In 1977, the University of Kentucky College of Agriculture and the Kentucky Cooperative Extension Service launched an experimental videotex project named Green Thumb. The initiative aimed to build an inexpensive network terminal that could connect to a standard telephone line and an ordinary color television, delivering real-time weather forecasts and market updates directly to farm living rooms. Motorola Semiconductor in Austin, Texas, won the contract to design the logic board, while Tandy Corporation's Computer Division signed on to manufacture the hardware.
Silicon Integration at Motorola
Motorola engineers built an initial prototype terminal in 1978 around two core components: the new Motorola 6809 microprocessor and the MC6847 Video Display Generator chip. The prototype successfully retrieved remote data and displayed color graphics on consumer television sets. However, the circuit board contained too many discrete support chips to be commercially viable, pushing projected manufacturing costs beyond what agricultural users could afford.
To solve this cost bottleneck, Motorola consolidated the discrete logic chips into a custom integrated circuit: the MC6883 Synchronous Address Multiplexer, or SAM. The SAM chip took over dynamic RAM control, clock synchronization, and memory bus multiplexing, replacing dozens of smaller components. Paired with the 6809 CPU and the MC6847 display generator, this unified three-chip architecture became the core engine of the commercial AgVision terminal.
From AgVision to VideoTex
The simplified chip set allowed Tandy to manufacture the terminal efficiently, but the market for specialized farming receivers remained limited. Seeing potential in broader information retrieval networks, Tandy repurposed the AgVision hardware into a general-purpose terminal branded as the VideoTex. Sold through Radio Shack stores around 1980, the VideoTex retained the original molded housing, internal dial-up modem, and display interface.
Repurposing the Shell into a Microcomputer
Tandy engineers realized that minor modifications could convert the dedicated VideoTex terminal into a fully programmable home computer. They removed the internal modem board and added hardware interface ports for cassette tape storage, serial communication, and two joysticks. They also cut an expansion slot into the right side of the plastic case to accept ROM program cartridges and peripheral controllers.
Removing the modem left an unused hole in the top casing where the red modem activity light had originally sat. Assembly lines covered the opening with a decorative sticker specifying the computer's memory configuration. On July 31, 1980, Tandy announced the TRS-80 Color Computer. Priced at $399, the base entry system shipped with 4 kilobytes of dynamic RAM and 8 kilobytes of Microsoft BASIC stored in read-only memory.
Extending the Lifespan of a Niche Architecture
By modifying a design originally commissioned to deliver farm data, Tandy transformed a specialized videotex terminal into a flexible platform known as the CoCo. The underlying Motorola chip architecture proved adaptable enough to support multiple generations of hardware upgrades, keeping the Color Computer line in active production through 1991.
AI-generated editorial illustration; not a documentary photograph.
Source: TRS-80 Color Computer
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