The TRS80 Model 100 Gets A Brain Transplant

We’ll forgive you if you were busy in the ’80s, and missed the TRS80 Model 100. It was a portable version of the original, ran on four AA batteries, and even had an integrated acoustic coupler which proved handy for workers on the go. However, time is rarely kind, and [Trammell] had come across a non-functional example for just $20. It was time to bring this relic screaming into the modern age.

The motherboard was toast, so [Trammell] decided to wire up a Teensy++ directly to the Hitachi HD44102 display driver chips. Being an older LCD, the display needed a negative bias voltage, so a few diodes, capacitors and a PWM line stepped in to create a charge pump. There was no character generator on board, so the heavy lifting is all handled by the Teensy itself. The keyboard was a simple enough matrix design, so that was wired straight up.

[Trammell]’s work with this iteration got as far as acting as a USB serial terminal, and there was some work done on VT100 emulation. However, according to Twitter, the next stage involves an iCE40 FPGA and some music with which we’re altogether too familiar.

[Trammell] owns a working Model 100, too – employed in some modem experiments, no less.

45 thoughts on “The TRS80 Model 100 Gets A Brain Transplant

    1. finally got one after all these years of dreaming. i was inspired buy a friend that recently picked up a sears version of the 2600 which he used to dream about in the christmas catalog. of course, he had an apple 2e that collected dust :(. i was the computer guy and he was more of the ‘face’ man. i did get Egons computer in 7th grade.

        1. Not strictly a “clone”, since it was made by Atari, just with the Sears casing option, almost identical to Atari’s case except utterly boring plain black. Because at the time Sears would only sell own-branded stuff in their catalogue. Which seems bizarre even for 1977, positively sinister now.

          There were also Sears versions of the first few games, again with even boringer plainer packaging than the original plain boring packaging, simple sans-serif (fuuuuutuuuure ’70s!) text on black background. Even futuristicer than OCR-A!

          I would have liked a Model 100. But I had to make do with superior Cambridge Z88! Beautiful machine, the hardware is totally Sinclair in elegance and minimalism, and none of the efficiencies and money-savers come at a cost to the software or to the user, unlike, say, the ZX81 or even Spectrum. The rubber-mat keyboard of the Z88 is great, easy to touch-type, nice travel, nice movement, just about full-size keys, and it wipes clean! Nicer than most modern laptop keyboards which are more mechanically complicated.

          Then it’s OS fits together very elegantly too, very extensible. Finally, the Z88 is where the effort to get a C compiler for Z80 started, in the 1990s with Garry Lancaster, which lots of other machines now benefit from.

          But that’s a completely different (better!) machine. Maybe this upgrade to the Model 100 helps. Except the screen res is less than half of the Z88’s. Sorry!

  1. The TRS 80/100 was–and still is–such a beautiful machine and marvel of engineering that I think know a full-blown, detailed story about it–including the very powerful add-on programs created for it–would generate a lot of interest.
    Get someone who really knows the 80/100. No, you can’t have, or even borrow, my TRS 80/100 Engineering Manual.

    And please–no hacks allowed, for any reason.

  2. I still have mine. 33 years old and it still works fine. And that keyboard still is great. It even has the control key in the place god intended. I made a 32k SRAM ‘disk’ for it, and wrote a 127-byte driver in 8085 assembly to access it. Sadly, that driver has long evaporated, as has my floppy drive and my 8085 assembly brain cells…

    A Teensy and/or the FPGA would be a great replacement. That poor 8085 was pretty taxed, shoving those pixels around. Impressive screen update speed out of that FPGA. I had no idea the LCD interface could go that fast.

    1. I have 2 of them in my “one of these days” box right next to the paper tape punch and the dot matrix printer. One of them works great, the other has some sort of problem the details of which I can’t remember because – 33 years :-)

    2. I have one I still occasionally use to diagnose RS232. It has great built in features for this. But the need is declining as everything goes wireless and USB. However, great for working on retro stuff.

  3. I’ve been considering making a “note taking” keyboard with nothing but a 16×2 LCD display and a teensy or something like it. The hardest part would be getting data off of it, but a USB stick or the like might not be difficult. Or saving files to a cloud service with an ESP32. The old radio shack keyboard computers were very neat, and it’s a great form factor.

