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.
- audio/video-intensive applications
- SLM605 or SLM804 lazer printers
- several ACSI storage devices
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
- 1Mx4bit dynamic RAM chips (16 pieces).
Suitable part numbers (subject to typing errors) include:These should all be 20-pin DIL chips.
- HM514410AT-8
- KM44C1000BV-8
- uPD424400AGS-80
- MB814400A-80
- TC51440AFT-80
- 74ALS244 chip
- 47 Ohm resistor
Assembly Instructions
Onboard memory chips U500 to U515 are currently 256Kx4bit (M5M44256BP-70).
- Remove chips U500 to U515 (20-pin DIL chips).
- Connect pin 5 of all 16 chips' pads to the 47 Ohm resistor.
- Connect the resistor to the 74ALS244 pin 12.
- Connect 74ALS244 pin 8 to MCU pin 34 (MAD9 address line).
- Connect 74ALS244 pins 1, 10, 19 to the ground.
- Connect 74ALS244 pin 20 to +5V.
- Install the new 1Mx4bit memory chips.
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
* Last 2 MB of motherboard unavailable, if an ST RAM card is added (see above)
Motherboard Card = 8 MB - 8 MB 2 MB 8 MB * 8 MB 14 MB * [ 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