Radio Shack’s Original TRS-80 Family of Computers

In 1977, there were three home computers: The Apple II, the Commodore PET, and the TRS-80, which was sold at 3,500 Radio Shack stores across the United States. Apple and Commodore used the MOS Tech 6502 CPU in their computers, but Radio Shack chose the Zilog Z-80.

TRS-80 Model I

The TRS-80 Model I was originally known as the TRS-80 Micro Computer System and retroactively given the Model I designation after later models were introduced. It came to market on August 3, 1977 and remained in production until it was completely displaced by the TRS-80 Model III.

The computer itself is in the keyboard, which retailed for US$399 with 4 KB of memory. The monitor only displays output from the computer, and the cassette recorder is used to save and load software from cassette tape. The full system sold for US$599.

The Start of the Microcomputer Industry

January 1985 cover, Popular Electronics, featuring MITS AltairThe personal computing revolution was barely underway in 1977. The MITS Altair 8800 had been featured on the January 1975 cover of Popular Electronics, the first kit computer, which sold approximately 2,000 units during it first month on sale. Anticipating sales of perhaps 200 units, MITS was swamped trying to keep up with demand and sold 10,000 units that year.

MITS Altair 8800

This was back in the days when computers had switches on the front – lots and lots of switches. Built-in keyboards were a thing for the future.

The Altair was the first computer to use Microsoft BASIC, and it was built around a 2 MHz Intel 8080 CPU, the first popular microprocessor.

MITS’ first competition was the IMSAI 8080, which was also the first personal computer clone, as it was designed to be 100% compatible with the MITS Altair, which was supply constrained. The IMSAI shipped in December 1975 and sold 17-20,000 units through 1978.

Southwest Technical Products Corporation (SWTPC) produced a kit in 1975 that used the Motorola 6800 CPU, later to be replaced by the 6809 CPU that would go on to power some early personal computers and inspire the development of the 6502 CPU used in early Apple, Commodore, and Atari computers.

The MOS KIM-1 was the first single-board computer, designed as a development system for the new MOS 6502 processor. When it launched in 1976, it was the first assembled single-board  computer available.

The Apple 1, also a single-board computer, was introduced later in 1976 with a production run of 2,000. Buyers needed to add a keyboard, case, and power supply, but at least the logic board was fully assembled!

TRS-80 and Personal Computing

In 1977, the first assembled personal computers came to market, and Radio Shack was one of the first companies to market. Its TRS-80 went on to be the most popular computer of the late 1970s and early 1980s; some estimate it sold five times as well as Apple computers during that period.

Tandy/Radio Shack got into the computing business after its buyer, Don French, purchased a MITS Altair 8800. He thought Radio Shack could sell computers, and with the CB craze winding down, Radio Shack needed a new market. But how many could it sell? Nobody knew, and guesses ranged from 50,000 per year to 3,000 or less. To play it safe, they built 3,500 – one for each Radio Shack store. That way they could be used in house if they didn’t sell.

The TRS-80 was an instant success. Tandy received six sacks of mail asking for more information, over 15,000 people called in to place orders, and 250,000 were put on the waiting list after paying a $100 deposit. Even 50,000 units a year had been a conservative guess!

Although it took until mid 1978 before Radio Shack could catch up with demand, it was eventually able to stock the computer in its stores and have demo units set up. Radio Shack dominated the personal computing revolution.

TRS-80 Model I

Radio Shack TRS-80 Model IThe TRS-80 ran a 1.77 MHz CPU (vs. 1 MHz for Apple and Commodore), and Tandy used the newer Zilog Z-80 CPU instead of the older, less sophisticated Intel 8080 used by MITS and IMSAI. The Model I originally shipped with 4 KB of memory, and Tandy later bumped that to 16 KB.

The Model I could display 32 or 64 text characters across its screen, more flexibility than Apple or Commodore offered with their fixed 40 character displays. Then again, the TRS-80 only had 16 rows of text, while the Apple had 24 rows and the PET had 25.

Radio Shack offered an Expansion Interface (EI) for the Model I, although it did not use the industry standard S-100 bus. The EI had a floppy disk controller, room for up to 48 KB of additional memory, a serial port, and a parallel port, along with a second cassette interface. Radio Shack first offered floppy drives in 1978, and disks had a formatted capacity of 85 KB. Unfortunately, TRSDOS could be unreliable, and many early hobbyists developed ways of making it write to disk more reliably.

The Model I had two versions of BASIC, the programming language offered with all early personal computers. Level I BASIC fit in 4 KB of ROM, while the more powerful Level II BASIC used 12 KB of ROM. Level I BASIC was developed by Tandy and based on Tiny BASIC; Level II BASIC was licensed from Microsoft

The Model I is estimated to have sold 200,000 units during its production life.

TRS-80 Model III

TRS-80 Model IIIThe TRS-80 Model III was introduced in July 1980 and had built-in drive bays, a better keyboard, built-in support for lower-case text, and a slightly faster CPU that ran at 2 MHz. Unfortunately, the new disk drives were incompatible with disk from the Model I. BASIC and TRSDOS remained buggy.

Because of its all-in-one design, the Model III was a much better fit for the education market. With third-party CP/M support, it was also a popular business computer.

TRS-80 Model IIYes, there was a TRS-80 Model II, but it was a completely incompatible business-oriented machine with 4o or 80 characters and 25 rows of text, 8″ 500 KB floppy disks, and had its own, incompatible version of TRSDOS. It had been introduced in May 1979, included 32 KB or 64 KB of memory, and used a 4 MHz Z-80 CPU. Tandy had a bad habit of using similar names for unrelated computers.

TRS-80 Model 4

TRS-80 Model 4

With the TRS-80 Model 4, introduced in April 1983, Tandy moved to a 4 MHz CPU and included the ability to run CP/M as a standard feature, and it is possible to install 128 KB of RAM. Better yet, Tandy finally moved away from the silver spray painted cases of previous models to a clean white look.

Tandy shipped TRSDOS 6 with the Model 4, which was based on LDOS by Logical Systems. The hardware is also backward compatible and can run Model III software when booting from a Model III version of TRSDOS.

Tandy also built a totable version, the Model 4P. There was also a Model 4D, which included 360 KB double-sided floppy drives. This marked the end of Tandy’s Z-80 TRS-80 line of personal computers.

Other TRS-80 Models

Tandy loved the TRS-80 name and used it on a wide array of personal and business computers. In addition to the four models covered above, Radio Shack also sold the following TRS-80 families:

  • TRS-80 Color Computer, 1-3. Based on Motorola 6809 CPU, from 1980 through 1991 this was Tandy’s line of home computers with color support. We hope to cover these in a future article.
  • TRS-80 Pocket Computers, PC-1 through PC-8. These were rebadged Sharp and Casio devices.
  • TRS-80 Model 100TRS-80 Model 100 and 102. The Model 100 (right) was released in 1983, built by Kyocera, and had 8 lines of 40 characters on its display. It was very popular among journalists in the days before laptops and sold 6 million units. The 102 was a thinner, lighter version of the 100.
  • TRS-80 Model 2000 – see our article on Tandy’s PC Compatible Computers. Only the Model 2000 used TRS-80 as part of its name.

Further Reading

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