A Mainframe Computer For The Modern Age

The era of mainframe computers and directly programming machines with switches is long past, but plenty of us look back on that era with a certain nostalgia. Getting that close to the hardware and knowing precisely what’s going on is becoming a little bit of a lost art. That’s why [Phil] took it upon himself to build this homage to the mainframe computer of the 70s, which all but disappeared when PCs and microcontrollers took over the scene decades ago.

The machine, known as PlasMa, is not a recreation of any specific computer but instead looks to recreate the feel of computers of this era in a more manageable size. [Phil] built the entire machine from scratch, and it can be programmed directly using toggle switches to input values into registers and memory. Programs can be run or single-stepped, and breakpoints can be set for debugging. The internal workings of the machine, including the program counter, instruction register, accumulator, and work registers, are visible in binary lights. Front panel switches let you control those same items.

The computer also hosts three different microcodes, each providing a unique instruction set. Two are based on computers from Princeton, Toy-A, and Toy-B, used as teaching tools. The third is a more advanced instruction set that allows using things like emulated peripherals, including storage devices. If you want to build one or just follow along as the machine is constructed, programmed, and used, [Phil] has a series of videos demonstrating its functionality, and he’s made everything open-source for those more curious. It’s a great way to get a grasp on the fundamentals of computing, and the only way we could think of to get even more into the inner workings of a machine like this is to build something like a relay computer.

49 thoughts on “A Mainframe Computer For The Modern Age

      1. Anyone not familiar with the origin of the neologism ‘blineknlights’ / ‘blinkenlighten’, it was used in a famous, Fractured cod-German sign which is now almost ubiquitous in computer rooms.

        Blinkenlights is the correct spelling for the lights on the control panel, it now being a technical term absorbed into the grammar of the computer world – especially we old-timers who remember fondly the big iron kept in our glass dinosaur pens.


        Alles turisten und nonteknischen lookenspeepers!
        Das komputermaschine ist nicht für der gefingerpoken und mittengraben! Oderwise ist easy to schnappen der springenwerk, blowenfusen und poppencorken mit spitzensparken.
        Ist nicht für gewerken bei dummkopfen. Der rubbernecken sightseeren keepen das cottonpicken händer in das pockets muss.
        Zo relaxen und watschen der blinkenlichts.

  1. I remember seeing in person a small (just a printed circuit board a few inches square) 8-bit “computer” that was programmed with toggle switches one byte at a time and as I recall only had some LEDs for an output display … but for the life of me I can’t remember what it was called. It predated any other personal computers that I’m aware of. Any help?

      1. RCA the extinct Radio Corporation of America, made the 1802, in about 1976.
        To help people use it they had the Microtutor 1 and the Microtutor 2 about 8 by 5 inches.
        These were quickly replaced with bigger Ram arrangements, and the switch’s were replaced with Hex buttons, and the LEDS, were ignored as Terminals and TVs became more involved.
        But People still sell old ones now and then, and may even build some,
        as there is a group of over 500 members who take a keen interest, called https://groups.io/g/cosmacelf/files
        In my opinion the Company RCA wanted a simple machine to get vital communications to stay working, and this simplicity is why people still like it.
        For example the 1802 was the first 8 bit micro used for spark timing under the bonnet, in warm rumbling conditions, also RCA was involved with relatively simple Medium Wave Coast Guard radio.
        The main engineer, worked on it even at home, had set his sights on simplicity for his Daughters.
        Joe Weisbeckers elder Daughter Joyce is famous for being the first computer games maker,
        using a still used language on other platforms called Chip-8, (Chip-8 has changed a bit but only a bit, I think) that Joe also made.

    1. Yeah me, too. Like the above panel a lot! I suppose that is why I did hack together an old Star Trek interface with the lights and dials, and switches :) . Also have the 1/3 size PDP11/70 as well. Computers with flashing lights, knobs and switches just seems ‘right’ :) .

  2. I still have a PDP-10 front panel in storage. I’ll have to check out the interface. It came from a machine I actually ran back if the day. It would be cool to bring it back to life, flashing lights and all.

    1. I have the front panel to a Data General NOVA3 in my office. With key. And I have the schematics (for the panel only). One day…I’ll light it up.

