KB10014: Tandy Models Compared – TRS-80, Color Computer, and Model 100

Tandy Corporation (through its Radio Shack retail chain) produced several distinct families of personal computers before shifting to IBM-compatible machines. The TRS-80 line, the Color Computer series, and the Model 100 portables were each built on different architectures and aimed at different markets. This article covers the non-IBM-compatible Tandy systems.

TRS-80 Model I / III / 4

The original TRS-80 line was built on the Zilog Z80 processor and was one of the "1977 trinity" of personal computers alongside the Apple II and Commodore PET. All models in this family used a monochrome display and were aimed primarily at the business and hobbyist markets.

TRS-80 Model I (1977)

The original TRS-80 Micro Computer System used a Z80 CPU at 1.774 MHz with 4 KB of RAM (expandable to 16 KB in the base unit, or 48 KB with the Expansion Interface). It included a modified version of Level I BASIC in 4 KB of ROM, later upgraded to Level II BASIC in 12 KB of ROM for more capable programming. The display was a 64×16 character monochrome screen (128×48 block graphics). Storage started with a cassette interface; the Expansion Interface added floppy drive support, parallel printer port, and additional RAM. At $599 for the base system with monitor, the Model I vastly exceeded Radio Shack's sales expectations. One well-known issue was that the Model I generated significant radio frequency interference (RFI), which eventually led to FCC regulation changes and contributed to its replacement.

TRS-80 Model III (1980)

The Model III was designed as the Model I's successor, primarily to address the RFI issues by integrating the computer, monitor, and (optionally) floppy drives into a single enclosure. It used a Z80 CPU at 2.03 MHz—a modest speed increase—with 4 KB of RAM standard (expandable to 48 KB). The display improved to 64×16 or 80×24 characters (a significant upgrade for word processing). It added built-in lowercase character support, a faster cassette interface (1,500 baud vs. the Model I's 500 baud), and improved reliability overall. The Model III ran approximately 80% of Model I software but used an incompatible disk format, which frustrated users who had invested in Model I floppy-based programs.

TRS-80 Model 4 (1983)

The Model 4 was the final evolution of the Z80-based TRS-80 line. It used a Z80A CPU at 4 MHz (nearly double the Model III's speed) with 64 KB of RAM standard, expandable to 128 KB via bank switching. The display supported 80×24 characters with inverse video. The Model 4 booted its own TRSDOS 6 operating system and could also run CP/M without hardware modification, opening up a large library of business software. Crucially, the Model 4 could boot with Model III operating systems and run virtually all Model III software in compatibility mode. A later Model 4 "Gate Array" version used custom chips to reduce component count and cost. A portable version, the Model 4P, was also produced.

TRS-80 Color Computer (CoCo) Series

The Color Computer is an entirely separate architecture from the Model I/III/4 line. Where those machines used the Z80, the CoCo used the Motorola 6809E processor—a more advanced 8-bit CPU with features like hardware multiply, position-independent code, and a full complement of 16-bit operations. The CoCo was not software-compatible with the TRS-80 Model I/III/4 at all. Despite sharing the TRS-80 name, these are fundamentally different computer families.

Color Computer 1 (1980)

The original CoCo used a Motorola 6809E CPU at 0.895 MHz with 4 KB of RAM (later 16 KB or 32 KB configurations). It generated video through the Motorola 6847 VDG (Video Display Generator) chip, producing a 256×192 graphics display with a limited palette or 32×16 text characters (uppercase only). Color BASIC, supplied by Microsoft, was included in ROM. The original CoCo had a chiclet-style keyboard that was widely disliked. Video output was RF only (to a television), and sound was a single-bit DAC through the TV speaker. A cartridge port allowed ROM-based software, and the serial I/O "bit banger" port served for cassette storage and serial communication. The original retail price was $399 for the 4 KB model.

Color Computer 2 (1983)

The CoCo 2 maintained full software and hardware compatibility with the CoCo 1 while making incremental improvements. It shipped with 16 KB or 64 KB of RAM (eliminating the 4 KB option), featured an improved full-travel keyboard, and used a revised motherboard with fewer chips. Externally it was styled in a white case with a more modern appearance. The CoCo 2 ran the same 6809E at 0.895 MHz with the same 6847 VDG. Extended Color BASIC was standard on 64 KB models, adding disk and graphics commands.

