Happy 41st Birthday, TRS-80 Model I

Forty-one years ago today, Radio Shack announced the TRS-80 Microcomputer System, which would later become known as the TRS-80 Model I to differentiate it from later models.  The company didn’t quite know what to expect, demand-wise, so they manufactured 3,500 units, thinking that if they didn’t sell, the existing retail stores could use them to keep track of inventory. Soon after the announcement, sacks of mail and thousands of phone calls resulted in a huge waiting list wanting to purchase a system. Each of those folks put down a $100 deposit to hold their place in line. They sold 10,000 units the first month, 55,000 units the first year, and a total of about 200,000 units total.

My TRS-80 Model I

At that time, I had recently discovered programming on an old Wang 2200 minicomputer sitting the corner of my high school’s library. It lacked a usable manual, so I contracted with the school board to write a manual, which would be used as a mini-course textbook the following year. At the time, I knew I wanted/needed my own computer, and had been carefully studying the available kits. But my interest was focused on programming, and not on firing up the old soldering iron. I had built many electronic kits over the years, but with a computer, I wanted to get right to the good stuff – writing code. The fully-assembled TRS-80 product appeared on the market just as I was about to make a final purchasing decision.

Just a few days after its announcement on August 3, 1977, I ordered my TRS-80 (much later dubbed the Model I, after the Model II was announced). Mine was the very first computer sold by my local Radio Shack store. It was on back-order for several weeks.

I had ordered a system with Level II BASIC and 16K of RAM, but they shipped a 4K Level I system, with the promise of additional RAM and the upgraded BASIC ROMs to follow in a few weeks. (Level I BASIC was based on Palo Alto Tiny BASIC. Level II BASIC was a much more capable dialect, written my Microsoft.) The system included a bulky keyboard unit (which housed the computer), a video display (a stripped-down RCA black-and-white CRT television), a desktop power supply brick, and one cassette recorder for program and data storage. The CPU was a Z80 running at 1.77MHz (that’s MegaHertz, not GigaHertz).

The keyboard suffered from keyboard bounce, frequently resulting in unwanted repeated characters while typing. To alleviate this problem, Radio Shack soon distributed a “Keyboard De-Bounce” cassette, which you had to load each time you booted the system. To save a $1.50 in costs, the designers had decided not to provide lowercase characters, so everything was typed and displayed in uppercase. Radio Shack (and others) would later release a kit to add lowercase capability. That was the first modification I added, with a kit, some solder, and trace-cutting.

The computer had no official audio output, but with some programming effort, you could generate sound effects, music, etc., by controlling the cassette output port. (And like many microcomputers in those days, the computer emitted sufficient EMI to play sound through a nearby AM radio.) I built a little box that allowed me to monitor the cassette output as audio, gain manual control over the cassette motor, and also resolve a ground-loop problem.

The system grew over time, as systems do, to become the hodgepodge of components you see in the photo. The gigantic and incredibly loud dot-matrix printer only printed on rolls of paper, and then only in uppercase, even if the document contained lowercase characters. (At that time, text appearing in all uppercase was not considered to be the equivalent of shouting, and would not trigger riots and wars.) The voice synthesizer, perched atop the video display, was memory-mapped to a 32-character area in the lower right corner of the display, so that anything you displayed in that area would be treated as a spoken phoneme. The system had an Expansion Interface (below the video display), which contained an additional 32K of RAM (for a total of 48K), RS-232 interface board, printer port, floppy controller, and power supplies. I added two single-sided single-density 5.25″ floppy drives, but to save $110, I got a bare MTI drive and power supply as the second drive. Two cassette drives appear at the far right, for loading, saving, and backing up cassette-based software. The little switch box perched on top of one of the cassette drives was something I cooked up to add audio monitoring capability and to solve a ground loop problem. What you don’t see in the photo above are:

  • An acoustic coupler modem, with a maximum speed of 300 baud
  • A joystick
  • Stacks of cassette-based and floppy-based software
  • Stacks of programming and technical reference manuals
  • A VoxBox voice recognition unit, which had a press-to-talk CB microphone attached
  • An Electric Crayon color graphics peripheral, which attached to the printer port of the Expansion Interface
  • A light pen, that could be used to interact directly with the video display

There were some design issues in the Expansion Interface that would cause occasional lock-ups and lost data. This was just a fact of life, so I would frequently save my work. Another source of flaky behavior (lockups, random reboots, etc.) was oxidation of the expansion port edge card, which required frequent cleaning. I eventually got tired of doing this, so I added something called “Gold Plug 80,” offered by E.A.P. Company, which was a set of gold-plated contacts soldered directly onto the edge card on both the keyboard and expansion units. This simple fix made life with the system much more predictable and pleasant.

Level II BASIC turned out to be very powerful, but it suffered from the overhead of being interpreted. I was able to squeeze more performance out of the system by learning Z80 assembly language, bypassing the BASIC interpreter. Initially, this meant hand-converting the assembly instruction mnemonics into equivalent hexadecimal values, and entering them through TBUG monitor. Later on, I got the Editor/Assembler, and was able to write in assembly language using the actual instruction mnemonics. Much later on, I picked up a Fortran compiler.

I took it all with me to college, and used it to write simulations, test out algorithms, edit papers, generate presentations, etc. This was well before the Internet was generally available, so my dial-up connections included the campus mainframe computer, a new service known as CompuServe, and a handful of BBSs.

