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Official Backup Memory Cart

Variations

The Sega Saturn Official Backup Memory cart uses an AT29C040 512K x 8bit IC. Some carts used the 32-pin variant of the AT29C040 while others used a 40-pin variant. The AT29C040 is the same underlying silicon in both cases, the 40-pin variant simply has 2 NC pins in either direction as can be seen in the 32-pin photo.

 40-pin AT29C040 Version

 32-pin AT29C040 Version
 
 

Memory Mapping

A 74HC00 is used to perform simple address decoding on the Sega Saturn Memory Cart.
  • When A23(A32) = '1' AND #ACS1(B48) = '0', the 74HC244 is selected and driving the databus, it does not use the #ARD(B47) signal
  • When A23(A32) = '0' AND #ACS1(B48) = '0', the AT29C040 is selected, read or write is controlled by #ARD(B47) and #AWR0(A49)

Bus Width

The databus on the Sega Saturn's cartridge connector is 16-bits. This means that the A0(B45) pin on the cartridge connector is mostly useless. Because of this A1 of the cartridge connector is connected to A0 of the AT29C040 - do not be confused by this is you try to line up the above photos with the Sega Saturn Schematics.

Size Register

The presence of the 74HC244 on this board, and it's wiring, makes me believe that this is a size register (just like the Sega CD Backup RAM Cart). The 74HC244 is wired to output 0x01 on the databus when it is selected. A great experiment would be to see how the Sega Saturn behaves when different values are used on the 74HC244. This would be quite simple on the 40-pin version (the one I happen to own) since the bits of the 74HC244 are all connected to giant PCB jumpers.

Boot Block Data

The Sega Saturn expects certain data to be present in the AT29C040, you can't simply install a new chip and hope everything will work out. The string "BackUpRam Format" is present in the first 256 bytes. Without this, the Saturn will recognize the Memory Cart but will refuse to format it.

Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

00000000 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000010 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000020 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000030 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000040 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000050 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000060 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000070 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000080 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000090 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000A0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000B0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000C0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000D0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000E0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
000000F0 42 61 63 6B 55 70 52 61 6D 20 46 6F 72 6D 61 74  BackUpRam Format
00000100 00 00 00 00 00 00 00 00 00 00 04 00 00 00 00 00  

Credit to RDC for 32-pin board scan and Boot Block data.