DIY Sega CD Saveram cart? Is there any way?

Is there any possible way to modify an existing genesis cart, or otherwise, to make your own saveram cart?

I never had the good fortune to get one and look inside, so I'm not entirely sure how complex the innards are...if it's just some type of flash RAM chips wired to the approriate pins, or if there's some kind off eprom controller onboard, or worse.....

Just a thought, feel free to flame me for my Sega CD technological illiteracy.
 

ExCyber

Staff member
Is there any possible way to modify an existing genesis cart, or otherwise, to make your own saveram cart?

It's been done.

I never had the good fortune to get one and look inside, so I'm not entirely sure how complex the innards are...if it's just some type of flash RAM chips wired to the approriate pins, or if there's some kind off eprom controller onboard, or worse.....

A famous spaceship told me that it can be done with just SRAM (and a battery, of course... I assume FRAM would work too if you can get it) and standard 7400 series logic chips.
 
Hrrmm.... Really? It's been done? FASCINATING!

If you don't mind my bothering you, can you tell me, are there any instructions written up anywhere, or at least a basic guideline how to go about making one? If not, I might have to hunt down the Spaceship in question, and confess my love. :
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I've got plenty of old genesis carts lying about, (with and without internal saves) just asking to be modified, and I'm sure I've got enough experience to construct it, if I can just find out how to do it...

In any case, thanks for replying!
 

ExCyber

Staff member
Really? It's been done?

It appears that way.

If you don't mind my bothering you, can you tell me, are there any instructions written up anywhere

Not really, as far as I know.

or at least a basic guideline how to go about making one?

Here's what was revealed to me, in full (this should be enough to get you started if you've got a good understanding of computer engineering and digital logic, but it'd still be some work):

The map for the ramcart is something like this:

0x400000-0x4FFFFF - memory size register (read only)

0x600000-0x6FFFFF - SRAM (mirrored)

0x7FFFF0-0x7FFFFF - Write register (write 1 to make SRAM writeable)

To decode that I used a 74LS138 plus the partial decoding for higher lines done by the megadrive hardware (cart chip select signal, as you mentioned).

To decode 0x7FFFF0-F I used a pair of 8 input NAND (74LS30) ports "ORED" with a 74LS32, which is also used along a JK flip-flop (74LS74) to control the write protection register. This register is implemented so the bit gets set (no write) after the 68k get a reset pulse. I've been using this cart for a while now and it works fine ...

And the most curious thing about it is the fact it is a 128k SRAM cart, so it has doubled capacity over the original cart sold by sega. SCD bios support carts with size up to 512kb. It's just a matter of putting the correct ID in the ID register.

0/nothing=8k; 1=16k; 2=32k; 3=64k; 4=128k; 5=256k; 6=512k

7=error (_No Format is put on the screen). I believe that there's also a value for a 1m ram cart, but for being used as work ram for expansion (DRAM memory), but I didn't got much biased on testing this much, as a such type of expansion is near the useless as no game were ever made for using a such type of work ram expansion...
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It's up to him to decide whether or not to elaborate further...
 
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