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New Hardware: Atari 400 RAM-Card 48/52 KB & external OS-ROM


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some weeks ago another ABBUC member ask me for a way to update his Atari 400 to 48K main memory. Until the end of 2016 mostly I prefer the usage of the genuine Atari 400 48K Factory Upgrade Kit - Atari part number CO61553. BEST Electronics offer them until somewhere up to the end of 2015 for approx. 25 USD each w/o any additional parts or manual. In 2016 the price rises up over 35 USD and now it´s sold out.


Due to this impossible mission to get more of the CO61553, I create my own solution - even better than the original ;)


Here´s the Atari 400 48/52 KB RAM-Card with "external" OS-ROM support using standard EPROM sizes:





This card is a full replacement for the 16 KB built-in RAM card. At least only two wires must be soldered at the bottom side of the Atari 400 mainboard. My RAM-card uses the same pinout like the CO61553 by Atari, but only 2 of the 4 wires suggested by Atari are really needed. But it works also, if you have already equipped an Atari 400 wit the CO61553, you can easily change to my RAM card, if you want.


The 74LS243 chip has to be removed, it´s not needed any longer. After soldering the 2 (or 4) wires, just plug in the DIL connector into the socket were the 74LS243 was before, the other end has to plugged on the new RAM card. That´s all.


The new RAM card uses SRAMs instead of DRAM and provides 48 KByte or 52 KByte main memory. When enabling the 52K RAM mode, the address range of $C000-$CFFF is also RAM. Please remind, that this area isn´t usable under BASIC (or, to be more precise, I didn´t know about that). A jumperfield on the PCB let´s you choose which size is active.


The second new feature is the possibility of using a standard 27128 or 27256 EPROM for the operating system (and math-pack). When the "external" OS-ROM mode is enabled, the three ROMs on the Atari 400 mainboard won´t be used anymore. You can remove them or leave it in their sockets. The EPROM´s first 6 KB are unused, then 2 KB math-pack will be followed by the 8 KB OS-ROM code.


A third jumperfield switches the highest address line A14 of the EPROM, so you can hold two different operating-system sets in one 27256 EPROM.


What you get?


- PCB "Atari 400 48K/52K RAM Card" with ENIG (Gold contacts) surface levelling. Picture shows only standard HASL, but I offer only the better quality with ENIG.

- Ready-for-use crimped ribbon cable for the 74LS243 socket to connect with the RAM card

- some wire for the 2 or 4 solder connections at the Atari 400 mainboard´s bottom

- Installation instructions with pictures in english, sent by email




- from 20 up to 25 euros each depending on the amount of orders

- Shipping costs to any place in the world for up to 5 pieces is 9 euros




- Please show your interest and how many you want to order here in this thread

- I will collect inquires until 2017 July 2nd (end of date)

- If 10 or more orders will be placed, everyone who´s interested get the payment details

- Shipping out at the end of July 2017


Best regards, Jurgen





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Are you saying I can put any OS on the EPROM? Can I put the XL OS on the 27128? If yes, can I select between two OS's if I use a 27256? Was there a reason it couldn't go to 64K with the right OS?

Edited by ACML
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Are you saying I can put any OS on the EPROM? Can I put the XL OS on the 27128? If yes, can I select between two OS's if I use a 27256? Was there a reason it couldn't go to 64K with the right OS?


No, any XL operating system won´t work without heavy patching. Main reason: The XL/XE OS assumes that PORTB is not used for joystick 3+4 input, it´s used for memory management. First no-go reason is the hardware test routine after power-up, which enables the Selftest part of the O.S. ROM ($5000-$57FF), jumps at a specific address and doing some memory checks. Without having memory cascade functions controlled by PORTB the OS will simply deadlock.


This is the same reason why the RAM "under the OS" ($D800-$FFFF) isn´t usable without heavy mods in an Atari 400. At XL/XE systems bit 0 of the PORTB is used to switch between ROM or RAM in the area of $C000-$FFFF (except H/W area, $D000-$D7FF), without these functionality using the RAM under the OS won´t work.


Yes, if using a 27256 type EPROM, you can put two OS on it. The jumper close to the left side of the EPROM is for selecting.


You can use any "Oldrunner" OS or special patched versions like the Omnimon. When the RAM-mode of this new card is set to 52K, only 10 KB (2K math-pack and 8K OS) are mapped between $D800-$FFFF, the area $C000-CFFF is RAM. Using the standard-mode of 48K enables 14K ROM feature, then $C000-$CFFF is also ROM. Maybe some mods of the "personality card" (from Atari 800 system) may work then also in an Atari 400. I didn´t have such ROMs, so it´s not an "guaranteed feature", but technically it will work :)



Edited by tf_hh
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