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TRS-80 Color Computer


Nov 19, 2013
Miami, Florida
The Radio Shack TRS-80 Color Computer (also marketed as the Tandy Color Computer and affectionately nicknamed CoCo) was a home computer launched in 1980.
Despite bearing the TRS-80 name, the "Color Computer" was a radical departure from the earlier TRS-80; in particular it had a Motorola 6809E processor, rather than the TRS-80's Zilog Z80. Thus, despite the similar name, the new machine was absolutely not compatible with software made for the old TRS-80.
The Motorola 6809E was a very advanced processor, but was correspondingly more expensive than other more popular microprocessors. Competing machines such as the Apple II, Commodore VIC-20, the Commodore 64, the Atari 400, and the Atari 800 were designed around the much cheaper MOS 6502 itself essentially an enhanced clone of the Motorola 6800. Some of these competing machines were paired with dedicated sound and graphics chips and were much more commercially successful in the 1980s home computer market. Steve Wozniak once commented that the 6502 was 1/4 the price of the Motorola 6800 when the original Apple was being developed in the late 70s. By 1986 prices for eight-bit processors had dropped dramatically from the late 70s, but the MC6809 was still just over twice the price of a MOS6502 (6809/6809E - $5.95; MOS6502 - $2.79).
The Tandy Color Computer line started in 1980 with what is now called the CoCo1 and ended in 1991 with the more powerful yet similar CoCo 3. It was one of the more powerful eight-bit computers of its day. All three CoCo models maintained a very high level of software and hardware compatibility, with few programs written for the older model not running on the newer. The converse cannot be claimed, obviously, due to the greater capabilities of the newer CoCo models.

[h=2]Origin and history[/h]

The TRS-80 VideoTex Terminal, circa 1980​

The TRS-80 Color Computer started out as a joint venture between Tandy Corporation of Fort Worth, Texas and Motorola Semiconductor, Inc. of Austin, to develop a low cost home computer in 1977.
The initial goal of this project, called "Green Thumb", was to create a low cost Videotex terminal for farmers, ranchers, and others in the agricultural industry. This terminal would connect to a phone line and an ordinary color television and allow the user access to near real-time information useful to their day-to-day operations on the farm.
Motorola's MC6847 Video Display Generator (VDG) chip was released about the same time as the joint venture started and it has been speculated that the VDG was actually designed for this project. At the core of the prototype "Green Thumb" terminal, the MC6847, along with the MC6809 Microprocessor Unit (MPU), made the prototype a reality by about 1978. Unfortunately, the prototype contained too many chips to be commercially viable. Motorola solved this problem by integrating all the functions of the many smaller chips into one chip, the MC6883 Synchronous Address Multiplexer (SAM). By that time in late 1979, the new and powerful Motorola MC6809 processor was released. The SAM, VDG, and 6809 were combined and the AgVision terminal was born.
The AgVision terminal was also sold through Radio Shack stores as the VideoTex terminal around 1980. Internal differences, if any, are unclear, as not many AgVision terminals survive to this day.
With its proven design, the VideoTex terminal contained all the basic components for a general purpose home computer. The internal modem was removed, and I/O ports for cassette storage, serial I/O, and joysticks were provided. An expansion connector was added to the right side of the case for future enhancements and program cartridges ("Program Paks"), and a RAM button (a sticker indicating the amount of installed memory in the machine) covered the hole where the Modem's LED "DATA" indicator had been. On July 31, 1980, Tandy announced the TRS-80 Color Computer. Sharing the same case, keyboard, and layout as the AgVision/VideoTex terminals, at first glance it would be hard to tell the TRS-80 Color Computer from its predecessors.
The initial model (catalog number 26-3001) shipped with 4 kB of Dynamic Random Access Memory (DRAM) and an 8 kB Microsoft BASIC interpreter in ROM. Its price was 399 USD. Within a few months, Radio Shack stores across the US and Canada began receiving and selling the new computer.

