This unit was designed to provide the Atari XL series of home computers:
600XL, 800XL (Unreleased 1400XL & 1450XLD) with expansion slots. The
XL Expansion System contained 5 8-bit PBI (Parallel Bus Interface) slots
which could accommodate a variety of expansion options. Also a control
line was added to the PBI to allow a "BUS MASTERING" card such as an alternative
CPU card to take over the Bus and in effect, take over the computer. This
would have allowed for alternative CPU configurations like an Intel 8086
processor for IBM compatibility or the 65816 processor which was the 16-bit
version of the 6502 processor used in all of the Atari 8-bit series computers.
The XL Expansion system appears to have been in a finished and completed form. The case was complete and although they are scarce for one reason or another the top covers were also completed. Several cards were completed or near completion and data on the product was delivered to all Atari dealers. The XL Expansion system would have opened the doors for the Atari 8-bits and allowed them to grow in all new ways. However in July of 1984 Atari, Inc was acquired by Jack Tramiel and like so many innovative Atari projects, the 1090 XL Expansion system made it to the garbage dumpster and not the Atari Retailers shelves.
The Atari 1090 XL had several expansion card specifications laid out for
it, however only a few were ever fully designed and built, here are images
of three of these Atari made EXPANSION