      Also recommend the PiDP-8/I for *almost* the feel of a real computer!

    2. The obsolescence guaranteed guy that did the pdp11 and pdp8 reproduction has a ka10 front panel and he demoed it at vcf sw a few months ago. Grew up on ka&ki machines and know sever that have pre-order and would love a real ka.

        1. Talking about a ka panel. Even paul allen had to convert their pdp10 use switching supplies, and pretty sure they had emulated disks tied to the massbus to keep the power and ac bills in check. One. Of the ones i used had ki with front panel and a custom 11/70 as io processor so two front panels for the win. Remember one day would not boot so wrote memory starting at each cabinet base address and figured out fist was dead, rearranged the addressing plugs on to of rack to move broken one to highest address, run ipl and booted fine just 64KW less ram.

    1. There are 2 rows of 32 switches.
      Top row is for loading any register plus a few misc functions like setting which microcode to load.
      Second row is for setting breakpoint on contents of any register.
      Work registers are 16-bit, accumulator is 32-bit.

  3. I worked at SAC headquarters in Omaha in 1970 on an ANFSQ31. It used two 1401s as pre processors mainly to spin the tapes. It was an octal system with the first and only drum storage I’ve ever seen.

  4. very cool piece of work

    Nonetheless, it isn’t a mainframe without a punch card reader and a requirement for an air conditioned room with a false floor with all the cables run “down under”.

  5. though not programmed through switches, I can assure you that mainframes are still alive and well! Many of the core systems on the planet still have a mainframe in the middle ie try and run a decent sized bank without one..

    1. Mainframes are one heck of a lot more compact than they were. Free-standing hard drives, enormous tape drives and punch-card readers are a thing of the past.

      Part of the computer-room soundscape was the beating of the line printer literally hammering out reports on music-ruled 14″ fanfold paper.

      These days, the sound of a computer room is mostly the air conditioning and the cooling fans.

      1. The free standing hard drive (DASD) is now a rack of ‘external’ DASD.
        Open reel tapes replaced by LTO tape drives, frequently in a robot juke box. (Or replaced by virtual tape…)

        The last functional punch card reader I saw was in 1990. A large blue computer company converted a pallet of 96 column cards to more modern media for a customer upgrading to more modern hardware. They had to pull a card reader off the local museum floor and hook it to the last s/38 on the raised floor. The pallet’s worth of data was returned on a couple of diskettes.

    2. It’s renamed to “cloud” to try to hide the fact it’s an old concept. Many apparently even forgot why we moved away from mainframes in the old days.
      The biggest difference was that back then, mainframes pretty much all existed for good technical reasons. Nowadays, while a lot of cloud exists for good technical reasons, the rest exists pretty much entirely to benefit the operators…

  6. Mainframes still power most of the banking world, although they have changed a lot since the mid 20th century. Mainframes are also common in the insurance industry as well as some hospitals. Mainframes are essentially just very powerful centralised servers.

    1. I would argue those are not mainframes. They are large cluster servers, you can take a node out of service and still have a fully functional computer sitting on the repair bench and not really effect the performance of the cluster.
      Its not quite the same as having something take up the space of a house just for A single computer

    1. That would be the late great UK’s Gerry Anderson’s touch.
      (never saw a Thunderbird have a random equipment failure –
      or any SHADO assets).

      In contrast to USA’s stuff in Irwin Allen’s “low-bidder” procurements
      (ie. equipment *suddenly* , for no real reason, let’s all the magic
      smoke out). Common story element to, Voyage To The Bottom Of The
      Sea, Lost In Space, Time Tunnel, etc episodes.

      1. Oh I definitely saw the magic smoke escape! 😳 In one instance, an IBM Tech bumped a bottle of pop sitting on top of a 360/50 (the top plate was perforated for the cooling exhaust). We were down for days before homework could run again. 😥

    1. Sorry, I didn’t post everything as wasn’t sure how much interest there would be. More details are available via email, but I’ll try and get a few more items uploaded.
      The MCU drives various chips so a combination of protocols are used, e.g. SPI for the 6 sd-cards, parallel i/f for the LCD module, bit-bang for the TM1637 7-seg display and MAX7219 led, etc.

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