It’s a very simple modification to the schematic design. We simply invert the B inputs and leave the A inputs as they are. Instead of adding A and B, this configuration will subtract B from A.
In part 6, we built a 1-bit full adder. In this part we’re going to expand upon that and build a complete 4-bit binary adder. I have made the assumption that you already know about the binary number system but in case you don’t, have a look here and do some searching to learn about it.
Back? Great, let’s get on with it. Grab your CPLD development board, a breadboard and the usual handful of LED, switches and resistors. We will need switches for this guide instead of buttons.