Understanding a 139 decoder to wire up 2 roms.

[darknezz19]

So I'm playing around with this 139 decoder and a couple 27c322's to try and make an 8MB Genesis cart but so far have not got it to work. Some questions have come to mind about how this should be wired up. This is going off a setup for a 6MB Snes game as a reference which has the address input line connected to A19 on each of the two flash chips. Then it uses the OE lines, one output to each chips line, to control which rom to enable. Pin 1 on the 139, which I believe just enables that side of the chips decoder outputs, is tied to mad-1 pin which is OE. Couldn't this pin just be tied to any 5v source though? Also the decoder output are setup so the first output goes to the second rom, and the second output goes to rom 1, why is that? I tried swapping these just to see if it would work the other way but it wouldn't.

Furthermore after looking at a Genesis pcb with 2 rom chips that uses a 139 to decode it's using the CE lines on the outputs instead of the OE lines. I'm just going off of pictures from romlab so it's hard to tell exactly how it's setup though. I have tried using both the OE, and then the CE for outputs to the 27c322s but it's hasn't made a difference. For the address input I've tried A19, A18, A20, both A20 and A19 together, A21, A22, A23, A24 and AS. Gave it my best shot, any advice?

Pictures for reference:
imgur.com/a/98uL5#5

Data Sheets:
www.nxp.com/documents/data_sheet/74HC_HCT139_CNV.pdf

[Deleted]

I'm not entirely sure you can use 2 4 MB eproms. The genesis hardware can bankswap banks of 512 kb except the first 512 kb which contains the bootcode and such. I think you'd be able use M27C160's without issue, though.

(someone correct me if I'm wrong)

[darknezz19]

Hi there elusive. So if I understand you corrrectly the genesis itself can bankswap up to 512kb blocks but except for the first 512kb, that is interesting. In my pictures posted there is a cart PCB that uses presumably 2x 42 pin 2MB roms and a 139 decoder, but it's impossible to tell what all pins it go to. I know for sure it's using the two rom /CE lines as outputs to switch between the two though. Also I have another cart that uses a 42 pin 2MB rom and another 40 pin 1MB rom and a pc74hc00p to switch between the two using the /CE lines on each chip as the outputs and A19 for the input.

pc74hc00p
pdf.datasheetcatalog.com/datasheet/NXP_Semiconductors/74HC_HCT00.pdf

[Deleted]

The MegaDrive doesn't have a banking mechanism built-in ( apart from a 32KB window of 68K memory mapped to the Z80 ). The 512KB banks with the first page being fixed that Elusive mentions are specifications of the SSF2 Mapper ( SEGA 315-5779 ), which is a on-cart chip used in Super Street Fighter 2.

More information can be found here.

[Deleted]

Titan is correct, I apologize for that. It's been some time since I read up on banking.

[darknezz19]

So there is no way to do it with a 139 because there would be no address signal that goes over 0x0FFFFF coming from the Genesis cart connector, not even a21-a24? I got a guy who says he did it before but doesn't remember how he wired it up unfortunately. Says he found the diagram on a Russian forum, but I guess if it were that easy Capcom would have done something like that in the first place.

I'm going to order a SSF2 cart and see if it's pin compatible with an mx29l3211, then try to change the rom data on it. One part about the mapping that is not clear is when it describes 'Bank 0 will always point to the first segment of ROM in order to not interfere with the vector table.' What does this mean? Thanks for the help.