This repository contains KiCAD projects and libraries for a replica of the Apollo Guidance Computer. The schematics have, for the most part, been digitized directly from the original schematics for the computer. The schematics have been peppered with metadata required for digital simulation using my agc_simulation repository.
All logic modules (A1-A24) have been digitized and appear to be operating correctly in simulation. Additionally, one custom module, fixed_erasable_memory, has been designed. I am currently working on obtaining self-test software to prove the correctness of wiring in the CPU. Once the simulations have passed the self-tests, I will begin with board layouts.
- Modern 74HC-series components are used due to the unavailability of RTL logic chips.
- I've taken advantage of the nearly-infinite fanout of 74HC chips to remove all fanout expansion gates.
- To reduce chip count, I've not limited myself to using only 3-input NOR gates; I also use NOT gates, 2-input NOR gates, and 4-input NOR gates, where applicable. Combined with the deletion of fanout expansion gates, the total chip count is roughly half of the original's.
- Again due to the nature of 74HC logic, outputs of NOR gates cannot be directly connected together as was possible with RTL. Where this is necessary (cross-module buses and >4 input gates), I've inserted fast open-drain buffers with accompanying pullup resistors. This gives very close to the same behavior, as the fan-in expander gates of the original were operating effectively open-collector lines.
- I've been fairly liberal about moving sections of logic around to make modules more stand-alone, and to condense related logic all into the same module. This facilited much easier development, and I anticipate it to make bench-testing individual modules easier when I've actually gotten boards manufactured. I also believe it to have significantly reduced the number of signals that will be going across the final backplane, although I have no numbers to back this up.
- The implementation for modules not hosted at the NASA Office of Logic Design site is custom, by me. Currently this includes the single board that replaces all of the memory-related modules (fixed_erasable_memory) and the DSKY boards. The memory board uses FLASH for fixed memory and MRAM for erasable memory.