TRS-80 Color Computer

From Academic Kids

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TRS-80 Color Computer II

The Radio Shack TRS-80 color computer (also called Tandy Color Computer, or CoCo) was a home computer based around the Motorola 6809E processor and part of the TRS-80 line.


Origin and history

The CoCo started life as a Motorola reference system, and was meant to be used as a Videotext terminal. In fact, a cut-down version of the CoCo was sold as a Videotext terminal using the same case and keyboard. The original CoCo came in 4 kB, 16 kB and 32 kB versions, though hobbyists quickly figured out how to make the 32 kB systems into 64 kB systems by enabling the second bank of RAM (which was disabled in the factory). The original battleship-gray case and chiclet keyboard of the CoCo I were dropped in favor of white and a full-travel keyboard for the CoCo II and III.

The main magazine about the CoCo was The Rainbow.

A cartridge-based system

The CoCo differed from the Z80-based Models I/II/III/4 and 4p by having a different processor (Motorola 6809e) and cartridge slot rather like the popular Atari 2600 VCS system. Consequently, many games and applications (including, in fact, a disk system) were released in cartridge form. Like the Model I, the Coco came with Standard and Extended BASIC (analogous to the Level I and II BASIC). A Disk Controller added Disk Extended BASIC (DECB). These BASIC Languages were licensed from Microsoft. It is interesting to note that DECB wasn't a true DOS (disk operating system), but added commands to the BASIC programming language. These commands directly accessed the disk drive controller.

The Dragon clone

A British clone of the CoCo was called the Dragon 32/64. An American company Tano, attempted to import these units into the U.S. but met with no success. The Dragon was a much improved unit with RGB Video (rather than the TV Output of the CoCo I and II, and much like the later CoCo 3), a Parallel Printer port (the CoCos only printed through a slower Serial port), and a better keyboard.

CoCo 3

In 1986, Tandy introduced the CoCo 3 which was meant to compete with the Amiga and Atari ST systems. Based on the faster 68B09e, and with improved graphics, the CoCo 3 was meant to be more of a gamers system. It came with 128 kB of RAM Standard, and could be upgraded to 512 kB. The graphics hardware went up to 640x225 resolution, and lower modes could display 16 out of 64 colors (though clever programming could display all 64 at the same time).

Besides Tandy's licensed Disk BASIC from Microsoft, additional operating systems were available for the CoCo line. These included the TSC FLEX operating system (distributed for the CoCo by Frank Hogg) and Microware's OS-9 operating system. Both systems turned the CoCo into a much more powerful system, and in the case of OS-9, made it multi-user/multi-tasking.

Tandy also released a Multi-Pak which allowed up to 4 cartridges to be mounted at the same time, a Voice Synthesiser, 300 Baud Modem Pak, and other accessories. The CoCo was the first Tandy computer to have a mouse available for it.

A popular accessory was a high-resolution Joystick adapter (designed by software whiz Steve Bjork). It was first used by a software package named Color Max 3 which was a clone of MacPaint but added support for color graphics. This was very desirable product for CoCo owners and, interestingly enough, the prototypes of the Macintosh Computer were built using the same Motorola 6809 Processor.

Description of different versions

There were three versions of the Color Computer:

Color Computer I - Grey Case (1980-1983)

The original version of the Color Computer was available with a choice of 4 kB, 16 kB or 32 kB of RAM, and regular or extended version of Microsoft's Color Basic interpreter software. It used a regular TV for display. Later, an upgrade to 64 kB of RAM was available. This was possible due to a motherboard revision -- earlier models can't be easily upgraded to 64 kB. A number of peripherals were available: tape cassette storage, serial printers, a 5.25 inch floppy disk drive, speech and sound generators, and joysticks. Towards the end of the CoCo 1 days, some units were manufactured using a white case and a different keyboard which was somewhere between the original "Chicklet" calculator type keyboard and the later "real" keyboard. (Early model CoCo 2s used this same keyboard.) The CoCo 1 had at least three popular motherboard revisions, known as "D", "E", and "F".hhy

Color Computer II - White Case (1983-1986)

This version of the CoCo featured a smaller case and improved keyboard. In later models of the CoCo 2, when the label changed to read "Tandy Color Computer" instead of "TRS-80", a different video chip allowed the display of lower-case characters. All were upgradeable to 64 kB of RAM (early models came with 16 kB standard). Radio Shack also sold the Color Computer 2 under the "Tandy Data Products" label as the "TDP-100". The 64 kB CoCo 2 could run OS-9 Level 1 from Microware.

Color Computer III - White Case (1986-1991)

This featured a Motorola 68B09E processor, and 128 kB or 512 kB of RAM. It used all the original peripherals, and most older software ran on it. Taking the place of the graphics and memory hardware in the CoCo 1 and 2 was an ASIC called the "GIME" chip, which supported various display modes (some text-only, some graphics, with up to 16 colors from a 64-color palette displayable at one time) and also handled memory mapping (in 8 kB blocks, which some developers considered outrageously large for a 64 kB address space) and RAM refresh functions. Microware further extended the Extended Color BASIC to support the new display modes and later provided a version of the OS-9 Level 2 operating system. This OS featured multitasking, windowing, and a more extensive development environment that included a bundled copy of BASIC09. C and Pascal compilers were available. (Various members of the CoCo OS-9 community enhanced OS-9 Level 2 for the CoCo 3 at Tandy's request, but Tandy stopped production of the CoCo 3 before the upgrade was officially released. Most of the improvements made it into NitrOS9, a major rewrite of OS-9/6809 Level 2 for the CoCo 3 to take advantage of the added features and speed of the Hitachi 6309.)