Overall, even with the variety of frustrations and reliability issues, the TRS-80 Model I experience was well worth it in terms of what I learned and what I was able to do with it. By the time I got my first IBM PC clone, a few years later, I was able to hit the ground running.

I have to give credit once again to my (now late) parents for seeing the value in purchasing a computer system (after much persuasion), and for their patience with me as I spent countless hours in front of this system.

The TRS-80 Dynasty

The TRS in the TRS-80 brand stood for “Tandy Radio Shack”, and the 80 initially referred to the Zilog Z80 CPU used in many, but definitely not all, of the computers sold under this brand.

Here’s a rundown of the TRS-80 models. There are many more differentiating details than I have included here. Some models were sold for several years, so the year listed is the year the model was introduced.

  • “Model I” (1977)
    • 77 MHz Z80 CPU, separate components: keyboard/CPU unit (with 4K or 16K RAM), video display, cassette recorder
    • An optional Expansion Interface supported: up to 32K additional RAM, up to four 5.25″ floppy drives, an optional RS-232 serial card, a second cassette interface, and printer port.
    • Here is a link to the first TRS-80 catalog.
  • Model II (1979)
    • 4 MHz Z80A CPU
    • This relatively expensive system was targeted at businesses, and had an 8″ floppy disk built in.
  • Model III (1980)
    • 2.03 MHz Z80 CPU, 32K or 64K RAM.
    • An all-in-one, upgraded version of the Model I, which was mostly but not completely compatible with Model I software.
  • Pocket Computer 1 (1980)
    • Effectively, a Sharp PC-1211.
  • Color Computer 1 (1980)
    • 895 MHz Motorola 6809E CPU
  • Pocket Computer 2 (1981)
    • Effectively, a Sharp PC-1500.
  • Model 12 (1982)
    • Replaced the Model II, and used half-height double-sided 8″ floppy drives.
  • Model 16 (1982)
    • Replaced the Model II, and used a 6MHz, 16-bit Motorola 68000 CPU, using the Z80A processor for I/O operations.
  • Pocket Computer 3 (1982)
    • Effectively, a Sharp PC-1251.
  • Model 4 (1983)
    • 4 MHz Z80A CPU
    • Another all-in-one, this was compatible with Model III software and some CP/M software.
  • Model 16B (1983)
    • This was the most popular commercial Unix-based computer of the day.
  • Model 100 (1983)
    • 46 MHz 8-bit Intel 80C85 CPU
    • Made by Kyocera, this was one of the first truly portable “notebook” computers, which could run on standard AA batteries, and had a built-in LCD display.
  • Model 4P (1983)
    • “Luggable” version, similar in size and weight to a sewing machine.
    • It could be booted in a separate mode to run Model III software.
  • MC-10 (1983)
    • 89 MHz Motorola 6803
  • Pocket Computer 4 (1983)
    • Effectively, a Casio PB-100.
  • Color Computer 2 (1983)
    • 895 MHz Motorola 6809E CPU
  • Model 4D (1985)
    • This was the last descendant of the original 1977 TRS-80 Model I computer.
  • Color Computer 3 (1986)
    • Up to 1.79 MHz Motorola 6809E CPU

In some cases, the TRS-80 models evolved into Tandy-branded models. For example:

  • The Tandy 6000 evolved from the TRS-80 Model 16B.
  • The Tandy 102 and 200 evolved from the TRS-80 Model 100.
  • The Tandy Pocket Computer line picked up where the TRS-80 Pocket Computer line left off.

There were many third-party TRS-80 clones as well, including (but not limited to) Lobo Max-80, LNW-80, and Aster CT-80, Dick Smith System 80, PMC-80, HT-1080Z, and TRZ-80. Some were only marketed in specific regions of the globe. The Color Computer line had many clones as well. (By the way, among enthusiasts, you’ll see the TRS-80 Color Computer line sometimes referred to as CoCo.)

Even though the PC-DOS-based IBM PC appeared on the scene in 1981, none of these TRS-80 models were compatible with PC-DOS/MS-DOS. There were several IBM-compatible (or mostly-compatible) MS-DOS-based computers sold by Radio Shack under the Tandy brand (e.g. Tandy 2000 in 1983 and the Tandy 1000 in 1985), but not under the TRS-80 brand.

Continued Interest in the TRS-80

Even now, 41 years after the first TRS-80 was announced, and decades after the last system in the TRS-80 family arrived, hobbyists and vintage computer enthusiasts continue to collect, share information about, and even develop new products for this family of computers.  Recently, an annual conference was started, called Tandy Assembly. Here are links to just a few sites that cater to those interested in the TRS-80:

I ran across an interesting book recently, entitled Priming the Pump: How TRS-80 Enthusiasts Helped Spark the PC Revolution. It relates several of the inside stories of how the TRS-80 came to be, and its influence on the personal computing industry in those early years. If you’re interested in the origins and influences of the TRS-80, I highly recommend this book.

The Fate of the TRS-80 Model I

The TRS-80 Model I was finally discontinued in January 1981, because it failed to meet the FCC’s Radio Frequency Interference standard.

My entire TRS-80 system was donated to the Dallas Public Library in 1985, to make room for more modern equipment. In a letter of thanks, the Director of Libraries wrote “We are currently exploring the best possible use for the TRS-80 Computer System.” I have often wondered if they did anything with it, or if it ended up in someone’s personal collection, a museum, a thrift store, or a landfill.