    1. Oh, heck, if you’re using a Teensy to make your note-taker, just configure it to emulate a USB stick too: Jam it in a PC, and it will look like a disk — easy to slurp the files.

    2. I’ll start by recommending the Z88, which you can still get, but they’re not dirt-cheap. They’re not bad though, still cheaper than they were at launch. And very, very capable machines, the software includes a word processor that’s also a spreadsheet, and works well as either or both! Uses a Z80 CPU with about 20 hours life from 4xAA’s, and of course an AC adaptor is available, or any old 6V one will do. Comes with 32K RAM but benefits from a 128K RAM pack in Slot 1.

      The main I/O is an RS232 port. There’s an app on board (about a dozen or so apps and accessories built-in actually) to send files through it, as well as a terminal emulator. So you could store your notes in RAM and RS232 them to a big computer later. Very comprehensive little operating system. Even multi-tasking, though background tasks just go to sleep. You can program it in on-board BBC BASIC, which includes a Z80 assembler, handily! You can put asm inline in your BASIC programs.

      But apart from that, I like your idea. There was an old PDA thing in the 1980s called the Agenda Microwriter. Spare parts were like gold dust to users. Had a puny alphabetic keyboard, but also 5 Chord Typing buttons. Where combinations of holding down the buttons generated letters etc. Was a fast way of typing one-handed without even taking it out of your pocket! Chord typing was gonna be the Next Big Thing in the 80s for a while. The Agenda had a little 2×16 (I think) display and 32K RAM, and was designed for the purpose you describe. I doubt you’ll find a working one but the design would be great to copy.

      If you’re gonna go with full keyboard (best for typing, if you touch-type (which we of course all do) ), I’d go with a bigger screen. There’s all sorts of functions I’d want to add. A small graphical display, maybe one of the calculator type 128×64-ish, which use almost no power. Or maybe a higher-res nicer one. Could use ugly tiny-pix font for zoomed out reading, nicer bigger font for close-up typing.

      And then an offboard RAM. Maybe even a serial one. And a serial EEPROM too for saving, unless you fancy an SD card slot, which takes care of getting data off, and storage. All this because MCUs don’t have much RAM on board.

      Then again, all that said, might just be the job for an app on the smartphone I’m sure you have. And perhaps a selection of Bluetooth keyboards, one for portable, one for usable. Beyond that, voice notes are probably easier.

      1. …They’re not bad though…”

        What an absolute blockbuster of a testimonial.
        You’re right, though; they’re not bad–they’re terrible. It had, probably, the worst screen of any machine of this class (and don’t even go here: 40 characters vs. 80 characters? What good is 80 characters on a very tiny, miniscule screen, one third of which is REALLY a “MENU” whose sole purpose in life is to make it look bigger to you. And, hence, hopefully to get you to overlook how small–and unreadable–the screen actually is.

        Simply compare pictures of the TRS 80 Model 100 against pictures of the Cambridge Z88. And THEN read PC Magazine’s review of the Z88, back in 1988.
        PC Magazine did not even attempt to salvage anything of its review by saying…

        “…They’re not bad though…”

        1. 80 characters PLUS a menu display at the left, and a small status display at the right. Actually 104 characters strictly, or 640×64 to get right down to it. Also fully usable for bitmap graphics, though you need machine code for that. Fortunately (did I mention?) that’s built-in, integrated into BASIC.

          The screen was small because the computer’s supposed to be portable. Which it was. Much more so than the Tandy. So that’s 2-nil so far. The screen was extremely clear and had great contrast, probably from using a supertwist LCD.

          I’ve seen both machines in real life, operating. I owned a Z88 for years. Used the arse out of it. Every day was a delight as you learned new stuff (with a programmer’s mind; plenty of mobile writers, journalists etc were happy just learning to word-process then sticking with it).

          They barely compare. The Z88 was years ahead, though it was also years later, so fair enough. Compared to laptops of the time, eg 2x720K floppy and you get 90 minutes til the battery dies, on TRULY sad and pathetic screens.

          I’ve never read PC magazine, why would their opinion matter?

          Compared to even your usual pissweak trolling, you haven’t done well. I notice you’re not advocating the good points of some rival machine. Is comparing pictures all you’ve done? Did you just look all this up before you posted?

