Mega Drive 32mbit repro whit sram help!

Hi im trying to make a sonic 3 complete repro for my personal use. i have a problem how should i wire it up so that the save feature works? i have searched alot but i guess im to stupid to figure it out. if some one could help me it would be much appreciated,

regards/tomas

 

looking at this, i need to ad a74ls139 (i will salvage on from a broken arcade board) but the q connection where is that?

 

and the rom connection i guess goes to pin A20 on the 27c322?

 

okay that did not work. anybody got a cad file or something? so i can se that i wired it correctly

 

My guess would be that goes to the ROM's /OE or /CE signal. If you connected that to an address line then it certainly wouldn't work.

 

will try that.

 

okay guess i should have paid more attention in school, but hey whos going to use that old electronics doooh.

any help to rewire these board to work i much appreciated and a small donation will be made. can give a Sonic adventure 2 promo disc? (DC) or maybe a dead or alive 2 promo cib (DC)

Board type 1
https://i.imgur.com/hw0iDTy.jpg


Board type 2
https://i.imgur.com/XDZrizI.jpg

 

If you mapped out the connections on these boards by using a multimeter you could figure out how things are wired which would help you figure out how to rewire or replace things to achieve what you want. Are these boards 16 mbit or less games that map SRAM to the fixed position?

Why not just find a board already set up for greater than 16mbit ROM with SRAM?

 

these are 16mbit boards and the only one i have, here https://www.famicomworld.com/forum/index.php?topic=11748.0 we have my type1 whit 32mbit sram to bad the picture qulity is not superb.

 

I can't tell you exact wiring but the added IC is some kind of latch/flip flop. Then they rewired some things to change decoding between ROM and SRAM so that the flipflop bit is involved when decoding SRAM. Maybe someone else has exact wiring, otherwise you'll need to figure out what is likely going on.

 

got in touch whit the owner of that cart hi will send me some better pictures later this week,

 

It looks like it provides a way of mapping out part of the ROM and putting the RAM in it's place. This makes sense, since the space mapped to the cart on a Megadrive is $000000 to $3fffff - which is a 4MB window. Since a 32Mbit cart will occupy the entire cartridge window, you need to do some sort of mapping to have both 32mbit of RAM and SRAM.

They seem to have chosen the following mapping:

$000000-$1fffff: Always ROM
$200000-$3fffff: Mappable between 2nd half of ROM and RAM

The other carts shown are presumably using smaller ROMs and can simply map the RAM somewhere in the cart space.

 

Right, that seems to be the way these carts work generally. If they aren't over 16 megabit they just map SRAM after ROM. But _PSX_ is wanting an exact wiring solution to convert one of these 16 megabit ROM + SRAM carts into one that uses a register/flipflop to toggle between the second 16 megabits of ROM and SRAM.

There is a thread here with more info on the subject, though it's more about how Sonic 3 works. https://forums.nesdev.com/viewtopic.php?f=23&t=17089

 

Okay i finaly did it!!!!! YES!!!!! you need a 74HC74 and an 74HC08 (or make an and gate whit a resistor and two diodes). if anyone want the schematic (realy bad quality) just send me a PM.

 

I agree. Post it here.

 

Yea post it here.

 

if someone could draw a god schematic of my really crappy one i will post it public here, so please send me a message if you can make a god schematic and i will send you my.

 

I'd just post it here anyway. Someone will redraw it

 

okay i post it hopefully some one will redraw it
the text that is in swedish translates to
lift leg 1 and 4 och IC4
Cut GND on A20 and A19
yellow wire to the hole where pin 1 from ic4 was
redwire connects leg 1 and 4 on ic4
on the backside of the cart if you can se it the redline shows where to cut a trace.

ENJOY if you post it any where give me credits