Which bios chip for a Sega CD II?

[wolfchild]

I want a US version of a Sega CD System, but I only have two PAL systems.

Now I`m thinking about to change the bios chip of one PAL system.

I have a US bios file which I found on a webpage.

Would it be possible to burn the bios on a chip and solder it into the Sega CD instead of the PAL bios? Which chip is the right one? And can I write the bios with a normal PC bios flasher?

Hope that it will be possible...

 

[ExCyber]

Would it be possible to burn the bios on a chip and solder it into the Sega CD instead of the PAL bios?

Yes.

Which chip is the right one?

Pretty much any parallel 16-bit EPROM/Flash of sufficient size should work if wired correctly. I'm not sure which kind of EPROM would be the closest match for the pinout of the stock ROM (maybe a 27C4096?). Flash chips tend to have a /WE line mucking up the address pinout.

And can I write the bios with a normal PC bios flasher?

That depends on what you mean by a "normal PC bios flasher". Most PC BIOS chips are 8-bit chips these days, whereas you need something that will handle a 16-bit chip. In theory you could use two 8-bit chips in parallel, but if you need to ask these kinds of questions, you're probably not prepared to figure out the wiring for that...

 

[wolfchild]

Thanks for your help!

I will try it, when I have a little time for that...

 

[computolio]

Anyone know what KIND of 16-bit EPROM is the closest equivalent to the one used in the Sega CD? The 27C200/27C400 and 27C2048/27C4096 have completely different pinouts.

 

[rt9342]

I believe the 27C200 to be equivalent to the 27C2100 (and the 27C400 equivalent to the 27C4100). These EPROMs should be available through some distributors. They are different from the 27C2048 and the 27C4096 because the 272048/274096 are strictly 16-bit, while the 27C2100 and the 27C4100 can be switched between 8-bit and 16-bit (there's a BYTE' line that selects Words [16-bit] if HIGH and Bytes [8-bit] if low - when in 8-bit mode, bits 8-14 on the data bus go to high-impedance and bit 15 allows you to choose the upper or lower byte of the address to be spit out of bits 0-7). If you can't find these chips, Jameco has a 27C160, which has the same pinout, except that it's a 42-pin chip instead of a 40-pin chip, so it has 2 additional pins on the left side (the side with the "U" indention). These pins are extra address pins - you could probably just ground them to make it backwards compatible with the other versions. (Note: some surface mount dual inline versions of the 27C160 are 44-pin, having 2 additional pins added to the left side that are not found on the DIP version - these 2 pins have no connection).

 

[rorirub]

Wouldn't you just get a "error, this is an ntsc system to be used in ntsc regions outside asia and japan" message if you put a usa bios into a pal console?

 

[ExCyber]

You'll get that if the SCD/MCD and Genesis/MD regions don't match one another (and it's not too hard to remove that check anyway).