Editing Atari 1090 (section)

== Announcement and specification ==

Atari announced the unit on 5 June 1983 as the "ATARI Expansion Box", alongside the CP/M add-on module and the 64K memory module for the 600XL (which became the [[Atari 1064]]). The press release gave it eight expansion slots, two RS-232 ports and a bi-directional parallel bus, and said the devices would be available in the fourth quarter at prices not yet announced.<ref name="press" />

By the time it was called the 1090 XL Expansion System, the specification had changed. The comp.sys.atari.8bit FAQ describes it as plugging into the PBI to provide two RS-232-C serial ports, one Centronics-type parallel output port and five internal 8-bit PBI card slots, and says it was intended to replace the Atari 850 interface.<ref name="faq" /> The Atari Museum gives the same five slots and adds that:<ref name="am" />

* the first specification had eight slots, including an SIO card so that the 800 and 1200XL could use the unit through an external buffer box called the "Julie FIFO";
* a control line was added to the bus so that a bus-mastering card, such as a different processor, could take over the computer; the museum names an 8086 for IBM compatibility or a 65816 as possibilities;
* the XL Expansion System technical specification is dated 11 May 1984.