[h=2]Differences from earlier TRS-80 models[/h]The Color Computer, with its Motorola 6809E processor, was a radical departure from the Z80-based TRS-80 Models. Indeed the "80" in "TRS-80" stood for "Z80". For a time, the CoCo was referred to internally as the TRS-90 in reference to the "9" in "6809". However this was dropped and all CoCos sold as Radio Shack computers were called TRS-80 in spite of the processor change.
Like its Z80-based predecessors, the CoCo shipped with BASIC, but in this case Tandy licensed Microsoft BASIC. As with the Z80 systems, there were multiple "levels" of BASIC. In the case of the CoCo they were the standard Color BASIC and Extended Color BASIC. Beyond that, Disk Extended Color BASIC came with the floppy controller, and on the CoCo 3, "Super" Extended Color BASIC extensions were added by Microware. Additional ROMs such as JDOS (Copyright J&M Systems) were available to enable the use of double sided disk drives.
The CoCo, which could display eight colors at once, was designed to be attached to a color television set, whereas the Z80 machines used monochrome computer monitors, often built into the case. The CoCo also featured an expansion connector for program cartridges (mostly games) and other expansion devices, such as floppy disk controllers and modems. In this way it shared some similarity to the Atari 2600, Atari 400/800 and other cartridge-capable game consoles and computers. Tandy also released a Multi-Pak Interface which allowed switching quickly among four cartridges mounted at the same time. This was similar in concept to the Model I's Expansion Interface.
Unlike some Z80 models, the CoCo did not come with a built-in floppy drive. The CoCo was designed to save and load programs and data from a standard audio cassette deck. Although slow (though at an average of 1500 bit/s it was faster than the Commodore 1541) and sometimes unreliable, this made the CoCo more affordable as a home computer. Tandy eventually offered a floppy disk drive controller for the CoCo as a cartridge. Both the CoCo and earlier TRS-80s shared the WD17xx series floppy disk controllers and 35 (later 40) track industry-standard floppy drives.
Even with the add-on floppy drive, the CoCo did not have a true DOS until third-party operating systems such as TSC FLEX9 (distributed for the CoCo by Frank Hogg Laboratories) and Microware's multi-user, multi-tasking OS-9 were available. However, a disk-based CoCo did contain Disk Extended Color BASIC on an internal ROM in the controller cartridge that gave the BASIC user the ability to save and load programs from the disk and store and retrieve data from disk in various ways.
Some non-program expansion cartridges included a sound/voice synthesiser (which led to the CoCo being used as an accessibility device for the disabled), 300 baud modem pak, RS232 pak, a hard drive controller, stereo music adapter, floppy disk controller, input tablet, and other accessories. The CoCo was the first Tandy computer to have a mouse available for it.

[h=2]Versions[/h]There were three versions of the Color Computer:
[h=3]Color Computer 1 (1980–1983)[/h]The original version of the Color Computer shipped in a large silver-gray case with a calculator-like "chiclet keyboard", and was available with memory sizes of 4K (26-3001), 16K (26-3002), or 32K (26-3003). Versions with at least 16K of memory installed shipped with standard Microsoft Color Basic or (optionally) Extended Color Basic. It used a regular TV for display, and TV-out was the only available connection to a display device.
The early versions of the CoCo 1 had a black keyboard surround, the TRS-80 nameplate above the keyboard to the left side, and a RAM badge ("button") affixed on the top and right side of the case. Later versions removed the black keyboard surround and RAM button, and moved the TRS-80 nameplate to the mid-line of the case.
Initial versions of the CoCo were upgraded to 32K by means of piggybacking two banks of 16K memory chips and adding a few jumper wires. A later motherboard revision removed the 4K RAM option and were upgraded to 32K with "half-bad" 64K memory chips as a cost-cutting measure. These boards have jumpers marked HIGH/LOW to determine which half of the memory chip was good. This was transparent to the BASIC programmer since in either configuration 32K of memory was available. As memory production yields improved and costs went down, many (perhaps most) 32K CoCo 1s were shipped with perfectly good 64K memory chips; many utilities and programs began to take advantage of the "hidden" 32K.
Radio Shack could upgrade all versions that shipped with standard Color BASIC to Extended Color BASIC, developed by Microsoft, for $99. BYTE wrote in 1981 that through Extended Color BASIC, Radio Shack "has released the first truly easy-to-use and inexpensive system that generates full-color graphics". Eventually the 32K memory option was dropped entirely and only 16K or 64K versions were offered.
In late 1982 a version of the Color Computer with a white case, called the TDP System 100, was distributed by RCA and sold through non-Tandy stores. Except for the nameplate and case, it was identical to the Color Computer.
At some point after this, both the Coco and the TDP System 100 shipped with a white case which had ventilation slots that ran the entire length of the case, rather than only on the sides. This ventilation scheme was carried over to the CoCo 2. Some late versions of the CoCo also shipped with a modified keyboard, often referred to as the "melted" keyboard, which had bigger keycaps but a similar rubbery feel.
A number of peripherals were available: tape cassette storage, serial printers, a 5.25 inch floppy disk drive, a pen and graphics tablet called the "X-Pad", speech and sound generators, and joysticks.

