3MB & 4MB PCBs (or easier to rewire smaller?)

[bender3455]

Hello all! I'm sitting at my desk, hard at work (otherwise known as playing), making some great repros for my collection. I'm now at the point of attempting a 32x game (Blackthorne), and have compared the pin outs of larger Genesis games with that of a few 32x games that I have lying around. Here is a 32x game (Star Wars Arcade, for the curious):



And here is a 3MB genesis cart:



The Genesis cart has a chip between the two ROMs that appears to be for bank switching between the two chips. The 32x board does not have this, which makes me believe it possibly does internal bank switching? Has anyone soldered dual type boards to make a working repro? Or, should I just use single chip boards and rewire A18-A20?

Oh, and the 32x reproduction is purely experimental at this point. If it uses internal bank switching, I'll have to figure out how that works, otherwise, the pin outs appear the same as standard Genesis.


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[Jazzmarazz]

The narrow chip between the two mask ROMs is the SRAM module. With games as small as 3MBytes, the individual chips are enabled and disabled with the higher address bits and the /LBW (low enabled, lower byte write) pin for the SRAM.

The 32x cart is actually using the 74hc139 as the bank switching device.

You can locate datasheets on most unknown devices by googling the numbers printed on them.
62256

[bender3455]

Thanks Jazz! I had a bit of a brain fart on the Genesis cart, and wasn't paying attention to the fact that there was a battery (which means SRAM present), and on the 32x board, I was thinking the 74hc139 chip was the encoder chip with no bank switching. I'll look up the data sheet and see if I can't come up with a workable solution! Results to be posted!


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