Does a Sega Naomi Dual Bios Mod Exist?

[gamemaster14]

I have a Naomi 2 Board along with usa and japanese bios chips. Is there any way to have both of the chips in the board and have some sort of switch?

I have read people saying it was possible but never saw anything solid.

 

[markfrizb]

What size is the EPROM?

 

[channelmanic]

It's a 27C160... you'll need a 27C322 but I don't recall how the pinouts are between the 2.

 

[markfrizb]

First, you'll need to see if the 27c160 is in 16 or 8 mode.
You'll have to check the BYTE pin and see if its high or low on the mother board. If it's high, then the 27c322 could be used as a multi bios by toggling a20 pin high or low with a switch.
If the byte pin is low, then it can still be done but not as easy....

 

[ToplessZ]

If someone finished my triple bios there would be, I had to many issues and then lack of interest

 

[ToplessZ]

first attempt proto type

 

[micksimmonds]

what issues did you run into?

I would love a multi BIOS Naomi, as would allow you to play all games without swapping bios

 

[ToplessZ]

Just lack of knowledge of the 27c160 eprom
I needed a way to shut down the other two, when I thought I had it , it seemed there was still interference with the other chips

 

[markfrizb]

Just lack of knowledge of the 27c160 eprom
I needed a way to shut down the other two, when I thought I had it , it seemed there was still interference with the other chips

Were you pulling the CE lines high of the 2 roms not being used and the CE low of the rom selected?

Did you have the byte pins high or low? This wouldn't matter if using the 3 individual roms. It would matter if using the 27c322.

If this rom is used as a 16 bit rom (probably is but I don't know for certain), then you can use a 27c322. Program the rom with the 2 BIOS's, then A20 pulled either high or low will select the different BIOS's.

3 BIOS's would require a bigger rom or multiple roms. The main issue is that the chip enables are pulled high on the 2 roms not being used and CE pulled low on the rom you select to use.

Also very important is that each rom has a decouple cap at its power pin.

Maybe a 47uF cap for the whole rom board too.

 

[Paladin]

A friend helped me set one up (did most of it actually). I can switch between HOTD2 and the last Japanese bios so I can play all the gun games in one cab. Pictures and description at the bottom of my web page here:
http://home.comcast.net/~dark.paladin/naomi

Somewhere online I read a post that said you can't get a 27c322 to work due to some hardware limitation of the Naomi, which is why the only multiple bios hacks you see utilize multiple EPROMs.

 

[markfrizb]

I can see it being more difficult if the rom is in 8 bit mode. Address line A20 on the 27c322 is the BYTE select pin on a 27c160. So if the 27c160 BYTE pin is low putting the rom in 8 bit mode, then a 27c322 would not work. That's the part I don't know..... If it is in WORD MODE (16bit) or BYTE MODE (8bit). Anybody here know??

Even if the Naomi bios is in 8bit mode, you could use a few 27c257's and a xxx32 ic's to have the 27c322 16bit outputs converted to 8 bit mode. Then toggle the A20 I mentioned earlier.

http://forums.nesdev.com/viewtopic.php?f=12&t=4877&start=45#p124330

Or you could make an adapter with a "tsop40 (am29f032b flash rom) to dip42" that will do the same thing. This way though would require the skills of smt soldering.