Author Topic: MEGA CD/SEGA CD SRAM Cartridge: Make your own  (Read 13804 times)

Offline l_oliveira

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MEGA CD/SEGA CD SRAM Cartridge: Make your own
« on: September 23, 2008, 01:31:30 pm »
At my country people who own or owned a SEGA-CD don't even know about the existence of a backup ram cartridge.

Back in 2002 I contacted a few people related to emulator development and I got a enormous help either from Charles McDonald or Steve Snake
(Sorry I don't really remember who sent me the info and the e-mails are long gone) by the form of a register map for the backup ram cartridge.
I've built two units and have been using them since 2002 with the only problem being the fact that I used Sanyo ML2430 MnO2 rechargeable cells (same kind you see on the Dreamcast) and they might discharge if the cartridge is not used for a long period of time. It lasts like three to five months but maybe it could last one year or more on full charge if I used a proper watchdog circuit such as the 8 pin Rohm chip used on SEGA cartridges.

128 kilobytes = 2045 blocks (2048 - 3 for the filesystem tables)

There's the circuit:

Offline Lawrence

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #1 on: September 23, 2008, 02:05:02 pm »
Nice work, very nice work indeed.

I'm gonna front-page this, if that's OK, but I have a quick request: can you upload the images as PNG or GIF?  JPGs look a little crap, for schems.

Offline l_oliveira

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #2 on: September 23, 2008, 03:09:17 pm »
The original pictures are bitmaps.

This zipfile contains the original files as I uploaded to SegaXtreme, back in 2002.


Offline Tiido Priimägi

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #3 on: September 24, 2008, 03:25:35 am »
Once I got bored and thought what would happen if I insert my 256KB SRAM cart into my MD (with !CartIn covered) and to my surprise, I could format it as 4096 block (actual number was smaller.....)... I just had 256KB with the !WE signal connected to !LDSW of 68K, worked nicely... now if that cart has some battery on it...

Looking at your schematic, I'm a bit amazed that my SRAM cart actually worked as SRAM cart for MCD(2)... Thank you for the schematics !!!
Mida sa loed ? Nagunii aru ei saa ;)

Offline l_oliveira

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #4 on: September 24, 2008, 03:49:12 am »
Without the write protection circuitry/ ID byte the RAM will be detected and formatted, but the data in it will be lost if you start a game or reboot the system...

That happens because the MEGA-CD standard has support for expansion RAM (normal volatile memory 16 bit wide) and it happens to be on the same region of the 68000 memory.

The detection attempt of such memory causes the SRAM contents to be corrupted if no protection circuitry exists.

Ah and I'm almost sure that having no ID byte will cause the SRAM to be detected as 8KB (64 blocks).

Offline Tiido Priimägi

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #5 on: September 24, 2008, 04:25:00 am »
I got 4096... I can post a pic. My SRAM cart : http://www.hot.ee/tmeeco/DWNLOADS/SRAMCART.RAR
My cart's purpose was to be able to run my own ROMs on real HW before I built my flashcart ( http://www.hot.ee/tmeeco/EEPROM/INACT2.JPG )
« Last Edit: September 24, 2008, 04:26:33 am by Tiido Priimägi »
Mida sa loed ? Nagunii aru ei saa ;)

Offline Ashen

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #6 on: November 10, 2008, 09:11:39 pm »
http://www.megaupload.com/?d=DTWXAZ06
Schematic diagram, PCB foto (top and bottom side) and datasheet or all IC'c...
It's reversed from Genuine MegaCD SRAM Backup Cart. Second SRAM IC is not installed - only pad's placed on PCB (it shown on a foto).
« Last Edit: November 10, 2008, 09:37:54 pm by Ashen »
Perfect F.O.E.

Offline orgasmotron

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #7 on: February 09, 2010, 03:15:47 am »
hello
i'm a bit confused about picture number03
621000 sram
A0 - A16 goes where (D1 -  D15) there is more pin (17 pin left on the sram)

Offline l_oliveira

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #8 on: February 09, 2010, 07:59:43 am »
They go to A0-A16 on 68000 BUS. (for 16 bits CPU A16 means 256 KB, not 128 as usual but we're connecting only half of data bus so 128kb here)
Also the cart connect only 8 bits to the data bus. D8-D15 are left unconnected on cartridge slot.

Offline orgasmotron

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #9 on: February 09, 2010, 03:59:02 pm »
can you confirm if i have the good pinout
i took all the pinout from the catridge slot using gamesx cart slot pinout info.








Offline juanmiglesias

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #10 on: August 22, 2017, 04:26:15 am »
HI there, long time, are the schematics and board still available somewhere else? I can host them if you still have them.
thanks.

Offline khouji

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Re: MEGA CD/SEGA CD SRAM Cartridge: Make your own
« Reply #11 on: September 19, 2017, 12:55:43 pm »
HI there, long time, are the schematics and board still available somewhere else? I can host them if you still have them.
thanks.

What do you need the schematics for? I have some of it.