The following document was submitted by Jim DeClercq.

Installing 8 MB of ST RAM on the Motherboard

Usually, TT RAM is considered faster and more versatile. However, there are cases where more ST RAM is desirable: These situations all require buffering through ST RAM, since DMA transfers are explicitely banned from TT RAM.

While Atari once produced an 8 MB expansion board, those have always been expansive and hard-to-find. Fortunately, there is another solution: upgrading the motherboard to 8 MB of ST RAM.

It involves replacing the RAM chips on the motherboard and adding a few discrete components.

Parts required

Assembly Instructions

  1. Remove chips U500 to U515 (20-pin DIL chips).
  2. Connect pin 5 of all 16 chips' pads to the 47 Ohm resistor.
  3. Connect the resistor to the 74ALS244 pin 12.
  4. Connect 74ALS244 pin 8 to MCU pin 34 (MAD9 address line).
  5. Connect 74ALS244 pins 1, 10, 19 to the ground.
  6. Connect 74ALS244 pin 20 to +5V.
  7. Install the new 1Mx4bit memory chips.
Onboard memory chips U500 to U515 are currently 256Kx4bit (M5M44256BP-70).
Pin 5 is not used on those, but it's the A9 address line on 1Mx4bit chips.

New Adresses

This should give 8 MB of onboard ST RAM in the same address range as an 8 MB card would normaly use:

Address Function
Motherboard only 000000 - 7FFFFF 8 MB Motherboard

If the upgrade is augmented with an ST RAM card, then the address changes to

Address Function
with ST RAM card 800000 - FFFFFF 8 MB Motherboard

...which overlaps several system addresses:

Address Function
Address Conflicts E00000 - EFFFFF TOS ROM
FA0000 - FBFFFF Cartridge
FF8000 - FFFC06 I/O

Therefore, the last 2 MB of the onboard ST RAM will be "lost".

Memory Configurations

Standard

Motherboard Card =
2 MB - 2 MB
2 MB 4 MB
8 MB 10 MB

Modified

Motherboard Card =
8 MB - 8 MB
2 MB 8 MB *
8 MB 14 MB *
* Last 2 MB of motherboard unavailable, if an ST RAM card is added (see above)

[ Webmaster's note: this modification was prepared from circuit schematics. We do not know of anyone who actually performed it, but it should work. If you complete this upgrade sucessfully, please let me know. ]


../ST RAM
../Memory