Sunday, October 30, 2016

RC2016/10 -- Happy, Happy, Joy, Joy!

After several hours of wrapping wires, I completed the wiring of my COSMAC ELF replica. At that point, I flipped it over and stuffed the various parts into the sockets. The ICs installed without problems, while the discrete components required a little bending and trimming to fit into the sockets but were not really a problem. Two 0.1" jumpers (a.k.a. "shunts") were required by the design yet not included in the package, but luckily the local Radio Shack is still in business and had them in stock. With everything in place, I attached the battery, flipped the switch, and...nothing happened! Now what?

Follow The Process

The exact procedure for loading code in to the COSMAC ELF is not entirely intuitive, so I reviewed all of the available information to ensure that I was following the correct process. Convinced that I was trying to do the right things and yet still not getting any results, I broke out the schematics and started my analysis...

After basic circuit "first aid" like checking power and ground lines and ensuring that the clock was ticking, I started trying to isolate specific bits of functionality on the board to make sure that flipping switches was sending the right signals to the correct pins on the CPU. While poking around, I found that the output of one of the inverters did not change when it was supposed to change. A bad chip? Well, not so fast...

Chip Test

Long ago I picked-up an EPROM programmer that includes the ability to test logic chips, including a variety of 74xx and 40xx chips. I took advantage of this piece of kit by testing the 40106 chip that seemed to be producing the wrong output. Oddly enough, the chip worked fine in the tester! I took the precaution of testing most of the other logic chips on board, all of which proved to be working fine. Hmmm...

The culprit...
Circuit Inspection

Reassured that the chips were working fine, I refocused my attention on the possibilities of bad wiring. Not surprisingly, I found one wire out of place. Despite having checked along the way during construction and further checked all the connections afterwards, one wiring mistake had escaped detection. Oh well, at least it was just the one!

With that one wire removed and replaced correctly, the machine is working! Too bad I don't have any COSMAC ELF software lined-up for Halloween. But for your amusement, I have included my recreation of the classic COSMAC ELF construction photo above... :-)

Anyway, this project is mostly done. I'll probably still wire-up the expansion port and maybe secure a few bits here and there with some hot glue or something in the near future. Hopefully there will also be some bigger and better ELF or RCA1802 projects in the future as well. But no matter what, I hope that you will continue to stay tuned!


  1. You did well to have only a single wiring snafu! Am looking forward to reading your future ELF adventures

  2. You did well to have only a single wiring snafu! Am looking forward to reading your future ELF adventures