TRS-80 Model II Lives Again

A lot of people had a Radio Shack TRS-80 Model I. This was a “home computer” built into a keyboard that needed an external monitor or TV set. Later, Radio Shack would update the computer to a model III which was a popular “all in one” option with a monitor and even space for — gasp — floppy disks. But the Model II was not nearly as common. The reason? It was aimed at businesses and priced accordingly. [Adrian] got a Model II that was in terrible shape and has been bringing it back to life. You can see the video of how he’s done with it, below.

The Model II was similar to the older “Trash 80” which had been used — to Radio Shack’s surprise — quite often by businesses. But it had more sophisticated features including a 4MHz CPU — blistering speed for those days. It also had an 80×25 text display and a 500K 8-inch floppy drive. There were also serial and printer ports standard.

There were a few interesting features. The floppy drive’s spindle ran on AC power and if the computer was on, the disk was spinning. In addition, there was bank switching so you could go beyond 64K and also you didn’t have to share your running memory with the video display. In theory, the machine could go beyond 64K since half the memory was bank switchable. In practice, the early models didn’t have enough expansion space to handle more than 64K physically.

The machine had to have a floppy to boot and in a previous video, the computer failed to boot correctly. Given it was stored poorly for years, that isn’t too surprising. You’ll get to watch the machine being torn down, schematics examined, and ICs tested. However, eventually, the computer seemed to fix itself. If you have any experience with this kind of thing, you can guess what happens next. It fails again, of course.

Theorizing that the parts that tested good were temporarily healed by heating, [Adrian] replaced the chips, but it took a bit of work to find out it was a bad disk controller.  Once he knew the bad area, it was relatively easy to find the bad chips.

If you have fond memories of the model I, why not build one? We love seeing these old machines restored instead of gutted, although sometimes there is little choice.

15 thoughts on “TRS-80 Model II Lives Again

      1. 5.25″ high density drives also spun at 360rpm. that’s why the controllers have to be able to handle 300khz data in addition to 250khz and 500khz; 250khz/300rpm in a 360rpm drive gives you 300khz data.

  1. The Model Ii at the time was an incredible office system with several options. One was an external 3 bay Floppy and they even sold a desk it fit into. Another was the external up to 35 MB external hard drive ( I had one on my Model 4 running my TBBS based bulletin board). The main option that made it an office system was ARCNET which enabled up to 255 TRS80s to be networked. Printer and file sharing was available! Glad to see some focus on the largely ignored Model II. Good article!

  2. “…the older “Trash 80” which had been used…”

    Armor suit on: The TRS 80 was not “Trash 80”, but rather “Treasure 80” :)

    (and yes, we all know TRS was the abbreviation for Tandy Radio Shack)

  3. I have a warm place in my heart for the Mod II. I began my career programming the Mod II (and later the 16B running Xenix) for a medical company.

    One comment. NEVER power up that computer with a floppy disk in the drive. It has the tendency to corrupt the disk.

    Back in the day, I went so far as to build a latching relay circuit in an outlet box, which, in the event of power interruption, would prevent the machine from powering back up until you had a chance to remove the floppy. You’d have to then press a momentary-contact reset switch to re-energize the latching relay.

    This accessory ended our problems wirh corrupted disks.

  4. In the late 1980s I managed to purchase a pallet full of these at a government surplus auction for the princely sum of USD$1. Not particularly useful until I discovered they were fully populated with 4164, 41256, and 414256 DRAM chips, which were pricey and hard to come by at the time (a shortage, IIRC). Managed to fully populate 640KB, 2MB on my EMS expansion board, and my EGA card on my first “powerhouse” computer.

  5. What I found interesting (besides the process of debugging) it that just ‘one’ bad gate on a chip caused the whole computer to not work correctly. Think about that…. The processors of today has billions of gates that have to work correctly and that’s not counting all the circuits on the motherboard and peripherals…. And we take it for ‘granted’ that it WILL work for days, years on end. In fact (see reviews on CPUs, motherboards, hard drives), some complain when a CPU doesn’t work or goes bad after 6 months, or a bad motherboard slips through the cracks… To me, it in mind blowing that our systems just ‘work’ at all!!!

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