With this circuit, your Atari 800 XL computer will have 512 KB of additional RAM that is Rambo/Compy compatible. Thanks to the additional RAM, you will have the opportunity to use new generation games and demos that require extra RAM on your Atari 800 XL computer.

The original design belongs to Matthias Reichl (https://www.horus.com/~hias/atari/), I thank him for this wonderful project. I redrew the original design so that it could be mounted on the Atari 800 XL. The circuit consists of two separate parts: RAM card and MMU connection card. During PCB production, the two will be produced combined. You will break the MMU card from the RAM card and use it.

Assembly:

Disassemble SALLY, disassemble PIA, disassemble POKEY, Disassemble the MMU chip. Replace the RAM card in the old location of SALLY and PIA. Replace the MMU card and connect the RAM card and MMU card with a dupont cable. (I added the assembly pictures.)

NOTE: Unfortunately, since the keyboard cover does not close, it is necessary to solder the POKEY chip and 512 KB RAM card to the ATARI PCB without a socket. If you want, you can make the MMU card socketed.

512 KB RAM Card Jumper Modes:

J1   J2   Mode

Open  Open  ext RAM Off

Open  Closed 256 KB ANTIC access

Closed Open  256 KB 

Closed Closed 512 KB