  4. I think it’s a pretty big stretch, calling the Model 100 “a portable version of the original”. Pretty much had nothing in common with the Model 1. Different CPU, no common peripherals between them, nothing. It WAS a remarkable machine, though, in those pre-laptop days.

  5. I’ll be a lot more impressed when he gets that all put inside the original case.
    But just the same, there are a couple projects that are too popular for their own good – you can’t get them right now without waiting. First is the ELLO 2M, which is a PIC32-based BASIC computer with not only a LCD screen, but also a prototyping area, built on a stack of PCBs. The other is Olimex’s TEREX-I, a completely hackable ARM-based laptop. I think I’d take an ELLO 2M over a Model 100.

    1. There’s gobs of room in the Model 100 case: easily enough for another quite large PCB to sandwich in there — that’s where I put a ramdisk of four 28-pin DIPs and some glue logic on veroboard. A Pi Zero or Teensy would be trivial to stuff in there.

      1. There was a company, I can’t remember a name, which made peripherals and a case extension. Either they offered a plastic bottom piece that was deeper than the original m100 piece, or a plastic piece that fit over the existing bottom. There was a very large number of companies selling third party hardware.

        I think one company offered a “base station” which was a fancier computer, the M100 then being used as a terminal. The Radio Shack unit added video and a floppy drive, but no extra computing. I remember thinking that a CMOS 6809 would be great, a few years later higher density CMOS ram was available, so you could swap it in. The M100 was a tad early, each if the 8K memory modules was a board with four 2K surface mount RAMs and some interface logic, soon after one could get 24 pin CMOS ram with 8K or more.

        Of course what I liked least was the 40 by 8 lines of text, kind of big but too few characters.


  6. The N key decided to stop working a couple days ago on my ‘200. Thinking it was fixable, I desoldered it, undid the snap clips, only to find a remote control style carbon pad that contacts the goldish colored lands. Lucky for me, the CTRL keyswitch on a parts machine fixed it

  7. What would be neat is a new PCB and color display. Put something suitably powerful in it, along with a replica of the original on a FPGA, with original and variable ‘turbo’ speeds. Of course it should be able to interface to original peripherals, and give access to non-dialup network to the M-100 replica.

    1. If you’re going to replace the CPU and the LCD, then why not just run an emulator on a normal laptop? The form factor frankly sucks, with the non-tilting screen. The keyboard was fantastic for a portable computer of the day (and a huge improvement over my typewriter), but by itself really isn’t worth building a new machine around.

      1. “…The form factor frankly sucks, with the non-tilting screen…”

        That’s precisely why Olivetti came out with their version WITH a tilting screen–the Olivetti M10.

        [That manual may be on-line also; it’s called “M10 Portable Computer General Service Manual“]

  8. the real trick would be to do a less invasive mod stealing the bus from the bottom or back to use it with a raspi or ice40 in some form of dma or something instead of gutting a literal work of art.

  9. I still have my TRS80 model 100 with upgraded memory and it still works. In a way, it was one of the first home computer with GPIO available. This took the form of an 8155 parallel IO interface. Not full GPIO but it could be used to interface to such things as stepper motor drivers. My last project on it was back in the early eighties. It was a G code to stepper motor interface for a Unimat 7 lathe that I had converted to CNC control. It used discrete H drivers controlled by the 8155. The code was written in basic and so it took some careful coding to make it work at any useful speed. The project faded out when I tried to move to servo motors and needed more compute power than the model 100 had to offer. I moved on to home control using a Tangerine Microtan 65 6502 based SBC. I also still have that, with a number of expansion boards. I need to see if it still runs.

    1. Well it did have a 300baud modem inside, which could directly connect to the phone line, or connect to an optional acoustic coupler. I don’t think at the time many computers came with a modem, though the portable nature of the device probably meant acoustic coupling was more useful than direct.


      1. Even in the 1980s, modular telephone jacks were nearly everywhere. Even before that there were the big four-pin square outlets, which could replace the original direct-wired junction boxes, and there were adapters to plug the RJ11 plugs into. Acoustic couplers weren’t really for convenience; they were made because you didn’t have to get them certified by Ma Bell to hook them up directly to a phone line. So yes, there were still acoustic couplers in the late 1980s, but these were throwbacks to before the Bell breakup.

