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Hey this might seem like a weird request, but I was wondering if anyone else has ever modded a SMS II (preferrably the US version) to have a different BIOS? Reason I ask this is because I would like to put 1.3 on my system, as I like the SE-GA intro better than the plain logo of the Model II's default BIOS. That, and I grew up playing Snail Maze.

I've seen a few references on SMSpower to changing the BIOS on these, but it was mostly to change the built in game to Sonic or Alex Kidd, I couldn't find anyone putting a lower revision BIOS on one.

Anyway, I hope someone can help.

Makes sense.

From memory, the SMS II BIOS has the same pinout as the PROM in a 128KB cartridge with no mapper chip. Unless it has Sonic build in, then it's a 2mbit PROM and has the same pinout as a 256KB cartridge.

I think a EPROM with 1.3 BIOS burnt on it should drop straight in to a non-Sonic SMS.

The pinouts can be found in this forum post:
https://www.smspower.org/forums/18536-NeedHelpForW27c512EEPROM32kbGamesAnd28PinCartridgeNoMemoryMapper

asynchronous wrote

Makes sense.

From memory, the SMS II BIOS has the same pinout as the PROM in a 128KB cartridge with no mapper chip. Unless it has Sonic build in, then it's a 2mbit PROM and has the same pinout as a 256KB cartridge.

I think a EPROM with 1.3 BIOS burnt on it should drop straight in to a non-Sonic SMS.

The pinouts can be found in this forum post:
https://www.smspower.org/forums/18536-NeedHelpForW27c512EEPROM32kbGamesAnd28PinCartridgeNoMemoryMapper

Thanks this is helpful - is there a source for a 1.3 bios eeprom? There used to be an option for this on console5, but I don't see it anymore.

I'm totally unfamiliar with this, but I wonder if there will be an error because Bios 1.3 first checks the card slot and then cartridge slot.

No, all the BIOSes do that, even on SMS2. The I/O chip supports it, but the result is a disconnected bus so it acts the same as a slot with nothing inserted.

Author	Message
Perfect! Joined: 01 Mar 2020 Posts: 18	'Downgrading' US NTSC SMS II to BIOS 1.3? Posted: Wed Dec 13, 2023 5:34 pm
Perfect! Joined: 01 Mar 2020 Posts: 18	Hey this might seem like a weird request, but I was wondering if anyone else has ever modded a SMS II (preferrably the US version) to have a different BIOS? Reason I ask this is because I would like to put 1.3 on my system, as I like the SE-GA intro better than the plain logo of the Model II's default BIOS. That, and I grew up playing Snail Maze. I've seen a few references on SMSpower to changing the BIOS on these, but it was mostly to change the built in game to Sonic or Alex Kidd, I couldn't find anyone putting a lower revision BIOS on one. Anyway, I hope someone can help.

asynchronous Joined: 14 Aug 2000 Posts: 742 Location: Adelaide, Australia	'Downgrading' US NTSC SMS II to BIOS 1.3? Posted: Thu Dec 14, 2023 3:29 am
	Makes sense. From memory, the SMS II BIOS has the same pinout as the PROM in a 128KB cartridge with no mapper chip. Unless it has Sonic build in, then it's a 2mbit PROM and has the same pinout as a 256KB cartridge. I think a EPROM with 1.3 BIOS burnt on it should drop straight in to a non-Sonic SMS. The pinouts can be found in this forum post: https://www.smspower.org/forums/18536-NeedHelpForW27c512EEPROM32kbGamesAnd28PinCartridgeNoMemoryMapper

Perfect! Joined: 01 Mar 2020 Posts: 18	Posted: Thu Dec 14, 2023 5:18 pm
Perfect! Joined: 01 Mar 2020 Posts: 18	asynchronous wrote Makes sense. From memory, the SMS II BIOS has the same pinout as the PROM in a 128KB cartridge with no mapper chip. Unless it has Sonic build in, then it's a 2mbit PROM and has the same pinout as a 256KB cartridge. I think a EPROM with 1.3 BIOS burnt on it should drop straight in to a non-Sonic SMS. The pinouts can be found in this forum post: https://www.smspower.org/forums/18536-NeedHelpForW27c512EEPROM32kbGamesAnd28PinCartridgeNoMemoryMapper Thanks this is helpful - is there a source for a 1.3 bios eeprom? There used to be an option for this on console5, but I don't see it anymore.

gallos_11 Joined: 07 Jun 2010 Posts: 195	Posted: Fri Dec 15, 2023 6:17 pm
gallos_11 Joined: 07 Jun 2010 Posts: 195	I'm totally unfamiliar with this, but I wonder if there will be an error because Bios 1.3 first checks the card slot and then cartridge slot.

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14745 Location: London	Posted: Sat Dec 16, 2023 12:02 am
	No, all the BIOSes do that, even on SMS2. The I/O chip supports it, but the result is a disconnected bus so it acts the same as a slot with nothing inserted.

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