This model also had an RGB output, allowing direct attachment to RGB analog monitors.

The 6809 in the CoCo 1 and 2 ran at 0.895 MHz; the CoCo 3 runs at that frequency by default, but is software controllable to run at twice that rate. Those are one fourth and one half of the color burst frequency used in color television. (Synchronizing the CPU clock to the color burst was common in home computers and video game consoles of the time; even the original IBM PC ran at 4/3 color burst frequency.) This technique was no doubt convenient in lowering part count, but it limited how designers of the computer's successors could adjust the clock rate. Tandy took many other shortcuts in CoCo design, eating CPU cycles to cut the part count. The most notorious were probably the "bit banger" serial port and the "high-res mouse interface," which put the CPU through a busy wait loop while a capacitor discharged to figure out the position of the mouse, so that unless you were actively using the mouse, you learned to move it to the upper left hand corner of the screen.

Third-party companies such as DISTO and Cloud-9 ( have done considerably more with the CoCo than Tandy perhaps thought possible. For example, one can with third-party hardware attach IDE and SCSI drives to the CoCo, as well as upgrade the memory up to 2 MB of RAM. The CoCo still has a small but active user community.

The OS-9 divide

There is/was a major division of CoCo users into two groups: those who used OS-9 and those who "used" DECB (Disk Extended Color BASIC); the quotes are present because many if not most non-OS-9 programs for the CoCo used DECB only as a loader and for disk I/O, beating directly on the hardware for everything else. That meant that not carrying on every wart and shortcut in the original CoCo design would break non-OS-9 CoCo applications, whereas with OS-9 one would need only rewrite device drivers. This perceived requirement of total backwards compatibility killed off at least one attempt to improve on the CoCo--Frank Hogg's "Tomcat" TC09 fizzled out while Chris Burke was attempting to make it simulate all the details of CoCo hardware--and probably killed them all; if there were an archive of the CompuServe OS-9 SIG messages, Kevin Darling's cri de coeur directed to DECB users with the subject line "You're Killing the CoCo!" would be a useful link. Tandy threw away a significant opportunity--one should recall that a 1.8 MHz 6809 processor readily outperformed the 4.77 MHz 8088 in the original IBM PC, and people have run the Hitachi 6309 at 5 MHz.

CoCo 4

For many, the discontinuation of the CoCo 3, and the CoCo line in general was very distressing. In general, Tandy did not take the CoCo very seriously. They failed to market the CoCo as the powerful and useful machine that it was, and offered customers no hint at the massive third party software/hardware market that grew to fill the void.

A few third party hardware companies attempted to continue the CoCo torch, but the lack of decent backwards compatibility to the CoCo 3 failed to entice much of the CoCo community over to the new machines. These new machines did help to pave the way for OS-9 68K.


Frank Hogg Labs introduced the Tomcat TC-09 in June of 1990, which was somewhat compatible with the CoCo 3, but was mostly only able to run OS-9 software. A later version called the TC-70 (running on a Motorola 68000 family chip) had strong compatibility with the MM/1, and also ran OS-9 68K.


The Multi-Media One was introduced in July of 1990, ran OS-9 68K on a 15 MHz Motorola 68070 processor with 3MB RAM, and had a 640x208 graphics resolution as well as supporting a 640x416 interlaced mode. It included a SCSI hard drive, stereo A/D and D/A conversion, and an optional MIDI interface. It is estimated that about 200 units were sold.


The AT306 (also known as the MM/1B) was a successor to the MM/1.

Delmar System IV/Peripherial Technology PT68K-4

Peripheral Technology produced a 16 MHz Motorola 68000 system called a PTK68K-4, which was sold as a kit or a complete motherboard. Delmar sold complete systems based on the PT68K-4 and called the Delmar System IV. The PT68K-4 has the footprint of an IBM PC, so it will fit in a normal PC case, and it has seven 8-bit ISA slots. Video was provided by a standard IBM style monochrome, CGA, EGA, or VGA video card and monitor, but for high resolution graphics the software only supported certain ET4000 video cards. It appears that most users of this system used/uses OS-9, but there are several operating systems for it, including REX (a flex like OS), and SK*DOS. Dan Farnsworth, who wrote REX, also wrote a basic interpreter that was fairly compatable to DECB, but it was too little, too late to be of interest to many CoCo users. There was also a card available called an ALT86, which was basically an IBM XT compatable computer on a card, which allowed the user to run DOS programs on it. In fact, you could run both the 68000 and the ALT86 card at the same time, if you didn't need access to the ISA bus from the 68000 side of it.

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