Mystery 1802 Computer Was A Homebrew Project

[CelGenStudios] has an impressive collection of vintage hardware. One that really struck us came from a thrift store in Canada, so the original provenance of it is unknown. It looks like someone’s handmade interpretation of a SOL-20. There’s a wooden and sheet metal box containing a keyboard looted from an old dedicated word processor (back when a word processor was a machine, not a piece of software). Inside? Some vintage-looking hand-drawn PC boards, including a backplane with two boards. One contains an RCA 1802 and a little bit of memory. There’s also a video card with more memory on it than the CPU.

We loved the 1802, and we disagree with [CelGenStudios] that it “wasn’t that popular.” It was super popular in some areas. The CMOS processor was popular in spacecraft and among homebrew builders. There were a few reasons for that. Unlike some early CPUs, you didn’t need much to bootstrap a system. It would run on 5V and had a “DMA” mode to key data in with just a few simple switches and buttons. You didn’t need a ROM-based monitor to get the system to work. In addition, the design could be low power, and the static design meant you could slow or stop the clock for very low power compared to many other systems of the day.

Inside the box was also a tiny board that was a mystery. That is until he noticed that it had a connector that would fit a Commodore cassette deck. The keyboard cable needs rewiring, but otherwise, the machine works. There is a ROM with a monitor since the DMA mode wouldn’t understand the video and keyboard.

The real mystery is who made this computer and why? It seems like it might have been a prototype for a failed business. Or perhaps it was just a labor of love from someone with a lot of talent. If this computer looks familiar to you, he’d love to hear from you.

The 1802 had a very logical instruction set thanks to being the design of one person, [Joe Weisbecker]. You can still build a simple computer using one. Of course, you can also emulate it with an Arduino.

Thanks [Stephen Walters] for the tip!

29 thoughts on “Mystery 1802 Computer Was A Homebrew Project

  1. Because it was available in silicon on sapphire, The 1802 became the processor for a lot of satellites, there’s a whole open source operating system that was used for the amateur radio ones. The resources still exist in some corner of the AMSAT website.

    1. I worked at a branch of RCA’s Advanced Tech Lab in Camden NJ from 1978-1981 on a few projects coding 1802 assy lang for some secure projects. Also did a short stint at RCA Princeton Labs and got to meet the guy who designed the 1802 (Jerry Herzog) there. I was aware that the RCA ATMAC bit slice processor was available on SoS, but never heard of 1802 on SoS. AFAIK the ATMAC was intended for internal use on space applications – but perhaps it was also commercially available.

    2. The normal version already was good enough for satellite applications, if memory serves.

      What’s nice about the 1802 is its very wide range of operation voltages, I think.
      I know of no other CPU that’s somewhat forgiving.

  2. The “RCA COSMAC Elf” was written up in Popular Electronics back in late 1970’s. It was in maybe three issues… I read the 1802 was also used in ocean buoy sensors. Its low-power demand was a plus when the nearest AC outlet was miles away.

    Personal computers before FCC Part B looked so much nicer, like with wooden sides!!

  3. For those kind-of-interested in the 1802, there are simulators on the Web, from simple hex-key/7-segment display models to microcomputers running BASIC, FORTH, and a primitive Disk OS.


  4. Want to see some documentation about many aspects of the 1802?
    The BMP802 – Brussels MicroProcessor, the 2nd book published in 1980.
    A good entry into many aspects of this wonderful project.

    BMP802 Design Ideas Book

    A couple of years ago, I initiated the availability of an 1802 ( limited capabilities) for FPGA
    FIG FORTH Manual – implemented by Steve Teal on Steve Teal’s own 1802 in VHDL for FPGA.
    What a great job he did.

    and there is much more he designed

    And not to forget the Radio Amateur’s book by Dr. Meinzer

    And there is a very active COSMACELF user group.

    You can program it easily in HEX as many instructions cover the 16 registers.
    And the short test program 7a 7b 30 00 we all know
    Switch the Q output ( with LED) on, switch it off, and jump back to addtess xx00.
    It is the first CMOS processor, and the advantage of low power unfortunately came then with a disadvantage of silicon area – but do not forget: (nearly) all processors nowadays are manufactured in CMOS.

