NQSAP Breadboard Computer
The Not Quite Simple As Possible Computer
Inspired by:
- Understanding Digital Computers - Radio Shack (Forrest Mims)
- Digital Computer Electronics - Paul Malvino
What is a TTL computer and why build one?
The breadboard computers that people are building now are a chance to learn the gritty details of computer architecture. They are functional computers built with no microprocessor - every part of them is created from simple digital logic, like gates, decoders, counters, and memories. The resulting computer is extremely basic, suited to tasks like numeric calculations or very simple games. But they are computers, and with enough breadboards and wire, they could theoretically do just about anything.
The interesting thing about these computers is that they never really have to be finished. There’s always another enhancement that could be added on: new instructions, a larger memory space, or an interrupt controller.
I like to think of it like a ship in a bottle - it’s not about seaworthiness or cargo capacity, it’s about crafting something just for the enjoyment of building it.
How did I get here?
As a teenager, I found the Understanding Digital Computers book at the local Radio Shack and was immediately fascinated by its description of the PIP-1 computer - an extremely basic 4-bit machine. This book then led me to the library to find a copy of Paul Malvino’s Digital Computer Electronics, the book that defines the SAP-1, SAP-2, and SAP-3 computers.
At that point, I was already building electronic kits and wiring simple logic circuits on breadboards, but didn’t have the resources (or the skills!) then to actually build a SAP. Still, the knowledge of computer architecture from those books was an excellent foundation for later assembly language programming on Z-80 TRS-80s and 6502 assembly on Ohio Scientific Challengers.
When I finally did build a Netronics Explorer/85 computer from a kit, the PIP-1 and SAP-1 helped me to understand the inner workings of the 8085.
Flash forward many years later to a career writing embedded software for computer communication equipment. In my spare time, I started playing around again with 8-bit computers. The first project was to dust off the Netronics Explorer/85 and then build a software compatible clone on a breadboard. A supporting project for this was TommyPROM, an Arduino-based EEPROM programmer that has been used as the basis of the Loader and the Microcode programmer for NQSAP.
After the 8085 reboot project, I soldered some great 8-bit computer kits, including the Gigatron, PiDP/11 (not 8-bit, but still cool), and the IMSAI 8080 Replica from The High Nibble.
The kits were a nice trip back to soldering and front-panel switches, but they didn’t really offer much to think about - the design was already done, the parts just needed to be assembled.
Finally, while browsing the Homebrew Computer Club on Facebook, I found the Ben Eater 8-bit computer and instantly recognized the SAP-1 that I’d read about so many years ago. This was definitely going to be my next project! The NQSAP is my version of a TTL breadboard computer, with its own features and quirks.