TRS-80 Color Computer | Code Like It's 198x

Tandy’s Color Computer was built from Motorola’s catalogue. Its processor, the 6809E, its memory controller, the 6883 SAM, and its video chip, the MC6847, were designed to work together, and Motorola published them as a set. Tandy sold the result through its Radio Shack stores from September 1980 for $399, with 4K of RAM and a Microsoft BASIC.

Reviewers were unkind. The first CoCo had a calculator-style keyboard and a 32-by-16 screen that showed lower case as inverse capitals, and it was often compared, unfavourably, with the Atari 800. Owners called it the CoCo, and critics often called it a toy. It outlasted almost every machine it was compared with. Tandy sold three generations over eleven years, and the same chip set, copied almost exactly, became Britain’s Dragon 32.

Fast facts

Unveiled: 31 July 1980; in Radio Shack stores from September 1980.
Launch price: $399 with 4K; $599 with 16K and Extended Color BASIC.
Maker: Tandy Corporation, sold through Radio Shack.
CPU: Motorola 6809E at 0.89 MHz; a 68B09E switchable to 1.78 MHz in the CoCo 3.
Video: MC6847 VDG in the CoCo 1 and 2; Tandy’s own ACVC chip, better known as the GIME, in the CoCo 3.
Generations: CoCo 1 (1980), CoCo 2 (1983), CoCo 3 (1986).
Last sold: 1991.

Model comparison

Model	Year	RAM	CPU clock	Video
Color Computer	1980	4K to 64K	0.89 MHz	MC6847
Color Computer 2	1983	16K to 64K	0.89 MHz	MC6847
Color Computer 3	1986	128K, expandable to 512K	0.89 or 1.78 MHz	ACVC (GIME)

Motorola’s reference design

F. G. Swygert’s 1993 history of the machine, Tandy’s Little Wonder, traces it to work Tandy and Motorola did together in the late 1970s, including “Green Thumb”, an information service for Kentucky farmers that used terminals Radio Shack built with Motorola. Swygert writes that the 6847 video chip was ready by late 1977, and that Motorola’s new SAM chip then replaced the network of chips that had made the first design too expensive. The SAM divides a 14.31818 MHz crystal to produce the 6809E’s 0.89 MHz clock and shares one bank of RAM between the processor and the video chip. Tandy’s technical reference manual explains how. There are no data lines to the SAM, so each of its control bits has two addresses: a write to the even one clears the bit, and a write to the odd one sets it.

Microsoft wrote the BASIC. Color BASIC came in 8K of ROM, and Extended Color BASIC added another 8K with graphics commands such as PMODE, LINE and CIRCLE. Programs came on cassette or on plug-in ROM cartridges, which Radio Shack called Program Paks.

Support from Tandy was thin at first. Early buyers got a manual with only 13 of its planned 24 chapters. Other people filled the gap. In July 1981 Lawrence (Lonnie) Falk printed the first issue of The Rainbow, two photocopied pages. It grew into the main CoCo magazine and was still running when Tandy withdrew the machine ten years later.

The CoCo and the Dragon

In 1982 the Welsh Dragon 32 went on sale with the same processor, SAM and video chip, and a Microsoft BASIC. Inside the Dragon told its readers that much of the book applied to the Tandy machine as well. The two were close, not identical. Their BASICs used different codes for keywords, so a CoCo program saved in the usual way would not run on a Dragon; saved as ASCII text, it usually would. Machine code that called the BASIC ROM needed converting, because the Dragon’s routines were in different places. In 1983 Dragon Data even sold the Dragon 64 in the United States, through Tano Corporation of New Orleans, in direct competition with the CoCo. When Dragon Data went into receivership in 1984, Tandy was among the companies reported to be interested in buying it.

The CoCo 2

The Color Computer 2 went on sale in 1983, with the same chip set. Radio Shack’s 1983 prices started at $239.95 for 16K. Later CoCo 2s were made for Tandy in Korea, and prices kept falling. By December 1986 a 64K CoCo 2 with Extended BASIC cost $99.95.

