Originally spec'd out in Spring of 1983. The Atari 1090 XL Expansion 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 design and specification went through several revisions.   Originally it was spec'd to have 8 expansion slots, to include an SIO interface card for use with the original 800 and 1200XL computers (Or a 400 if sufficient memory was installed) and would have an external buffering box between it and the computer system called the "Julie FIFO" box - FIFO meaning "First In First Out".

The XL Expansion system appears to have been in a finished and completed form.   For a number of years in the 1990's Best Electronics had a large supply of the 1090XL chassis' and the 64K memory cards which they sold for a low cost.   They did not include the metal case cover however.   These were bought by the company through parts sales from Atari Corp which choose not to produce the product and had redesigned the XL series computers into lower cost XE series computers.

The product design was complete and plugging in an XL Expansion box to an XL computer with a 1064 memory card in the box will increase the XL memory by 64K.   A Z80 CP/M card - called the 1066 is in the Atari Museum, but no software or drivers are available so its not able to be tested.    It is reported that the Z80 CP/M would have worked with the 80 Column card in the box to give the XL computers not only CP/M capability, but also in 80 columns on the screen.   The Atari Museum has a partial assembled 80 column board, their are no schematics available to complete its assembly.   The National Video Game History Museum in Texas has a 1090 Expansion system with almost all of the proposed expansion cards in it.   

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, click click here: EXPANSION CARDS.