idea for those.

As far as the displays not showing anything? That's a different problem. They come with a lamp test pin. Use it to verify that your displays are wired up correctly. The outputs are open-collector, so you must be using a common-anode 7-segment display device and NOT a common-cathode device, for example. There are lots of possible problems there. Also, if you have a voltmeter use it to probe the voltages at each of the binary lines going in to make sure they are values you expect. Etc. But mostly, forget the rest of the circuit and just focus on making sure your digit displays actually work. If they don't, nothing else will. And they are easy to test. So do that FIRST. Before screwing around with the adder stuff.