The Trials Of Digital Design Class

Late last week, we saw a rather clever combination lock build that used only a single 74xx logic chip. [J. Peterson] read this post, and in a battle royale of geek one upmanship sent us a write up of the logic chip computer he built nearly 30 years ago at the University of Utah.

Around 1982 or 1983, [J. Peterson] took the Digital Hardware Lab at the University of Utah. The class was split into two semesters; during the fall semester, students would build a four digit, stack-based calculator that could add and subtract. That may sound easy, but everything – including reading the keyboard, multiplexing LEDs, and performing the mathematical operations – was done with gates and latches.

After Christmas break, the poor souls who had just finished their calculator were presented with another challenge due in four short months. The calculator built during the fall would turn into a full-blown computer, functionally similar to a PDP-8.

After months of work, and seeing the 70 people who showed up on the first day of class in September dwindle down to a handful in late April, [J. Peterson]’s computer was complete. The test program ran through a couple iterations, and the computer was immediately disassembled.

An awesome tale of digital design from only a generation ago. And you thought Verilog was hard.

28 thoughts on “The Trials Of Digital Design Class

    1. Yeah that’s what I thought as well… like, “WTF!??”

      I have all of the really awexome stuff I did in college still “assembled” and working.

      Now, they may have done this so you couldn’t SELL your project to lower level classmates… so many people did that in my class, effing lazy bastards.

    2. From the writeup… The materials belonged to the university and were used again by the next student.

      Not from the writeup… That kind of stuff wasn’t cheap in those days. If the students had to supply their own parts I’m guessing that most of them wouldn’t be able to afford it.

      1. Hmm, I see now. In my microprocessors class, we were issued MC68HC11 dev kits with a matrix keypad and a few cables. I actually took mine OUT of the cardboard box, and hand-crafted a wooden “suitcase” for it complete with it’s own battery power supply, analog power good meter, 40×2 LCD, keypad, and cable storage. Still have it today.. worth the effort and money to create. I wonder how many of my classmates still have their MC68HC11 in their cardboard box (probably none)?

  1. This is difficult, but most of the better EE programs require something along the same lines. In my digital design class we implemented a similar simple computer at every level from gate level ICs to schematic capture on an fpga, to optimized VHDL…

  2. Brings back memories of my (slightly simpler) AS level electronics project: A score counter with high-score storage and 7seg display, without a microcontroller. Lots of complicated counters, shift registers and logic involved!
    Values would be parallel loaded from the counter into one pair of shift registers (each storing one BCD digit) and then both sets of registers would be shifted 16 times, with the value of each bit being compared and a flip-flop being updated if a bit was greater than the corrosponding bit, resulting in an integer comparison, enabling parallel load from the first set of registers to the second if the first was greater.

    Next year, during A2 electronics, I discovered microcontrollers and haven’t attempted to build anything quite so mad since.

  3. some of us were too ignorant to know it was hard. In my case and on my own, I once made this LOGO interpreter inside the 16 Kb of my Sinclair. Of course it had to be typed in every time (I never got the cassette tape thing to work reliably), and its command set was limited (it DID let you “to” new procedures, though), but what really made it possible was that I didn’t know any better.

    Later on, life and contact with my betters has shown me that attempting such a thing. Even hacking the IBM Logo to make it run in Spanish for the use of my students when I eventually got myself an 8088 I wouldn’t dare any more, if for no better reason that now I am aware such thing would be illegal (in those days, it was the company that sold me the computer that actually gave me – no charge – software, as long as I brought my own 5 1/4 floppys (manually notched so as to use both sides))

    oh well. Now the MSP430s are rather overwhelming… :-) love it!

      1. Oh the good old Timex Sinclair! I killed mine in a similar fashion to yours, although I would not use “real” next to the word “keyboard” for my hack ;}
        Granted I was only 8 years old at the time, but my Sinclair had the membrane keyboard (not the chicklet key model) and had a row of dead keys.
        Turns out there was a slight burn mark on one of the two keyboard ribbon cables, and eventually it caused a crack in the plastic that held the traces.

        I ended up making a “keyboard” from a huge 3×4 foot piece of cardboard, with aluminum foil glued down for rows, and bent paperclips taped down for keys.
        Each paperclip had a wire wrapped around the end, and with the bend, they acted like springs. Pressing down made the end touch the foil and made the matrix contact.

        The connector was also made of paperclips, with just one piece unbent at 45 degrees. Lots and lots of tape between each for spacing. Man did those connectors have play ;} Fit fine once plugged in (and taped) more or less.

        As I recall, each key wasn’t only a letter/symbol, but had a basic token on them as well. I had something like a 4 inch square area with no tape on it under each key to write in each key function.

        Man I wish I still had that thing, or at least pictures…

  4. That’s nothing!
    Back in my day we had to melt our own sand to make transistors and we had to harness lightning for power to run them.

    Off topic,
    last night I saw “Call Northside 777”, a B&W movie where Jimmy Stewart played a newspaper reporter.
    During the last reel or so, they give a demonstration of the “WirePhoto” facsimile system.
    Though I doubt the part where a background portion of a photo is blown up 140x to reveal the [spoiler deleted].

  5. We had design a basic computer with some basic instructions only using nand gates. We implemented it on an FPGA, which was good, I don’t imagine anyone had enough breadboard space + chips to implement the discrete solution.

  6. This stuff fascinates me to no end! But every time I try something on this level I get buried by the sheer difficulty of actually debugging something like this. Where the mind knows how to do it but the hardware won’t cooperate, that’s where I stop.

    1. Don’t get too discouraged. It is tough. As a lone Captain on the sea myself, I can easily come to HaD and get discouraged very quickly. Just remember many of these guys are pros or have pro training of some sort (school, mentor, parents) that you may not have had access to. They also seem to all have CNC machines, 8 oscilloscopes, and makerbots if one were to believe the posts (or anything on the internet for that matter). If it makes you feel any better, I watch tutorials and solder in my lap and get frustrated too whenever things work on the bread board and assembled and once the last screw goes in the case…. crickets-dead. Ugh undo screws and start over lol. Very common, man. Hang in there and keep tinkering. If you notice most of the comments on here today, you’ll see what I mean. There are some “pros” at AVR that have no idea how TTL or basic solid state electronics work. We all have a lot to learn and share with each other as everyone is good at something ;) Finally-the ghost in the machine-makes or breaks any project lol. Some projects are seemingly angry to be “alive” lol. take care and keep tinkering :)

  7. After reading this article and web page, I heard the wizzip, wizzip, sound of my wire wrap gun in the distance.

    I know its still in a box in the garage,somewhere.

    Poor thing, I bet its lonely and cold out there.


    1. I don’t do any more wire wrap designs these days, but still use my wire wrap tool to add wires to a header, or wire up some cable adapters from time to time. I don’t miss using the manual wirewrapping tools, and still have the electric one I made from a mini battery operated screwdriver ;-)

      1. Oh, and I did show my 6 year old son how to wire wrap the other day. He found it quite interesting. Now if he had to do a few 10,000 more connections he might start to think of better ways ;-)

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