Building A 4-bit TTL Computer

When [GG] was 12 years old, he was introduced to BugBooks, the wonderful ‘introduction to digital design’ books from the early 1970s. It has always been a dream of [GG] to build the TTL computer featured in the BugBooks, and now that he has the necessary time and money available to him, the Apollo181 has become a reality.

[GG]’s computer is built around a 74181 ALU, an exceptionally old-school chip that provides the core of a computer in a neat 24-pin chip. With a 256-byte RAM and a few additional logic chips, [GG]’s computer is an exceptional piece of engineering able to perform 625,000 instructions per second when clocked at 2.5 MHz.

This isn’t [GG]’s first homebrew computer build; last year we saw his incredible Z80 minicomputer. Now we can’t wait to see what’s on tap for next year. After the break, you can check out [GG] loading in operands and operators into his computer and letting the Apollo181 churn away on its program.


5 thoughts on “Building A 4-bit TTL Computer

  1. Nice job!

    That is one BIG piece of perfboard!

    I like how he properly added together all the propagation delays of all the gates to compute the maximum speed.

  2. as a TTL child I still remember most of the 74xx series chips – working for a company that built computers based on that technology, I’m impressed that this is still workng today and that there are people enjoying this ancient technology (as everyone seems to hurry for GHz clock rates and GB of memory space – not talking about TB of disk space :-)

  3. Outstanding work! I’m a big fan of tiny-bit home built CPU’s. For me the trouble with acquiring speed with TTL chips is a poor clock circuit. I’ll have to spend some more time on this website.

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