Nibbler – Getting the parts organized

The circuit for the 4-bit computer called Nibbler is simple. It only has 17 ICs, which means it can be easily assembled on a prototype board with wirewrap sockets. Or so I thought. First, it’s tough to find the variety of wire wrap sockets needed. Amazon actually sells a few, but you have to watch what you purchase because Amazon’s search engine doesn’t know the difference between a DIP socket and a DIP wire wrap socket.

The next problem is finding the chips. Most of them are obsolete. Oh, the TTL logic chips are still common enough to be in high quantities at places like Amazon or Jameco, but the memory chips are beyond obsolete and difficult to find. I purchased EEPROMS that are larger in size, and I plan to wire the extra address pins to ground. Keep in mind that the board takes 3 EEPROM chips, but one of them is used to contain the program you want to run. In order to change programs, you can either erase the chip and burn it again, or you can buy extra chips and put different programs on each chip (ala interchangeable modules like the old game machines).

Things to be aware of

If you’ve never built a circuit before, there are a couple of things to keep in mind. First, there’s power. I bought a 5V DC adaptor that can handle 6 amps. It’s overkill, but I’m not taking chances, and the power adaptor was available, so I bought it. When wire wrapping, you can daisy chain your power across chips, but I’m going to use an in-line wire wrap bar along the bus on my prototype board. I’ll use one for VCC (+5V) and one for ground. Then, I’ll run one or maybe two wires per pin to each chip. That way, I don’t have the total current going through the first feeder wire that daisy chains them together.

I purchased mostly 74LS series TTL chips. These use a lot more power than the 74HCT chips, as specified in the original article. The HCT chips are almost impossible to find. Jameco has only a few chips that I can use, but they don’t have the 74HCT173s or 74HCT163s in stock. Anyway, I’ll try the LS chips first and see how they perform. If the timing difference between the chips specified and what I’m using is a problem, then I’ll go looking for the correct chips. I have to keep that in mind.

I couldn’t find a vendor that was selling the clock module that they specified, so I substituted a 2Mhz clock (which is a little slower). I should be OK with that.

First Steps

I purchased the chips, and I have most of the wire wrap sockets (the rest of my 14-pin sockets come on Monday, 4/8/2024). I already have the board, the LCD module, and other components. I’ve already started arranging the wire wrap sockets on the board to determine where they are best placed. When I’m happy with that, I’ll solder them in place (just two pins at opposite ends).

The next piece is to focus on the power. I’ll put in the wire wrap strips:

Then, I’ll place all the bypass capacitors near each chip. They are all 0.1 uf in size. I usually solder one side of the capacitor to the VCC pin, and then wire wrap the other side to ground for each chip.

Then, I’ll hook up the power connector and wire all the VCC and grounds to each chip socket.

The final step will be to plug in all chips and the display. Then, I’ll hook up my oscilloscope to the power and watch the power level with all the circuits powered up. At this point, the computer will do nothing because it’s not wired, but I can make sure the power isn’t noisy or getting dragged down to 4.5 volts or something crazy like that.

Oh, there is one more step: Take pictures. In the next article, I’ll show the completed board to that point. I’ll take pictures along the way, too, so I can show details if necessary.

Eagle CAD

The plans for the Nibbler are in Eagle CAD format, which is produced by AutoDesk. Unfortunately, that product is about to be discontinued (just my luck). June 7th is the end of that product, and they have a page with more information here. I saved it to PDF on an 11×17 Tabloid sheet. I can use that as my electronic reference if I need to see a clean schematic on my computer screen. At least I don’t need to dig around for a program that can read PDF.