Understanding a 139 decoder to wire up 2 roms.

darknezz19

New Member
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:

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

Extra Hard Mid Boss
darknezz19 said:
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?
If the /E pin of the '139 is tied high, all of its outputs would be permanently high as well. See the function table on page 4 of the datasheet.

Try drawing a schematic of what you're trying to do, it's very difficult to follow just from a description.
 
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