Doubling The CPU Speed Of The TRS-80 Model 100 With A Mod Board

The TRS-80 Model 100 was released in 1983, featuring an 80C85 CPU that can run at 5 MHz, but only runs at a hair under 2.5 MHz, due to 1:2 divider on the input clock. Why cut the speed in half? It has a lot to do with the focus of the M100 on being a portable device with low power usage. Since the CPU can run at 5 MHz and modding these old systems is a thing, we got a ready-made solution for the TRS-80 M100, as demonstrated by [Ken] in a recent video using one of his ‘daily driver’ M100s.

This uses the board design from the [Bitchin100] website, along with the M100 ROM image, as one does not simply increase the CPU clock on these old CPUs. The issue is namely that along with the CPU clock, connected components on the CPU bus now have to also run at those speeds, and deal with much faster access speed requirements. This is why beyond the mod board that piggybacks on top of the MPU package, it’s also necessary to replace the system ROM chip (600 ns) with a much faster one, like the Atmel AT27C256R (45 ns), which of course requires another carrier board to deal with incompatible pinouts.

After this, we find that the system RAM also has access speed issues, due to the power-saving logic that’s part of the original board. The mod board deals with this by bypassing part of the logic and triggering the power-up signal (A*) much sooner, so that even the original base system RAM can scrape by. This then only leaves the UART which objects to being fed a 5 MHz signal, as this ruins its clocking ability. For this, the mod board also has a fix: a 2.45 MHz output pad.

With the mod fully installed, and the model 100 closed up again, it is confirmed that the system is indeed faster. Unfortunately, it also breaks some games, like Starblaze 100, but since you can switch between 2.5 and 5 MHz mode on the command line, this may not be such a dealbreaker.

Thanks to [Stephen Walters] for the tip.

13 thoughts on “Doubling The CPU Speed Of The TRS-80 Model 100 With A Mod Board

  1. The technically identical Olivetti M10 was my travel companion for many years. I modded it to use rechargeable batteries and to support international characters at the time. So sad, this mod comes 40 years late. I’d definitely have enjoyed it.

  2. One of the best videos I’ve seen in a long time. Great hack.
    The only downside to this mod–leaving aside the time, labor, and cost to do it–is that one ends up with a TRS-80/100 which cannot be re-sold for what it would have sold for, had it been left in its ‘Classic’ TRS 80/100 state.

    1. These aren’t as valuable or “collectible” as you would think – one I picked up at a hamfest for a little over $100 USD, and another on Ebay for about the same amount. Both are stock, and have the maximum of 32K memory (they came in various memory configs originally – 32K was the official max).

      Looking at Ebay quickly (I haven’t been to a hamfest in a while), aside from a few sellers who seem to think they have gold on their hands, most are going for sub-$200, which seems reasonable.

      Also – it’s quite likely that one with the mod might be worth more than one without it, at least in the M100 community. For a purist, it’s a different story.

      This may all change in the future (long after I’m gone, probably – this isn’t a computer to buy for a retirement investment – lol) – but for now, these machines are still in reasonable pricing range for anyone who wants one to try out.

      For anyone who does want to – do your research, of course. Note also that you may want to re-cap the system (some people do – some absolutely have to), and you’ll probably need to replace the internal battery (keeps the internal state “live” while changing the AA batteries it runs on), as the original tends to leak (which is a whole ‘nother ball of wax) – just like old PC mobo batteries. There is a simple mod out there to replace that battery with a small supercap, which is (imho) a better long-term solution (the trick is finding such a cap of the right specs that will fit in the space – but they’re out there).

      Probably the greater downside to this mod is the fact that so many changes need to be done to get it installed (not a simple “plug-n-play” kind of thing at all), coupled with the fact that when operating on one of these old buggers, you could find yourself battling other bugs introduced by messing around internally (particularly the LCD and keyboard flex connections and cables).

      Even so, it certainly does sound fun and intriguing to try it out!

  3. Try a Allwinner H618 Quad-Core CPU Banana Pi M4 ZERO 64 bit for <$50 … running Ubtuntu, Debian, Android …?

    "Why I remember back in ….. "

    You guys young people masquerading as old people?

    1. “You guys young people masquerading as old people?”

      Sure. I do that all the time.
      Grew up with my dad’s tech..

      And now I do know it better than him.
      More than that, I see value were he doesn’t.

      Well, ok, sometimes he comes back to his senses and realizes what he had and how great technology in his days sometimes was.

      Radio tubes, FETs and laser tubes are some part of that, for example.
      When he sets his smartphone aside and starts drawing some tube schematics on paper, he’s truly young again.
      His mind starts to working, there’s life in his eyes.

      But I suppose all in all that’s not too surprising?
      The mid-late 20th century tech simply was amazing, also because it did overcome limitations in a clever way.
      – In contrast to today, were we always use brute-force (raw computing power).

      See, there are teens/pre-teens out there today who’re into NES games, GB games or buildingn new DOS and Windows 98 PCs.

      While simultaneously, some old farts have forgotten about the details.

      It’s those who need the newest entertainment tech too feel young,
      while same time the young people may now be into collecting Audio CDs, VHS or Laserdiscs, be it for sheer curiosity.

      Man, I wished more of you old guys would realize this and open your mind!

      Mixing old and new technology isn’t just plain useful, it also unites generations of people. 😃❤️

  4. Ha. At the time these things were made they were a semester class homework assignment. This merely means that a single person could completely implement an entire system from hardware and write the BIOS. THAT was why TRS-80, Timex Sinclair, IBM PC were such great fun.

    Yes. It brings light to my eyes to write a VT100 emulator. The world was simple. Tubes were great.

  5. I had several of these when I was a child that my grandma had picked up for me at garage sells along with soldering irons and anything 80s/90s tech related i wanted. Man I miss radio shack. I opened them up and piggy backed the ram chips from one that was busted. If I remember correctly it worked but it went back to an earlier version BASIC/MSDOS. I wish we had resources like this in those days. I may still have some old external disk drives and cassette recorders if you guys want me to check my dads garage. I remember they weighed about 25 lbs a piece. Idk what kinda condition those ribbons will be in. Lol

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