        1. I’m sure I’m not the only one to have used the acoustic couplers on a payphone, or a work phone with those weird 6-pin modular jacks, or at my parents’ house that had only hard-wired wall phones.

          I recall $300 CompuServe bills from those days (more than rent!). Nope, not nostalgic about it, not even a little bit.

          1. Good point about pay phones. I hadn’t thought about that. I knew people in the BBS days who were paying huge phone bills, but I lived in San Jose at the time, and there were plenty of local numbers for BBSs, and I knew from the start that Compuserve’s (and others’) rates were too high to be viable services.

  10. I still have mine, the NEC PC8201A variant. Still worked last time I powered it up. For many years in the 90s, it lived under the seat of my Pontiac, as my mileage log and general note-taking machine. I’d transfer the text files over RS232 to my desktop for safekeeping.

    It would be fun to throw a little ESP-modem or something in there and telnet into an IRC/Slack bridge. That keyboard would be great for chat.

  11. One of THE best books, bar none, on the TRS-80 model 100 is Inside the Model 100 by Carl Oppendahl, ISBN 0-938862-31-6. [Full official title: Inside the TRS-80® Model 100]

    From the back cover: “Commented ROM routine listings, detailed hardware schematics, and numerous figures are provided throughout the book. Inside the TRS-80 Model 100 also contains invaluable appendices and is fully indexed. No Model 100 owner, whether novice or expert, should be without this book”.

    From the front cover: “8085 Assembly Language Programming; Advanced BASIC Programming; Hardware Overview–Keyboard,LCD, Printer Interface,Cassette I/O,Beeper, Power Supply, RS-232, Modem”.

    As an indication of how thorough this book is, the there is an index of 8080, 8085, and Z80 opcodes, with their hex and decimal equivalents (‘MOV M,A‘ ?–0x77); a memory index (if you really want to know, memory address 0x6D69 is discussed on p. 243 of the book); and-get this–a complete INDEX of ROM routines! Howzat again? Let’s say you want to know where the the ROM routine for a ‘block move’ is. Easy–that information is in Appendix B, and the address is 0x5A62.

    You can’t have my copy of this, either. But, then again…

  12. Dont remember as fondly as some. Expensive 8KB machine. Glad someone recycling. Display was horrible. Backlight was whatever headlamp/flashlight was wearing. low angle of viewing. Software slow even for that time and that processor. If didnt have updated ROM typing above certain speeds was garbage and caused garbage. Agree was one of the better keyboards but needed protecting. Trying to get rom and finding out there was a couple another story. Modem needed tweaking to get even close to 150 nevermind 300. Tho some of that piss poor networks in the way back and out. RJ-11/14 or rj9 or rj22 plentiful? Ya ok. Acoustic coupler and tweaked amp came in real handy but dont tell ma bell. Of course youd have to shout at her if did. Overrated battery life. But thats not new at all . Carry fresh alkalines. Leave on charger overnight or ur cads may not get charge in particular if have aftermarket ram modules that arent really LP.
    In contrast to luggables at the time had weight, size, and portability excellence but thats about it.

  13. The first true laptop came along shortly after the TRS 80/100, and helped hasten its end–the Toshiba T-1000. Full non-backlit 80 column display; MS-DOS 2.11 in ROM–one of the first ; fits in a briefcase; full complement of ports; 512 KB memory–$799.00, and Toshiba sold all they could make. I bought a 768 KB memeory expansion card, and happily paid $299 for it (I used it as an electronic FDD); and an internal modem card. Toshiba made extensive use of CMOS (80C88, etc.), and the entire unit ran on four 1.2-volt NiCads–yup: 4.8 volts (and that’s on a full charge).

    MS-DOS 2.11 was the very first DOS to support 3.5″ floppy drives, because Toshiba absolutely needed these to keep the size of this machine small. DOS 2.11 was chosen because Toshiba wanted a version (being in ROM) that was “tried-and-true”, absolutely frozen, and would never change. Great choice: mine still works flawlessly.

    We need an article on the Toshiba T-1000: the first real, true full-featured portable laptop computer.

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