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  • Joined: 11 Jul 2009
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SMS II Bios Replacement
Post Posted: Sun Nov 03, 2019 12:31 pm
Hi all, got a bit of an odd one here. I have a PAL SMS II with Alex Kidd bios. I have removed the bios and fitted a socket. Socket is confirmed working with original Alex Kidd Mask rom. I am looking to replace the bios with an eprom containing the Japanese bios.

I am using the GQ-4X4 programmer.

I have the following eproms available

3x Hitachi HN27C64G
2x ST M27C128
2x ST M27C512

All are erased with UV Light and Verified blank with GQ software.

I have programmed all 7 chips with the Japanese bios but they all produce a black screen when inserted into the console.

On the HN27C64G the file size was 8KB so filled the rom

On the M27C128 the file size was doubled up with hex editor so is 16KB filling the rom.

On the M27C512 i used the hex editor to insert the file a few more times resulting in a 64KB again filling the rom.

All files were verified working with Kega Fusion.

As they are 8bit machines i have not byteswapped the bios images.

All chips result in black screen.

Reading the chips back in the software and saving the output verifies fine and works fine in Kega.

Here's the kicker though, i have a Japanese bios in a model 1 i fitted a long time ago on one of the same M27C512 which i've pulled and tried in the SMS II and it works fine.

I then tried reading that chip and duplicating it to one of the other 512's but again black screen.

I can't imagine i have 7 faulty eproms although i suppose its possible.

Anyone have any ideas?
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  • Joined: 31 Oct 2007
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  • Location: Estonia, Rapla city
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Post Posted: Sun Nov 03, 2019 1:23 pm
The original mask ROM has a mapper in it and because of it the pinout does not match that of an EPROM, some additional effort is needed to make things work.
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  • Joined: 11 Jul 2009
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Post Posted: Sun Nov 03, 2019 1:59 pm
Thanks for your reply

I have a 27C512 with the Japanese bios in the system at the moment from a different system and it boots and plays the Japanese Space Harrier themed bios as it should.

I have now erased the 2x 27C512 and cloned the working chip and that now boots (no idea why it didn't the first time)

So does that mean i can use a 512 but not a 64/128/256?

what work is required to use the other chips?
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  • Joined: 14 Aug 2000
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SMS II Bios Replacement
Post Posted: Sun Nov 03, 2019 2:32 pm
For the Hitachi HN27C64G and ST M27C128 pin 27 is /PGM and on the BIOS socket this is connected to A14 so that may be putting the EPROM into programming mode instead of read mode.

For the M27C512 - which you got working the second time round - pin 1 is A15 on the EPROM and /WR on the socket so that looks to work fine if the image is burnt 4 times on the EPROM which you have done.
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Post Posted: Sun Nov 03, 2019 3:04 pm
What you said about the 128 made sense so i tried it again leaving pin 27 out of the socket, still get a blank screen, i then tried grounding it but again blank screen.

Anyway the 512 is working so that's fine with me.

Now i've gotta figure out why a seperate system is in a boot loop. It boots to the Sega logo but never loads the built in game or a cartridge
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SMS II Bios Replacement
Post Posted: Mon Nov 04, 2019 1:43 am
The /PGM pin for these EPROMs is active low so you probably have to tie it high not leave it floating or grounded.
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Post Posted: Mon Nov 04, 2019 4:40 am
It depends on a particular EPROM, some require it to be tied to 5V, some don't care when VPP is not applied (which won't happen outside programming equipment). Datasheet will tell exactly what you need to do with these pins.
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SMS II Bios Replacement
Post Posted: Wed Nov 06, 2019 11:59 am
You are correct as usual TmEE.

I checked my SMS. I have A14 still connected to pin 27 on the BIOS socket. NM27C128Q EPROMs will happily work like this. TI TMS27C128 EPROMs on the other hand won't.
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