 Look what I got in the mail from EasyEDA, boards for the 4-digit hex display. These are four-layer boards, and were about 7 US dollars each. If there were only two layers, it would have been about 40 cents each, so I definitely went luxury. There's not much need to pay attention to signal integrity here. I laid out the board so that the displays go in the center, all the resistors go on the other side under the displays, and the Atmel chips, one for each digit, go on the side. The spacing of the connectors is also breadboard friendly. I put the circuit board under my Amscope inspection microscope, and solder all the SMD resistors and capacitors first. I'm using .4mm solder here. There are only 40 SMD components, so it's not too bad to hand solder. I could have gotten a stencil for the 34 components on one side, and saved some time, but I'd only do that if I were making a lot of these. Anyway, next I use the programmer to program the Atmel chips. There's an adapter for programming surface mounts. Once the four devices are programmed, I solder them in, then the seven-segment displays, and finally the pin headers. Now it's time for the smoke test. I pet the cat, and turn on the power. That worked. Great. Now I'll hook up the inputs. It seemed to work okay, but then I noticed that blanking the display wouldn't blank one of the digits. I also found that one of the inputs was non-responsive. Not so great. So after resoldering the problematic pins and getting no improvement, I took out a fresh board and the multimeter, tested the connections for continuity, and found none when there should be. At this point, it could be a defect in the PCB manufacturing, or a fault in the design. I looked at my board in Kaikad, and sure enough I was missing those connections. I had failed to run DRC before output to Gerber. My bad. Well, nothing that a couple of bodge wires won't fix. With those in place, I retested and the board works as intended. I fixed the problem in Kaikad and generated new Gerbers. The Kaikad project and Gerber zip file are all in the GitHub project linked below. The next step will be to connect these to the data and address lines of my TMS-9900 CPU project, and that will be the subject of the next video. Thanks for watching.