Color Computer 3 (1986)

The CoCo 3 was a major upgrade built around the custom GIME (Graphics Interrupt Memory Enhancement) ASIC, which replaced both the 6847 VDG and the SAM (Synchronous Address Multiplexer) chips from earlier models. The GIME added dramatically improved graphics: resolutions up to 640×225 with 16 colors from a 64-color palette, plus an MMU (Memory Management Unit) that could address up to 512 KB of RAM (the base was 128 KB, expandable to 512 KB). The CPU remained a 6809E, now running at 1.79 MHz (double the CoCo 1/2 speed, though the "fast" mode introduced some peripheral timing incompatibilities). The CoCo 3 operated in an "all-RAM" mode, copying ROMs into RAM at startup for improved performance. It ran an enhanced Super Extended Color BASIC and was compatible with nearly all CoCo 1/2 software. Two GIME chip revisions were produced: the original 1986 "Tequila" (with some known bugs) and the 1987 "Tortilla" (with fixes). The CoCo 3 supported OS-9 Level II, a Unix-like multitasking operating system that took advantage of the expanded memory and MMU.

Model 100 / 102 / 200 Portables

Yet another separate product family, the Model 100 series were among the first true portable computers—lightweight, battery-powered, and instantly usable machines that anticipated the laptop category by several years.

Model 100 (1983)

Designed by Kyocera in Japan (and sold by various companies worldwide under different brands), the Model 100 was one of the most successful portable computers of its era, with over 6 million units sold across all versions. It used an Oki 80C85 CPU (a CMOS version of the Intel 8085) at 2.4 MHz with 8 KB to 32 KB of static RAM (expandable in 8 KB increments). The display was an 8-line by 40-character LCD (240×64 pixels). The ROM contained Microsoft BASIC (reportedly the last code Bill Gates personally wrote), a text editor, a telecommunications program, an address book, and a scheduler. A built-in 300-baud modem, RS-232 serial port, parallel printer port, and cassette interface provided connectivity. Running on four AA batteries for approximately 20 hours, the Model 100 became an indispensable tool for journalists, field technicians, and anyone who needed a portable writing and communications device.

Model 102 (1986)

The Model 102 was a refined version of the Model 100. It was thinner and lighter (about 3 pounds vs. 3.8 pounds), used the same 80C85 processor and LCD display, and ran the same software. The main improvements were a slimmer profile, a slightly updated case design, and the standard inclusion of 32 KB of RAM. The built-in modem was removed in some configurations. The Model 102 was functionally very similar to a fully-loaded Model 100 and maintained full software compatibility.

Model 200 (1984)

The Model 200 upgraded the display to 16 lines by 40 characters (240×128 pixels) in a clamshell form factor that folded shut to protect the screen. It included 24 KB of RAM standard (expandable to 72 KB) and added a second ROM bank for additional applications. The processor and basic architecture remained the same. The clamshell design made it more protective during transport, but the Model 200 was slightly heavier than the 100. It was compatible with Model 100 BASIC programs with minor adjustments for the larger display.

Quick Comparison Table

Model	Year	CPU	Speed	Base RAM	Family
TRS-80 Model I	1977	Z80	1.774 MHz	4 KB	TRS-80
TRS-80 Model III	1980	Z80	2.03 MHz	4 KB	TRS-80
Color Computer 1	1980	6809E	0.895 MHz	4 KB	CoCo
TRS-80 Model 4	1983	Z80A	4 MHz	64 KB	TRS-80
Model 100	1983	80C85	2.4 MHz	8 KB	Portable
Color Computer 2	1983	6809E	0.895 MHz	16 KB	CoCo
Model 200	1984	80C85	2.4 MHz	24 KB	Portable
Color Computer 3	1986	6809E	1.79 MHz	128 KB	CoCo
Model 102	1986	80C85	2.4 MHz	32 KB	Portable

Collector and Restoration Notes

TRS-80 Model I Expansion Interfaces are known for poor edge connector contacts—cleaning and reseating the connection between the computer and expansion unit resolves many intermittent issues. Model I and III power supplies should be tested before use, as overvoltage can damage the logic board. CoCo 1 and 2 units are generally reliable, but the RF modulator connection can degrade; composite video modifications are a popular upgrade. On the CoCo 3, the GIME chip is the single most critical component and is not reproduced (though FPGA replacements have been developed by the community). Model 100/102/200 units need their RAM backup batteries checked—a failed battery means losing all stored programs and data on power-off. The LCD panels on Model 100 units can develop dead lines, though the technology is generally long-lived. Capacitor replacement on all Tandy systems is less commonly needed than on Commodore products of the same era, as Tandy tended to use more reliable electrolytic brands.