    The disadvantage was, that for RCA this processor and the chips around it was not so important.
    I always compare it to the 68 000:
    If there had been more push, all PCs would be running on this architecture.
    But ARM has archieved a lot with their architecture in our mobiles and elsewhere.

  5. Man, I gotta give whoever built that some respect… that case is a thing of beauty, and a joy forever!

    It ticks all my checkboxes…
    A: Homebrew
    B: Vintage Computing
    C: U L T R A W I D E form-factor :D

    1. Thank you very much for the link to this article.
      It covers many aspects of this wonderful beast.

      Probably the longest serving processor in space,
      and many of them still active somewhere in our planetary system.
      I wonder which one is the longest active one.

  6. A fine CPU. And now let’s imagine how good a replica could be these days.
    Let’s think about it. It’s not miniaturization, I mean. This can do more bad than good and make the chip more vulnerable, likely.
    What I mean are new manufacturing processes, better optical lithography, increased accuracy, lower failure rates, new materials (semiconductive glass, plastic etc), self-healing traces etc.

    1. The machine was purchased in Vancouver. I can’t imagine that the builder would have been a member of a homebrew club in Ontario. Wouldn’t there have been one in a city as large as Vancouver? Ontario is an awful long way away.

  7. I built my own homebrew computer with the 1802 around 1981. I wrote a simple os for it that used a couple of 7 segment displays for output and a rotary dial telephone as the input device. It was a fun little project that was pretty cool for the time.

  8. I built a single board 1802 computer in 1978 in my final year in Electrical Engineering at the University of Toronto. It was kit designed by Eugene Tekatch with a whopping 256 bytes of memory and eight LEDs to read output in hex. Based on articles in the below mentioned Ipso Facto and Popular Electronics, I wired-wrapped 8k ,built a cassette tape interface, and after acquiring a KSR35 teletype, built an interface to it. Finally I received Quest Basic by cassette and starting coding Basic games and utilities.

    My brother in law, 13 at the time, came over and became fascinated with it. He studied engineering and worked for many years at IBM in Database design and AI. He credits his interest in computers to his fascination with my little project.

  9. It was amazing what you could do with no ram. 16, 16 bit registers that could all be program counters or index registers made it quite flexable.

    It’s big brother the 1805 added some much needed extwnded instructions as well as 64 bytes of ram.

    I developed and supported a multi node networked monitoring and control system bassed on the 1805 running at just under 1Mhz I also adapted a C compiler to use on it Later, wrote a small RTOS to better compartmentalize the code.

  10. It’s great to see that there is still so much interest in the quirky 1802 CPU that Joseph Weisbecker and Jerry Herzog developed for RCA.

    In 1980 I bought a Netronics ELFII from a schoolmate for $100. It had 256 bytes of RAM so I eventually added a 4K RAM card to it. I built a rudimentary sound card and I/O to interface with relays and the hope to connect to stepper motors. I also build a robot single board 1802 controller board in around 1982. I have no idea what became of it.

    I’ve still got a few of these ancient machines running. I’ve got an expanded Super ELF has 64K RAM, 32K EEPROM, two IDE hard drives (compact flash cards), a high speed UART/Serial port and a Ti TMS9918A video card. The CPU runs at 6MHz rather than 1.79MHz and it runs an OS called ELF/OS.

    There’s a very active group of 1802 enthusiasts, including people who were involved with the Ipso-facto newsletter and the Viper newsletter

    The best 1802 emulator is the EMMA 02. I use it sometimes just to test out a block of code I’m playing with

  11. Was the lead hardware engineer on a small team that built several non commercial products using the 1802. The software side of the team had access to a PDP 11/70 and they used the 11/70 macroassrmbler to generate the assembly code for the 1802. I don’t know the internals of the machine but I suspect it had a 1 (yes, one) bit ALU since it took 8 clock cycles to do an 8 bit add instruction.

  12. This system is not a mystery anymore. This computer was build by grade 12 students at Vancouver Technical School in 1983. I still have one. I ordered the keyboards from a surplus electronics magazine.

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