16k TRS-80 Color Computer I
I/O ports and cartridge slot on the TRS-80 Color Computer I
Late "white" model TRS-80 Color Computer I
Tandy Data Products TDP-100 (64K badge was added by user)
[h=3]Color Computer 2 (1983–1986)[/h]During the initial CoCo 1 production run, much of the discrete support circuitry had been re-engineered into a handful of custom integrated circuits, leaving much of the circuit board area of the CoCo 1 as empty space. To cut production costs, the case was shortened by about 25% and a new, smaller power supply and motherboard were designed. The "melted" keyboard from the white CoCo 1 and the TDP-100 style ventilation slots were carried over. Aside from the new look and the deletion of the 12 volt power supply to the expansion connector, the computer was essentially 100% compatible with the previous generation. The deletion of the 12 volt power supply crippled some peripherals such as the original floppy disk controller, which then needed to be upgraded, installed in a Multi-Pak interface, or supplied with external power.
Production was also partially moved to Korea during the CoCo 2's life-span, and many owners of the Korean-built systems referred to them as "KoKos". Production in the USA and Korea happened in parallel using the same part numbers; very few, if any, differences exist between the USA built and Korean built CoCo 2 machines.
Upgraded BASIC ROMs were also produced, adding a few minor features and correcting some bugs. A redesigned 5-volt disk controller was introduced with its own new Disk BASIC ROM (v1.1). This controller added the "DOS" command which was used to boot the OS-9 operating system by Microware.
Later in the production run, the "melted" keyboard was replaced with a new, full-travel, typewriter-style keyboard.
The final significant change in the life of the CoCo 2 was made for the models 26-3134B, 26-3136B, and 26-3127B (16 kB standard, 16 kB extended, and 64 kB extended respectively). Internally this model was redesigned to use the enhanced VDG, the MC6847T1. This enhanced VDG allowed the use of lower case characters and the ability to change the text screen border color. For compatibility reasons neither of these features were used and were not enabled in BASIC, however the resourceful user could enable them by setting certain memory registers. Midway during the production run of these final CoCo 2s, the nameplate was changed from "Radio Shack TRS-80 Color Computer 2" to "Tandy Color Computer 2". The red, green, and blue shapes were replaced with red, green, and blue parallelograms.

Early TRS-80 Color Computer 2 with "melted" keyboard
PAL version of the TRS-80 Color Computer 2
[h=3]Color Computer 3 (1986–1991)[/h]
The CoCo 3 was compatible with most of the CoCo 2's peripherals. Most older software ran on it. Taking the place of the graphics and memory hardware in the CoCo 1 and 2 was an application-specific integrated circuit (ASIC) called the "GIME" (Graphics Interrupt Memory Enhancement) chip. The GIME also provided additional features:
128k Tandy Color Computer 3, 26-3334

  • Output to a composite video monitor or analog RGB monitor, in addition to the CoCo 1 and 2's TV output. This did much to improve the clarity of its output.
  • A paged memory management unit which broke up the 6809's 64 kB address space into 8 x 8 kB chunks. Although these chunks were considered to be too large by many programmers, the scheme would later allow third party RAM upgrades of up to 2 MB (256 x 8 kB).
  • Text display with real lowercase at 32, 40, 64, or 80 characters per line and between 16 and 24 lines per screen.
  • Text character attributes, including 8 foreground and 8 background colors, underline, and blink.
  • New graphics resolutions of 160, 256, 320 or 640 pixels wide by 192 to 225 lines.
  • Up to 16 simultaneous colors out of a palette of 64 displayable at one time (unless programming tricks were employed to display more).
Omitted from the GIME were the seldom-used SAM-created Semigraphics 8, 12, and 24 modes. A rumored 256 color mode (detailed in the original Tandy spec for the GIME) has never been found.
Previous versions of the CoCo ROM had been licensed from Microsoft, but Tandy did not contract Microsoft to update the ROM. Instead, Microware provided extensions to Extended Color BASIC to support the new display modes. In order to not violate the spirit of the licensing agreement between Microsoft and Tandy, Microsoft's unmodified BASIC software was loaded in the CoCo 3's ROM. Upon startup, the ROM is copied to RAM and then patched by Microware's code. Although this was a clever way of adding features to BASIC, it was not without some flaws: the patched code had several bugs, and support for many of the new hardware features was incomplete.
Microware also provided a version of the OS-9 Level 2 operating system shortly after launch. This OS featured memory-mapping (so each process had its own memory space up to 64K), windowed display, 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 NitrOS-9, 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 (if the unit has the Hitachi CPU installed).

Typical CoCo 3 system​

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; OS-9 takes advantage of that capability. Some models of CoCo 1 and 2 were also capable of running at this higher speed, but this was not supported or guaranteed.
A popular accessory was a high-resolution joystick adapter designed by CoCo enthusiast Steve Bjork. While it did increase the resolution of the joystick/mouse interface by a factor of ten, it did so at the expense of CPU time. A modified version of this interface was included with a software package by Colorware called CoCo-Max 3, by Dave Stamp. This was a MacPaint work-alike but added support for color graphics. This was a very desirable product for CoCo owners and combined with a MacWrite-like word processor called MAX-10 (also by Dave Stamp and internally named "MaxWrite"), provided some of the functionality of an Apple Macintosh, but with color graphics and at a fraction of the cost.
This again limited the platform's potential as a game console. Early versions of the GIME had DRAM timing issues which caused random freezes. Due to bugs in the GIME some features that were problematic were marked as "reserved" or "do not use" in the programming and service manuals.
The power supply was marginal, and some would overheat if the system memory was expanded to the full 512 kB capacity due to the considerable heat generated by the additional static RAM on the optional daughterboard. Some CoCo 3 owners opted to add a small fan inside the case to keep it cool.