The CoCo 3

On 30 July 1986, six years and a day after the first CoCo, Tandy unveiled the Color Computer 3 at $219.95 with 128K. It kept the 6809, now a 68B09E that could run at 1.78 MHz, and replaced the SAM and VDG with a single custom chip. For older software it had a compatibility mode: setting bit 7 of its first control register, at $FF90, made it behave as a Color Computer 1 or 2.

Tandy’s service manual calls that chip the ACVC, or Advanced Color Video Chip (part TCC1014). CoCo users knew it as the GIME, “Graphics Interrupt Memory Enhancer”. It does far more than draw the screen:

ACVC can expand memory space up to 512K bytes. Having a built-in MMU (Memory Management Unit), it can support 2 banks of 256K-byte RAM, each with a 9-line address bus, even though the CPU possesses only 16 address lines.

The memory management unit (MMU) extends the 6809’s 16 address lines to 19. Two sets of eight task registers each pick an 8K segment of the larger memory for one eighth of the processor’s 64K address space, so an 8-bit processor can reach half a megabyte. The same chip generates the video, refreshes the RAM and times the whole machine.

The graphics grew accordingly. Tandy’s specification lists 320×192 in 16 colours and 640×192 in 4, with text at 40 or 80 columns in upper and lower case, and the chip can also show 200 or 225 lines. The colours come from 16 palette registers at $FFB0–$FFBF, six bits each, so 16 colours on screen from a choice of 64. On the RGB output the six bits are read as R1 G1 B1 R0 G0 B0; on composite video they are read as two bits of brightness and four of hue. Tandy also announced OS-9 Level II, a version of Microware’s multitasking operating system that could use the extra memory.

One old trick did not survive the new monitor. Many CoCo 1 and 2 games used “artifact” colours, extra colours that a composite television produced from fine patterns of pixels in the two-colour high-resolution mode (the MC6847 entry explains how). Tandy’s new RGB monitor showed them as they really were, in black and white.

Legacy

The CoCo 3 was the last model. Its price fell to $99.95 by December 1990, and the Color Computer’s last appearance in a Tandy catalogue was in 1991. The last Tandy advertisement for it ran in The Rainbow in May 1991. Swygert wrote his history two years later, for owners who still used the machine.

Sources

F. G. Swygert, Tandy’s Little Wonder: The Color Computer 1979-1991 (FARNA Systems, 1993): origins with Motorola and “Green Thumb”, the 6847 and SAM (p. 5); the July 1980 unveiling, September on sale, $399 with 4K, the keyboard, the reviews and the incomplete manual (pp. 5-6); The Rainbow’s first issue; the CoCo 2’s release in 1983; the CoCo 3’s unveiling, price and memory, the name GIME and OS-9 Level II; artifact colours on RGB monitors (p. 67); price lists and the last catalogue and advertisement (pp. 31-32); Alfredo Santos, “Enter the Dragon/64” (1983), on compatibility with the Dragon (p. 33); Tano’s Dragon 64 in the US.
Radio Shack, TRS-80 Color Computer Technical Reference Manual, 1981: the 6809E, the MC6883 SAM and its 14.31818 MHz master clock, the 0.89 MHz processor clock, and the SAM’s paired addresses.
Tandy Corporation, Color Computer 3 service manual: specifications (Section II); the ACVC (TCC1014), its compatibility bit in FF90 and its MMU task registers; system timing and RAM refresh; video modes and the FF99 video resolution register; the palette registers and their RGB and composite encodings.
Tandy Corporation, Color Computer 3 Extended BASIC, 1986: the 64 colours and the HSCREEN modes.
Radio Shack, Getting Started with Extended Color BASIC, 1984: the graphics commands, among them PMODE, LINE and CIRCLE.
Duncan Smeed and Ian Sommerville, Inside the Dragon (Addison-Wesley, 1983), preface: the Dragon and the Color Computer share the same processor and BASIC.
Dragon User, September 1983, pp. 19-21: Tano and the Dragon 64 in the US. August 1984, p. 8: Tandy’s interest in Dragon Data.