Discrete-Logic UART Keeps 8-Bit TTL Computer Connected

Pity the poor TTL computer aficionado. It’s an obsession, really — using discrete logic chips to scratch-build a computer that would probably compare unfavorably to an 80s era 8-bit machine in terms of performance. And yet they still forge ahead with their breadboards full of chips and tangles of wire. It’s really quite beautiful when you think about it.

[Duncan] at Shepherding Electrons has caught the TTL bug, and while building his 8-bit machine outfitted it with this discrete logic UART. The universal asynchronous receiver-transmitter is such a useful thing that single-chip versions of the device have been available since the early 1970s. [Duncan]’s version makes the magic of serial communications happen in just 12 chips, all from the 74LS logic family.

As if the feat of building a discrete logic UART weren’t enough, [Duncan] pulled this off without the aid of an oscilloscope. Debugging was a matter of substituting the 2.4576 MHz crystal oscillator clock with a simple 1 Hz 555 timer circuit; the reduced clock speed made it easier to check voltages and monitor the status of lines with LEDs. Once the circuit was working, the full-speed clock was substituted back in, allowing him to talk to his 8-bit computer at up to 38,400 bps. Color us impressed.

For more TTL computer goodness, and to see where [Duncan] got his inspiration, check out [Ben Eater]’s many discrete logic projects — his scratch-built 6502, a low-end video card, or even his take on serial communications.

18 thoughts on “Discrete-Logic UART Keeps 8-Bit TTL Computer Connected

  1. I wonder how many of these people who love playing with old school TTL/CMOS chips on breadboards would find it really worthwhile to get into FPGA but are too scared because it looks expensive/complicated. For what it’s worth, I fully recommend taking the FPGA dive.

    1. These are people who do it for fun, you know.

      Designing and building with 74-series chips is significantly more difficult than designing and building with a high-level HDL and an FPGA.

      And in many ways it’s much more satisfying if you’re not trying to meet a business goal just because of the sheer tactility of it all.

      1. For some, it is the other way around. It is far easier to poke around with 7400 chips on a breadboard until it “works”. Parts have predefined functions and are very well documented. There are few ways of doing things unless you are the type that only build circuits with NAND or NOR gates.

        In FPGA/CPLD, it would take a bit of top level design to figure out the functional blocks before you can code effectively. You have a lot of freedom, but sometime too much freedom to the point where you don’t know what to do.

    2. I don’t sit around and design in TTL on a regular basis, but when I need something that’s the route I’ll take. For me, I feel like I have to work my way over a big wall of time and money to move into FPGA. The barrier to entry is probably not as awful as it feels from here, but I haven’t yet had a hobby project that has compelled me to take the leap.

    3. I can’t answer for more than one (me), but I am into both. I’ve implemented the SAP-2 in each, as well as on PCB and written an emulator, all for the same cpu design.

      That said there are differences between each. For a person who maybe or maybe not has used an Arduino, and knows little to nothing of logic gates and CPU first principles, a massive breadboard design can be a far better “hands on” learning aid with little additional learning curve beyond the goal at hand.

      For designing something with a practical use, while breadboards might be involved early on, they are almost never the ideal end product. An FPGA might be a better fit, unless of course learning PCB layout is your next step, in which case it is less unrelated learning curve to duplicate a breadboard design to PCB form.
      Both are good skills to have before using an FPGA for something practical!

  2. I would say building with 7400, 4000, or 74HC family logic. Discrete logic is transistors, as you will seen in any EE text where logic circuits are introduced in the transistor section.

  3. “…or 74HC family logic. Discrete logic is transistors,…’

    Xistors require support circutry. It’s a shame that some.TTL only came in duals, triples & quads..Sometimes.only 1 gate was needed. Ground out the unused gate or… back to an Xisistor.

    Something long on my mind, has been what % of this and that TTL to stock in my buggout bag… my 2nd one, with sundry survival parts.

    But to deride TTL affectionados and almost promote a Chem-Mech RoBeetle?!? What’s up with that? What is the Tech Term equivalency to tech-racism? TIC (Tongue-In-Cheek)

    It was Apple whose fanless laptop overheaded w CMOS etc. But RETRO (ahem) fitted w TTL, generated enough heat such that convection carried the day. (A moment of silence, please.) The king is dead. Long live the king. (Hate to hear what he thinks of tube affectionados. LOL) I need a loaf of bread. Hmmm. Yeast and flour? Bacillous? Maybe I’ll print a loaf. It’ll last longer. No – too large. Print a sandwich! Break a RoBeetle or few over it – should be tastey!

    1. You can make everything from NAND gates, so a big pile of 7400 quad NAND should do it :-) But to make life easy add a variety of flip-glops and some some 7486 quad XOR. Me? I just have a big roll of #30 copper wire and some nails and tin snips yo use on food cans. Relay gates are the wave of the future and can handle post-apocalypse EMP, or whatever the current Emperor of China uses to subdue the World..

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