16MBit Snes repro help (and an additional question:-) )

[HexAnimus]

I've done a pretty exhaustive search and still can't seem to find the answer to my question.
I'm looking to do a snes translated reproduction of Shin Megami Tensei 1. I've already figured out to be what I believe is a suitable donor cart. Since SMT1 is a 16MBit ROM w/64 KBit SRAM loROM slowROM cart I've selected my cart and I'm almost ready to start. However, my hang up is the wiring. If this were a 32MBit cart I'd have no trouble since Seiken Densetsu 3 and a few others would follow the 32MBit wiring and need a 2to4 Line Address Decoder.

However, as I've not yet opened my Super Bases Loaded 3 cart (waiting on gamebits to be delivered), I'm not sure if the PCB has two 8 MBit mask roms or one 16 MBit mask. Is it better use a 27c160 eprom or two 27c080 eproms? If it is better to use the two 27c080s, how would you go about wiring it? Not sure if you use the Line Address Decoder and then only hook up 2 eproms to it (I've seen the 74ls139 used in repro carts that have either three or four 27c080s and they just leave the 4th slot blank on a 3 eprom setup).

Lastly, when making a 32 MBit cart is there any technical reason, outside of cost, that a 27C320 erpom can't be used in place of four 27c080 eproms? (please no recommendations for using a FlashCART or anything like that. I'm interested in the technical side of this to learn something new and have a cool project for myself)

[jeffro11]

The SNES is expecting 8bits of data. The 080 is the largest 8-bit chip available. (In DIL package)

A 74ls139 is a TWO to FOUR line decoder... You've already answered your question in your previous post.

If there is 1 ROM in your cart you'll have to use the decoder. If there is two, you wont have to.

Edit: Actually I lied. You can use the 2nd enable on the MAD-1 instead of the 74ls139 (if the cart has one, some do not)

[HexAnimus]

The SNES is expecting 8bits of data. The 080 is the largest 8-bit chip available. (In DIL package)

A 74ls139 is a TWO to FOUR line decoder... You've already answered your question in your previous post.

If there is 1 ROM in your cart you'll have to use the decoder. If there is two, you wont have to.

Edit: Actually I lied. You can use the 2nd enable on the MAD-1 instead of the 74ls139 (if the cart has one, some do not)

Ok... So if I'm understanding the 2to4 line decoder is being used to MUX? (interlace? not sure what the right term is in this application) the 4 chips down to two inputs for the MAD-1 and mask rom? And will work with 3 eprom chips. I've also seen a 3to8 used for 6 27c080s being used. It might just be that I need to read more and learn more about the function of the decoder and why its being used. I've yet to find any definitive guides for this kind of modification like there are for NES repros. Just a bunch of schematics and information haphazardly strewn about the 'net that I'm attempting to make sense of.

Regardless, it doesn't address my original question on how the two 080's would be WIRED in a 16MBit cart without the decoder or, if the decoder were needed, would it even work in a two 080 configuration.

Edit: Hopefully, I'm starting to understand this a little better. Seems like the 2to4 is being wired in reverse to make it an encoder for use with the MAD-1 chip on the snes cart PCB that is Hi-ROM enabled. In which case if there is no MAD-1 then its lorom only?

[jeffro11]

The decoder is used the same way with 3 or 4 chips as it used in a 2 chip configuration. You just wont be using all the inputs on it... Read the datasheet for more information.

[vlame]

is there a link to what i would need to do this myself?

or how to flash an SFC game with the American version?

[HexAnimus]

The decoder is used the same way with 3 or 4 chips as it used in a 2 chip configuration. You just wont be using all the inputs on it... Read the datasheet for more information.

Cool! That is exactly what I needed to know. I'll read up more on the Line decoders in general. Just didn't know that you didn't have to use all the inputs on them. I'll check out the spec sheet on them as well. Though as chip electronics isn't my forte I'm dubious that it will be of much use. Hence, my previous "simple" question. Thanks for answering though. Now I just have to wait for some tools and chips to get shipped and I can give this a go!

is there a link to what i would need to do this myself?
or how to flash an SFC game with the American version?

vlame,
I would recommend searching this forum, digitalpress, nintendoage, nesdev (under their snes dev subsection) and for snes repro. Also check out these sites:
http://snesdev.romhack.de/
http://caitsith2.com/index.htm
http://nintendoallstars.w.interia.pl/romlab/sneslab.htm

Not everything made sense to me at first and I'm sure I still don't understand all of it. But there is a wealth of information there. Just one note. When looking at the snesdev.romhack site. Do NOT automatically assume that the carts he lists are the only that can be used as donor carts. I prefer to use the older sports games that have the correct specs that are needed for the game in question. Can't remember where, but I found a spreadsheet that lists all the attributes of the snes games that can be used to identify easily acquired donor carts. At least that is what I started with. Lots of reading! Practice with soldering can help too if it isn't a skill you've developed just yet!

[jeffro11]

His wiring diagrams are also wrong/inefficient. But hey....

[HexAnimus]

His wiring diagrams are also wrong/inefficient. But hey....

Well he asked a question and I responded with the best knowledge that I had available. I asked about wiring, which is what I was having problems with. Heck, you could even mention which of the sites have bogus diagrams! If you have better wiring diagrams or know where they can be located I would love to see them so I don't in turn pass on bogus info. No need to be rude about it... ::shrug::

[jeffro11]

Not being rude. Just the truth. The best wiring diagram is the one you make yourself. Just go read the datasheets for the decoders, and the eeproms and you'll see that the snesdev diagrams are somewhat wrong (connecting pins that are marked as NC in the datasheets for example). That is seriously the best way to learn. Anyone can copy a diagram and see where to connect things. But wouldn't you rather know WHY your doing it?

[vlame]

a spreadsheet that lists all the attributes of the snes games that can be used to identify easily acquired donor carts

Nintendo Age Article