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Full-fledged ROM is made

A project log for 256-bit Programmable ROM

ROM module, made from switch banks, diodes, handful of TTL chips; organized as 16 words of 16 bits each

PavelPavel 02/07/2020 at 05:430 Comments

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.

Below are photos of the board:

In action, with the prototype as comparison:

Several views:

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