After learning about Full Adders and Ripple Carry Adders in ENGG*2410 — Digital Systems Design in the Fall semester of 2023, I wanted to build an 8-Bit ripple carry adder at home. The issue being that I do not own a Xilinx FPGA and therefore would need a different strategy than the one used in ENGG*2410. Luckily though I had a few breadboards and Quad 2-input Logic ICs. Specifically, XOR, AND, and OR logic chips. Each 14 pin DIP logic chip had 4 logic gates on it. Each gate has two input pins and an output pin. This leaves one more pin of the DIP package for a VCC connection and then one more for ground. The first step in the process was to plan out the 8-Bit ripple carry adder. A plan of the logic was made using AutoCAD.

Then a new block diagram was made that incorporated the pinout of the DIP ICs. This was again made using AutoCAD and like the above image it was exported as an SVG so that all the details can be zoomed in on. This new block diagram was made to get a better understanding for how to plan out the wires and connections that would be needed for the physical assembly of the 8-Bit adder.

Once this was done I began assembling the breadboards. I began by layout all the ICs I would need. In this project I made use of:

• 4 x 74x86 Quad 2-input XOR ICs
• 4 x 74x08 Quad 2-input AND ICs
• 2 x 74x32 Quad 2-input OR ICs
• 2 x DIP 8 Switches Board
• 2 x Full sized Breadboards
• 1 x Mini Breadboard
• 5 x 200Ω Resistors
• 8 x 10KΩ small Resistors
• 8 x 10KΩ larger Resistors
• 8 x Green LEDs
• 1 x Red LED
• Many many wires

The ICs were split up and placed evenly across the two full sized breadboards. The mini breadboard was used for the output to be displayed on the 9 LEDs. The 8 green LEDs being the output from sum bits S₀ to S₇ and the red LED displays the final carry bit C_out. This C_out bit could in theory also be considered as an S₈ bit when interpreting the binary result of the addition. The building process began by routing all wire as clean, straight and direct as possible. When only a few connections were made this was easy.

As more and more full adders were wired up and completed the connections started to get more out of hand an increasingly unmanageable. This project really makes me appreciate FPGAs design where the connection logistics are not important for the designer, and PCBs with elegant routing. Doing all the wire routing by hand is by far the hardest and most time-consuming part of this entire project. Near the final two full adders I also ran out of the stiffer flat laying wires and instead had to finish the project with jumper cables.

Overall, I am quite pleased with the 8-Bit Ripple Carry Adder. It took much longer than I anticipated and is a little more messy than I had hoped for. But in order to finish it within a reasonable amount of time and using the wire and tools I had on hand this was the most reasonable result. Given more time and some more wire and proper wire strippers I would probably have been abler to make a cleaner routes for the wires perhaps I will try to rebuild it with such tools. What I find most pleasing about it is watching the carry bits roll over into the output, this can be seen below.