The early machines: 400 and 800
Atari 400 (1979). Featuring a membrane keyboard and single-width cartridge slot cover.
Internal components of the Atari 800 without the heavy aluminum RF shielding. Although the main board has a card edge connector on the rear, it was inside the aluminum shield and unavailable for use.
Atari 800, internal components: * Plastic guide for installing cards/cartridges * Processor board (was hidden inside the aluminum shield) * OS ROM board * Interior of ROM cartridge * 16 kilobyte memory board * 16 kilobyte memory board * 16 kilobyte memory board * Main system board * External I/O and power supply board
Management identified two sweet spots for the new computers, a low-end version known as Candy, and a higher-end machine known as Colleen (named after two attractive Atari secretaries). The primary difference between the two models was marketing; Atari marketed Colleen as a computer, and Candy as a game machine or hybrid game console. Colleen would include user-accessible expansion slots for RAM and ROM, two 8 KB cartridge slots, RF and monitor output (including two pins for separate luma and chroma) and a full keyboard, while Candy used a plastic "membrane keyboard", non-accessible internal slots for memory, and only RF output for video.
At the time, the Federal Communications Commission (FCC) mandated that signal leakage protection in the television frequency range had to be extremely high. As the Atari machines had TV circuitry inside them, they were subject to this rule and needed to be heavily shielded. Both machines were built around very strong cast aluminum shields forming a partial Faraday cage, with the various components screwed down onto this internal framework. This had the advantage of producing an extremely sturdy computer, although at the disadvantage of added manufacturing expense and complexity. The FCC ruling also made it difficult to have any sizable holes in the case, which eliminated expansion slots or cards that communicated with the outside world via their own connectors. Instead, Atari designed the Serial Input/Output (SIO) computer bus, a daisy-chainable system that allowed multiple devices to connect to the computer through a single shielded connector. What internal slots existed were reserved for ROM and RAM modules.
An overarching goal for the new computer systems was user-friendliness. The Atari computers were designed to minimize handling of bare circuit boards or chips common with upgrades or even initial set up of other systems of that period. The computers were designed with enclosed modules for memory, ROM cartridges, and keyed connectors. The system did not require the user enter commands to boot the system. The OS, large and comprehensive for its time, would boot automatically loading drivers from devices on the serial bus (SIO). The DOS system for managing floppy storage was menu driven. When no software was loaded, rather than leaving the user at a blank screen or machine language monitor, the OS would go to the "Memo Pad" mode allowing the user to type using the built-in full screen editor.
Atari had originally intended to port Microsoft BASIC to the machine, as had most other vendors, intending to supply it on an 8 KB ROM cartridge. However the existing 6502 version from Microsoft was 12 KB, and all of Atari’s attempts to pare it down to 8 KB failed. Eventually they farmed out the work to a local consulting firm, Shepardson Microsystems, who recommended writing their own version from scratch, which was eventually delivered (on external cartridge) as Atari BASIC.
The machines were announced in late 1979 as the 400 and 800, although they were not widely available until November 1979, much closer to the original design date. The names originally referred to the amount of memory, 4 KB RAM in the 400 and 8 KB in the 800. However by the time they were released the prices on RAM had started to fall, so the machines were instead both released with 8 KB. As memory prices continued to fall Atari eventually supplied the 800 fully expanded to 48 KB, using up all the slots. Overheating problems with the memory modules eventually led Atari to remove the module’s casings, leaving them as "bare" boards. Later, the expansion cover was held down with screws instead of the easier to open plastic latches.
The Atari 400, despite its membrane keyboard and single internal ROM cartridge slot, outsold the full keyboard and RAM expandable Atari 800 by a 2-to-1 margin. Because of this, developers were generally unwilling to use the 800-only right cartridge slot.
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