Programmable ROM made of switches coupled with diodes. In some ways resembles organisation of ROM chips.
At first, a pathfinder was made, as a small self-contained unit, consisting of 4 banks of 8 switches each, addressable by 2-to-4 decoder, which has output signal displayed by column of 8 white LEDs - there are photos of it in gallery.
Afterwards, bigger unit was built, which is described in the log entry for this project.
Over a last couple of moths I constructed a ROM unit, which may be useful in further development of my computer project.
The unit is composed of 256 switches and 256 associated diodes, has 2 3-to-8 74hc238 decoders combined so they act as single 4-to-16 decoder, output buffer for each of 16 data bits (Schmidt invertor combined with regular one), and, as a bonus, the board also contains 74hc163 4-bit synchronous counter. With this arrangement, two ways of addressing are implemented.
1: one can just send an address, and after first rising clock edge the addressed data will be on outputs.
2: addresses can be auto-incremented, and new sequential data word will appear on each rising clock edge.
There are, as some useful extra, also LEDs, which show which word is chosen, and what data it contains.